hot water research paper

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Human physiological responses to immersion into water of different temperatures

Affiliation.

1 Faculty of Physical Education and Sports, University Hospital, Charles University, Prague, Czech Republic.
PMID: 10751106
DOI: 10.1007/s004210050065

To differentiate between the effect of cold and hydrostatic pressure on hormone and cardiovascular functions of man, a group of young men was examined during 1-h head-out immersions in water of different temperatures (32 degrees C, 20 degrees C and 14 degrees C). Immersion in water at 32 degrees C did not change rectal temperature and metabolic rate, but lowered heart rate (by 15%) and systolic and diastolic blood pressures (by 11 %, or 12%, respectively), compared to controls at ambient air temperature. Plasma renin activity, plasma cortisol and aldosterone concentrations were also lowered (by 46%, 34%, and 17%, respectively), while diuresis was increased by 107%. Immersion at 20 degrees C induced a similar decrease in plasma renin activity, heart rate and systolic and diastolic blood pressures as immersion at thermoneutrality, in spite of lowered rectal temperature and an increased metabolic rate by 93%. Plasma cortisol concentrations tended to decrease, while plasma aldosterone concentration was unchanged. Diuresis was increased by 89%. No significant differences in changes in diuresis, plasma renin activity and aldosterone concentration compared to subjects immersed to 32 degrees C were observed. Cold water immersion (14 degrees C) lowered rectal temperature and increased metabolic rate (by 350%), heart rate and systolic and diastolic blood pressure (by 5%, 7%, and 8%, respectively). Plasma noradrenaline and dopamine concentrations were increased by 530% and by 250% respectively, while diuresis increased by 163% (more than at 32 degrees C). Plasma aldosterone concentrations increased by 23%. Plasma renin activity was reduced as during immersion in water at the highest temperature. Cortisol concentrations tended to decrease. Plasma adrenaline concentrations remained unchanged. Changes in plasma renin activity were not related to changes in aldosterone concentrations. Immersion in water of different temperatures did not increase blood concentrations of cortisol. There was no correlation between changes in rectal temperature and changes in hormone production. Our data supported the hypothesis that physiological changes induced by water immersion are mediated by humoral control mechanisms, while responses induced by cold are mainly due to increased activity of the sympathetic nervous system.

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Article Contents

Introduction, physiological effects of dehydration, hydration and chronic diseases, water consumption and requirements and relationships to total energy intake, water requirements: evaluation of the adequacy of water intake, acknowledgments, water, hydration, and health.

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Barry M Popkin, Kristen E D'Anci, Irwin H Rosenberg, Water, hydration, and health, Nutrition Reviews , Volume 68, Issue 8, 1 August 2010, Pages 439–458, https://doi.org/10.1111/j.1753-4887.2010.00304.x

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This review examines the current knowledge of water intake as it pertains to human health, including overall patterns of intake and some factors linked with intake, the complex mechanisms behind water homeostasis, and the effects of variation in water intake on health and energy intake, weight, and human performance and functioning. Water represents a critical nutrient, the absence of which will be lethal within days. Water's importance for the prevention of nutrition-related noncommunicable diseases has received more attention recently because of the shift toward consumption of large proportions of fluids as caloric beverages. Despite this focus, there are major gaps in knowledge related to the measurement of total fluid intake and hydration status at the population level; there are also few longer-term systematic interventions and no published randomized, controlled longer-term trials. This review provides suggestions for ways to examine water requirements and encourages more dialogue on this important topic.

Water is essential for life. From the time that primeval species ventured from the oceans to live on land, a major key to survival has been the prevention of dehydration. The critical adaptations cross an array of species, including man. Without water, humans can survive only for days. Water comprises from 75% body weight in infants to 55% in the elderly and is essential for cellular homeostasis and life. 1 Nevertheless, there are many unanswered questions about this most essential component of our body and our diet. This review attempts to provide some sense of our current knowledge of water, including overall patterns of intake and some factors linked with intake, the complex mechanisms behind water homeostasis, the effects of variation in water intake on health and energy intake, weight, and human performance and functioning.

Recent statements on water requirements have been based on retrospective recall of water intake from food and beverages among healthy, noninstitutionalized individuals. Provided here are examples of water intake assessment in populations to clarify the need for experimental studies. Beyond these circumstances of dehydration, it is not fully understood how hydration affects health and well-being, even the impact of water intakes on chronic diseases. Recently, Jéquier and Constant 2 addressed this question based on human physiology, but more knowledge is required about the extent to which water intake might be important for disease prevention and health promotion.

As noted later in the text, few countries have developed water requirements and those that exist are based on weak population-level measures of water intake and urine osmolality. 3 , 4 The European Food Safety Authority (EFSA) was recently asked to revise existing recommended intakes of essential substances with a physiological effect, including water since this nutrient is essential for life and health. 5

The US Dietary Recommendations for water are based on median water intakes with no use of measurements of the dehydration status of the population to assist. One-time collection of blood samples for the analysis of serum osmolality has been used by the National Health and Nutrition Examination Survey program. At the population level, there is no accepted method of assessing hydration status, and one measure some scholars use, hypertonicity, is not even linked with hydration in the same direction for all age groups. 6 Urine indices are used often but these reflect the recent volume of fluid consumed rather than a state of hydration. 7 Many scholars use urine osmolality to measure recent hydration status. 8 , – 12 Deuterium dilution techniques (isotopic dilution with D 2 O, or deuterium oxide) allow measurement of total body water but not water balance status. 13 Currently, there are no completely adequate biomarkers to measure hydration status at the population level.

In discussing water, the focus is first and foremost on all types of water, whether it be soft or hard, spring or well, carbonated or distilled. Furthermore, water is not only consumed directly as a beverage; it is also obtained from food and to a very small extent from oxidation of macronutrients (metabolic water). The proportion of water that comes from beverages and food varies according to the proportion of fruits and vegetables in the diet. The ranges of water content in various foods are presented in Table 1 . In the United States it is estimated that about 22% of water intake comes from food while the percentages are much higher in European countries, particularly a country like Greece with its higher intake of fruits and vegetables, or in South Korea. 3 , – 15 The only in-depth study performed in the United States of water use and water intrinsic to food found a 20.7% contribution from food water; 16 , 17 however, as shown below, this research was dependent on poor overall assessment of water intake.

Ranges of water content for selected foods.

Percentage .	Food item .
100%	Water
90–99%	Fat-free milk, cantaloupe, strawberries, watermelon, lettuce, cabbage, celery, spinach, pickles, squash (cooked)
80–89%	Fruit juice, yogurt, apples, grapes, oranges, carrots, broccoli (cooked), pears, pineapple
70–79%	Bananas, avocados, cottage cheese, ricotta cheese, potato (baked), corn (cooked), shrimp
60–69%	Pasta, legumes, salmon, ice cream, chicken breast
50–59%	Ground beef, hot dogs, feta cheese, tenderloin steak (cooked)
40–49%	Pizza
30–39%	Cheddar cheese, bagels, bread
20–29%	Pepperoni sausage, cake, biscuits
10–19%	Butter, margarine, raisins
1–9%	Walnuts, peanuts (dry roasted), chocolate chip cookies, crackers, cereals, pretzels, taco shells, peanut butter
0%	Oils, sugars

Percentage .	Food item .
100%	Water
90–99%	Fat-free milk, cantaloupe, strawberries, watermelon, lettuce, cabbage, celery, spinach, pickles, squash (cooked)
80–89%	Fruit juice, yogurt, apples, grapes, oranges, carrots, broccoli (cooked), pears, pineapple
70–79%	Bananas, avocados, cottage cheese, ricotta cheese, potato (baked), corn (cooked), shrimp
60–69%	Pasta, legumes, salmon, ice cream, chicken breast
50–59%	Ground beef, hot dogs, feta cheese, tenderloin steak (cooked)
40–49%	Pizza
30–39%	Cheddar cheese, bagels, bread
20–29%	Pepperoni sausage, cake, biscuits
10–19%	Butter, margarine, raisins
1–9%	Walnuts, peanuts (dry roasted), chocolate chip cookies, crackers, cereals, pretzels, taco shells, peanut butter
0%	Oils, sugars

Data from the USDA national nutrient database for standard reference, release 21, as provided in Altman. 126

This review considers water requirements in the context of recent efforts to assess water intake in US populations. The relationship between water and calorie intake is explored both for insights into the possible displacement of calories from sweetened beverages by water and to examine the possibility that water requirements would be better expressed in relation to calorie/energy requirements with the dependence of the latter on age, size, gender, and physical activity level. Current understanding of the exquisitely complex and sensitive system that protects land animals against dehydration is covered and commentary is provided on the complications of acute and chronic dehydration in man, against which a better expression of water requirements might complement the physiological control of thirst. Indeed, the fine intrinsic regulation of hydration and water intake in individuals mitigates prevalent underhydration in populations and its effects on function and disease.

Regulation of fluid intake

To prevent dehydration, reptiles, birds, vertebrates, and all land animals have evolved an exquisitely sensitive network of physiological controls to maintain body water and fluid intake by thirst. Humans may drink for various reasons, particularly for hedonic ones, but drinking is most often due to water deficiency that triggers the so-called regulatory or physiological thirst. The mechanism of thirst is quite well understood today and the reason nonregulatory drinking is often encountered is related to the large capacity of the kidneys to rapidly eliminate excesses of water or to reduce urine secretion to temporarily economize on water. 1 But this excretory process can only postpone the necessity of drinking or of ceasing to drink an excess of water. Nonregulatory drinking is often confusing, particularly in wealthy societies that have highly palatable drinks or fluids that contain other substances the drinker seeks. The most common of these are sweeteners or alcohol for which water is used as a vehicle. Drinking these beverages is not due to excessive thirst or hyperdipsia, as can be shown by offering pure water to individuals instead and finding out that the same drinker is in fact hypodipsic (characterized by abnormally diminished thirst). 1

Fluid balance of the two compartments

Maintaining a constant water and mineral balance requires the coordination of sensitive detectors at different sites in the body linked by neural pathways with integrative centers in the brain that process this information. These centers are also sensitive to humoral factors (neurohormones) produced for the adjustment of diuresis, natriuresis, and blood pressure (angiotensin mineralocorticoids, vasopressin, atrial natriuretic factor). Instructions from the integrative centers to the “executive organs” (kidney, sweat glands, and salivary glands) and to the part of the brain responsible for corrective actions such as drinking are conveyed by certain nerves in addition to the above-mentioned substances. 1

Most of the components of fluid balance are controlled by homeostatic mechanisms responding to the state of body water. These mechanisms are sensitive and precise, and are activated with deficits or excesses of water amounting to only a few hundred milliliters. A water deficit produces an increase in the ionic concentration of the extracellular compartment, which takes water from the intracellular compartment causing cells to shrink. This shrinkage is detected by two types of brain sensors, one controlling drinking and the other controlling the excretion of urine by sending a message to the kidneys, mainly via the antidiuretic hormone vasopressin to produce a smaller volume of more concentrated urine. 18 When the body contains an excess of water, the reverse processes occur: the lower ionic concentration of body fluids allows more water to reach the intracellular compartment. The cells imbibe, drinking is inhibited, and the kidneys excrete more water.

The kidneys thus play a key role in regulating fluid balance. As discussed later, the kidneys function more efficiently in the presence of an abundant water supply. If the kidneys economize on water and produce more concentrated urine, they expend a greater amount of energy and incur more wear on their tissues. This is especially likely to occur when the kidneys are under stress, e.g., when the diet contains excessive amounts of salt or toxic substances that need to be eliminated. Consequently, drinking a sufficient amount of water helps protect this vital organ.

Regulatory drinking

Most drinking occurs in response to signals of water deficit. Apart from urinary excretion, the other main fluid regulatory process is drinking, which is mediated through the sensation of thirst. There are two distinct mechanisms of physiological thirst: the intracellular and the extracellular mechanisms. When water alone is lost, ionic concentration increases. As a result, the intracellular space yields some of its water to the extracellular compartment. Once again, the resulting shrinkage of cells is detected by brain receptors that send hormonal messages to induce drinking. This association with receptors that govern extracellular volume is accompanied by an enhancement of appetite for salt. Thus, people who have been sweating copiously prefer drinks that are relatively rich in Na+ salts rather than pure water. When excessive sweating is experienced, it is also important to supplement drinks with additional salt.

The brain's decision to start or stop drinking and to choose the appropriate drink is made before the ingested fluid can reach the intra- and extracellular compartments. The taste buds in the mouth send messages to the brain about the nature, and especially the salt content, of the ingested fluid, and neuronal responses are triggered as if the incoming water had already reached the bloodstream. These are the so-called anticipatory reflexes: they cannot be entirely “cephalic reflexes” because they arise from the gut as well as the mouth. 1

The anterior hypothalamus and pre-optic area are equipped with osmoreceptors related to drinking. Neurons in these regions show enhanced firing when the inner milieu gets hyperosmotic. Their firing decreases when water is loaded in the carotid artery that irrigates the neurons. It is remarkable that the same decrease in firing in the same neurons takes place when the water load is applied on the tongue instead of being injected into the carotid artery. This anticipatory drop in firing is due to communication from neural pathways that depart from the mouth and converge onto neurons that simultaneously sense the blood's inner milieu.

Nonregulatory drinking

Although everyone experiences thirst from time to time, it plays little role in the day-to-day control of water intake in healthy people living in temperate climates. In these regions, people generally consume fluids not to quench thirst, but as components of everyday foods (e.g., soup, milk), as beverages used as mild stimulants (tea, coffee), and for pure pleasure. A common example is alcohol consumption, which can increase individual pleasure and stimulate social interaction. Drinks are also consumed for their energy content, as in soft drinks and milk, and are used in warm weather for cooling and in cold weather for warming. Such drinking seems to also be mediated through the taste buds, which communicate with the brain in a kind of “reward system”, the mechanisms of which are just beginning to be understood. This bias in the way human beings rehydrate themselves may be advantageous because it allows water losses to be replaced before thirst-producing dehydration takes place. Unfortunately, this bias also carries some disadvantages. Drinking fluids other than water can contribute to an intake of caloric nutrients in excess of requirements, or in alcohol consumption that, in some people, may insidiously bring about dependence. For example, total fluid intake increased from 79 fluid ounces in 1989 to 100 fluid ounces in 2002 among US adults, with the difference representing intake of caloric beverages. 19

Effects of aging on fluid intake regulation

The thirst and fluid ingestion responses of older persons to a number of stimuli have been compared to those of younger persons. 20 Following water deprivation, older individuals are less thirsty and drink less fluid compared to younger persons. 21 , 22 The decrease in fluid consumption is predominantly due to a decrease in thirst, as the relationship between thirst and fluid intake is the same in young and old persons. Older persons drink insufficient amounts of water following fluid deprivation to replenish their body water deficit. 23 When dehydrated older persons are offered a highly palatable selection of drinks, this also fails to result in increased fluid intake. 23 The effects of increased thirst in response to an osmotic load have yielded variable responses, with one group reporting reduced osmotic thirst in older individuals 24 and one failing to find a difference. In a third study, young individuals ingested almost twice as much fluid as old persons, even though the older subjects had a much higher serum osmolality. 25

Overall, these studies support small changes in the regulation of thirst and fluid intake with aging. Defects in both osmoreceptors and baroreceptors appear to exist as do changes in the central regulatory mechanisms mediated by opioid receptors. 26 Because the elderly have low water reserves, it may be prudent for them to learn to drink regularly when not thirsty and to moderately increase their salt intake when they sweat. Better education on these principles may help prevent sudden hypotension and stroke or abnormal fatigue, which can lead to a vicious circle and eventually hospitalization.

Thermoregulation

Hydration status is critical to the body's process of temperature control. Body water loss through sweat is an important cooling mechanism in hot climates and in periods of physical activity. Sweat production is dependent upon environmental temperature and humidity, activity levels, and type of clothing worn. Water losses via skin (both insensible perspiration and sweating) can range from 0.3 L/h in sedentary conditions to 2.0 L/h in high activity in the heat, and intake requirements range from 2.5 to just over 3 L/day in adults under normal conditions, and can reach 6 L/day with high extremes of heat and activity. 27 , 28 Evaporation of sweat from the body results in cooling of the skin. However, if sweat loss is not compensated for with fluid intake, especially during vigorous physical activity, a hypohydrated state can occur with concomitant increases in core body temperature. Hypohydration from sweating results in a loss of electrolytes, as well as a reduction in plasma volume, and this can lead to increased plasma osmolality. During this state of reduced plasma volume and increased plasma osmolality, sweat output becomes insufficient to offset increases in core temperature. When fluids are given to maintain euhydration, sweating remains an effective compensation for increased core temperatures. With repeated exposure to hot environments, the body adapts to heat stress and cardiac output and stroke volume return to normal, sodium loss is conserved, and the risk for heat-stress-related illness is reduced. 29 Increasing water intake during this process of heat acclimatization will not shorten the time needed to adapt to the heat, but mild dehydration during this time may be of concern and is associated with elevations in cortisol, increased sweating, and electrolyte imbalances. 29

Children and the elderly have differing responses to ambient temperature and different thermoregulatory concerns than healthy adults. Children in warm climates may be more susceptible to heat illness than adults due to their greater surface area to body mass ratio, lower rate of sweating, and slower rate of acclimatization to heat. 30 , 31 Children may respond to hypohydration during activity with a higher relative increase in core temperature than adults, 32 and with a lower propensity to sweat, thus losing some of the benefits of evaporative cooling. However, it has been argued that children can dissipate a greater proportion of body heat via dry heat loss, and the concomitant lack of sweating provides a beneficial means of conserving water under heat stress. 30 Elders, in response to cold stress, show impairments in thermoregulatory vasoconstriction, and body water is shunted from plasma into the interstitial and intracellular compartments. 33 , 34 With respect to heat stress, water lost through sweating decreases the water content of plasma, and the elderly are less able to compensate for increased blood viscosity. 33 Not only do they have a physiological hypodipsia, but this can be exaggerated by central nervous system disease 35 and by dementia. 36 In addition, illness and limitations in daily living activities can further limit fluid intake. When reduced fluid intake is coupled with advancing age, there is a decrease in total body water. Older individuals have impaired renal fluid conservation mechanisms and, as noted above, have impaired responses to heat and cold stress. 33 , 34 All of these factors contribute to an increased risk of hypohydration and dehydration in the elderly.

With regard to physiology, the role of water in health is generally characterized in terms of deviations from an ideal hydrated state, generally in comparison to dehydration. The concept of dehydration encompasses both the process of losing body water and the state of dehydration. Much of the research on water and physical or mental functioning compares a euhydrated state, usually achieved by provision of water sufficient to overcome water loss, to a dehydrated state, which is achieved via withholding of fluids over time and during periods of heat stress or high activity. In general, provision of water is beneficial in individuals with a water deficit, but little research supports the notion that additional water in adequately hydrated individuals confers any benefit.

Physical performance

The role of water and hydration in physical activity, particularly in athletes and in the military, has been of considerable interest and is well-described in the scientific literature. 37 , – 39 During challenging athletic events, it is not uncommon for athletes to lose 6–10% of body weight through sweat, thus leading to dehydration if fluids have not been replenished. However, decrements in the physical performance of athletes have been observed under much lower levels of dehydration, i.e., as little as 2%. 38 Under relatively mild levels of dehydration, individuals engaging in rigorous physical activity will experience decrements in performance related to reduced endurance, increased fatigue, altered thermoregulatory capability, reduced motivation, and increased perceived effort. 40 , 41 Rehydration can reverse these deficits and reduce the oxidative stress induced by exercise and dehydration. 42 Hypohydration appears to have a more significant impact on high-intensity and endurance activity, such as tennis 43 and long-distance running, 44 than on anaerobic activities, 45 such as weight lifting, or on shorter-duration activities, such as rowing. 46

During exercise, individuals may not hydrate adequately when allowed to drink according to thirst. 32 After periods of physical exertion, voluntary fluid intake may be inadequate to offset fluid deficits. 1 Thus, mild-to-moderate dehydration can persist for some hours after the conclusion of physical activity. Research performed on athletes suggests that, principally at the beginning of the training season, they are at particular risk for dehydration due to lack of acclimatization to weather conditions or suddenly increased activity levels. 47 , 48 A number of studies show that performance in temperate and hot climates is affected to a greater degree than performance in cold temperatures. 41 , – 50 Exercise in hot conditions with inadequate fluid replacement is associated with hyperthermia, reduced stroke volume and cardiac output, decreases in blood pressure, and reduced blood flow to muscle. 51

During exercise, children may be at greater risk for voluntary dehydration. Children may not recognize the need to replace lost fluids, and both children as well as coaches need specific guidelines for fluid intake. 52 Additionally, children may require more time to acclimate to increases in environmental temperature than adults. 30 , 31 Recommendations are for child athletes or children in hot climates to begin athletic activities in a well-hydrated state and to drink fluids over and above the thirst threshold.

Cognitive performance

Water, or its lack (dehydration), can influence cognition. Mild levels of dehydration can produce disruptions in mood and cognitive functioning. This may be of special concern in the very young, very old, those in hot climates, and those engaging in vigorous exercise. Mild dehydration produces alterations in a number of important aspects of cognitive function such as concentration, alertness, and short-term memory in children (10–12 y), 32 young adults (18–25 y), 53 , – 56 and the oldest adults (50–82 y). 57 As with physical functioning, mild-to-moderate levels of dehydration can impair performance on tasks such as short-term memory, perceptual discrimination, arithmetic ability, visuomotor tracking, and psychomotor skills. 53 , – 56 However, mild dehydration does not appear to alter cognitive functioning in a consistent manner. 53 , – 58 In some cases, cognitive performance was not significantly affected in ranges from 2% to 2.6% dehydration. 56 , 58 Comparing across studies, performance on similar cognitive tests was divergent under dehydration conditions. 54 , 56 In studies conducted by Cian et al., 53 , 54 participants were dehydrated to approximately 2.8% either through heat exposure or treadmill exercise. In both studies, performance was impaired on tasks examining visual perception, short-term memory, and psychomotor ability. In a series of studies using exercise in conjunction with water restriction as a means of producing dehydration, D'Anci et al. 56 observed only mild decrements in cognitive performance in healthy young men and women athletes. In these experiments, the only consistent effect of mild dehydration was significant elevations of subjective mood score, including fatigue, confusion, anger, and vigor. Finally, in a study using water deprivation alone over a 24-h period, no significant decreases in cognitive performance were seen with 2.6% dehydration. 58 It is therefore possible that heat stress may play a critical role in the effects of dehydration on cognitive performance.

Reintroduction of fluids under conditions of mild dehydration can reasonably be expected to reverse dehydration-induced cognitive deficits. Few studies have examined how fluid reintroduction may alleviate the negative effects of dehydration on cognitive performance and mood. One study 59 examined how water ingestion affected arousal and cognitive performance in young people following a period of 12-h water restriction. While cognitive performance was not affected by either water restriction or water consumption, water ingestion affected self-reported arousal. Participants reported increased alertness as a function of water intake. Rogers et al. 60 observed a similar increase in alertness following water ingestion in both high- and low-thirst participants. Water ingestion, however, had opposite effects on cognitive performance as a function of thirst. High-thirst participants' performance on a cognitively demanding task improved following water ingestion, but low-thirst participants' performance declined. In summary, hydration status consistently affected self-reported alertness, but effects on cognition were less consistent.

Several recent studies have examined the utility of providing water to school children on attentiveness and cognitive functioning in children. 61 , – 63 In these experiments, children were not fluid restricted prior to cognitive testing, but were allowed to drink as usual. Children were then provided with a drink or no drink 20–45 min before the cognitive test sessions. In the absence of fluid restriction and without physiological measures of hydration status, the children in these studies should not be classified as dehydrated. Subjective measures of thirst were reduced in children given water, 62 and voluntary water intake in children varied from 57 mL to 250 mL. In these studies, as in the studies in adults, the findings were divergent and relatively modest. In the research led by Edmonds et al., 61 , 62 children in the groups given water showed improvements in visual attention. However, effects on visual memory were less consistent, with one study showing no effects of drinking water on a spot-the-difference task in 6–7-year-old children 61 and the other showing a significant improvement in a similar task in 7–9-year-old children. 62 In the research described by Benton and Burgess, 63 memory performance was improved by provision of water but sustained attention was not altered with provision of water in the same children.

Taken together, these studies indicate that low-to-moderate dehydration may alter cognitive performance. Rather than indicating that the effects of hydration or water ingestion on cognition are contradictory, many of the studies differ significantly in methodology and in measurement of cognitive behaviors. These variances in methodology underscore the importance of consistency when examining relatively subtle chances in overall cognitive performance. However, in those studies in which dehydration was induced, most combined heat and exercise; this makes it difficult to disentangle the effects of dehydration on cognitive performance in temperate conditions from the effects of heat and exercise. Additionally, relatively little is known about the mechanism of mild dehydration's effects on mental performance. It has been proposed that mild dehydration acts as a physiological stressor that competes with and draws attention from cognitive processes. 64 However, research on this hypothesis is limited and merits further exploration.

Dehydration and delirium

Dehydration is a risk factor for delirium and for delirium presenting as dementia in the elderly and in the very ill. 65 , – 67 Recent work shows that dehydration is one of several predisposing factors for confusion observed in long-term-care residents 67 ; however, in this study, daily water intake was used as a proxy measure for dehydration rather than other, more direct clinical assessments such as urine or plasma osmolality. Older people have been reported as having reduced thirst and hypodipsia relative to younger people. In addition, fluid intake and maintenance of water balance can be complicated by factors such as disease, dementia, incontinence, renal insufficiency, restricted mobility, and drug side effects. In response to primary dehydration, older people have less thirst sensation and reduced fluid intakes in comparison to younger people. However, in response to heat stress, while older people still display a reduced thirst threshold, they do ingest comparable amounts of fluid to younger people. 20

Gastrointestinal function

Fluids in the diet are generally absorbed in the proximal small intestine, and the absorption rate is determined by the rate of gastric emptying to the small intestine. Therefore, the total volume of fluid consumed will eventually be reflected in water balance, but the rate at which rehydration occurs is dependent upon factors affecting the rate of delivery of fluids to the intestinal mucosa. The gastric emptying rate is generally accelerated by the total volume consumed and slowed by higher energy density and osmolality. 68 In addition to water consumed in food (1 L/day) and beverages (circa 2–3 L/day), digestive secretions account for a far greater portion of water that passes through and is absorbed by the gastrointestinal tract (circa 8 L/day). 69 The majority of this water is absorbed by the small intestine, with a capacity of up to 15 L/day with the colon absorbing some 5 L/day. 69

Constipation, characterized by slow gastrointestinal transit, small, hard stools, and difficulty in passing stool, has a number of causes, including medication use, inadequate fiber intake, poor diet, and illness. 70 Inadequate fluid consumption is touted as a common culprit in constipation, and increasing fluid intake is a frequently recommended treatment. Evidence suggests, however, that increasing fluids is only useful to individuals in a hypohydrated state, and is of little utility in euhydrated individuals. 70 In young children with chronic constipation, increasing daily water intake by 50% did not affect constipation scores. 71 For Japanese women with low fiber intake, concomitant low water intake in the diet is associated with increased prevalence of constipation. 72 In older individuals, low fluid intake is a predictor for increased levels of acute constipation, 73 , 74 with those consuming the least amount of fluid having over twice the frequency of constipation episodes than those consuming the most fluid. In one trial, researchers compared the utility of carbonated mineral water in reducing functional dyspepsia and constipation scores to tap water in individuals with functional dyspepsia. 75 When comparing carbonated mineral water to tap water, participants reported improvements in subjective gastric symptoms, but there were no significant improvements in gastric or intestinal function. The authors indicate it is not possible to determine to what degree the mineral content of the two waters contributed to perceived symptom relief, as the mineral water contained greater levels of magnesium and calcium than the tap water. The available evidence suggests that increased fluid intake should only be indicated in individuals in a hypohydrated state. 69 , 71

Significant water loss can occur through the gastrointestinal tract, and this can be of great concern in the very young. In developing countries, diarrheal diseases are a leading cause of death in children, resulting in approximately 1.5–2.5 million deaths per year. 76 Diarrheal illness results not only in a reduction in body water, but also in potentially lethal electrolyte imbalances. Mortality in such cases can many times be prevented with appropriate oral rehydration therapy, by which simple dilute solutions of salt and sugar in water can replace fluid lost by diarrhea. Many consider application of oral rehydration therapy to be one of the significant public health developments of the last century. 77

Kidney function

As noted above, the kidney is crucial in regulating water balance and blood pressure as well as removing waste from the body. Water metabolism by the kidney can be classified into regulated and obligate. Water regulation is hormonally mediated, with the goal of maintaining a tight range of plasma osmolality (between 275 and 290 mOsm/kg). Increases in plasma osmolality and activation of osmoreceptors (intracellular) and baroreceptors (extracellular) stimulate hypothalamic release of arginine vasopressin (AVP). AVP acts at the kidney to decrease urine volume and promote retention of water, and the urine becomes hypertonic. With decreased plasma osmolality, vasopressin release is inhibited, and the kidney increases hypotonic urinary output.

In addition to regulating fluid balance, the kidneys require water for the filtration of waste from the bloodstream and excretion via urine. Water excretion via the kidney removes solutes from the blood, and a minimum obligate urine volume is required to remove the solute load with a maximum output volume of 1 L/h. 78 This obligate volume is not fixed, but is dependent upon the amount of metabolic solutes to be excreted and levels of AVP. Depending on the need for water conservation, basal urine osmolality ranges from 40 mOsm/kg to a maximum of 1,400 mOsm/kg. 78 The ability to both concentrate and dilute urine decreases with age, with a lower value of 92 mOsm/kg and an upper range falling between 500 and 700 mOsm/kg for individuals over the age of 70 years. 79 , – 81 Under typical conditions, in an average adult, urine volume of 1.5 to 2.0 L/day would be sufficient to clear a solute load of 900 to 1,200 mOsm/day. During water conservation and the presence of AVP, this obligate volume can decrease to 0.75–1.0 L/day and during maximal diuresis up to 20 L/day can be required to remove the same solute load. 78 , – 81 In cases of water loading, if the volume of water ingested cannot be compensated for with urine output, having overloaded the kidney's maximal output rate, an individual can enter a hyponatremic state.

Heart function and hemodynamic response

Blood volume, blood pressure, and heart rate are closely linked. Blood volume is normally tightly regulated by matching water intake and water output, as described in the section on kidney function. In healthy individuals, slight changes in heart rate and vasoconstriction act to balance the effect of normal fluctuations in blood volume on blood pressure. 82 Decreases in blood volume can occur, through blood loss (or blood donation), or loss of body water through sweat, as seen with exercise. Blood volume is distributed differently relative to the position of the heart, whether supine or upright, and moving from one position to the other can lead to increased heart rate, a fall in blood pressure, and, in some cases, syncope. This postural hypotension (or orthostatic hypotension) can be mediated by drinking 300–500 mL of water. 83 , 84 Water intake acutely reduces heart rate and increases blood pressure in both normotensive and hypertensive individuals. 85 These effects of water intake on the pressor effect and heart rate occur within 15–20 min of drinking water and can last for up to 60 min. Water ingestion is also beneficial in preventing vasovagal reaction with syncope in blood donors at high risk for post-donation syncope. 86 The effect of water intake in these situations is thought to be due to effects on the sympathetic nervous system rather than to changes in blood volume. 83 , 84 Interestingly, in rare cases, individuals may experience bradycardia and syncope after swallowing cold liquids. 87 , – 89 While swallow syncope can be seen with substances other than water, swallow syncope further supports the notion that the result of water ingestion in the pressor effect has both a neural component as well as a cardiac component.

Water deprivation and dehydration can lead to the development of headache. 90 Although this observation is largely unexplored in the medical literature, some observational studies indicate that water deprivation, in addition to impairing concentration and increasing irritability, can serve as a trigger for migraine and can also prolong migraine. 91 , 92 In those with water deprivation-induced headache, ingestion of water provided relief from headache in most individuals within 30 min to 3 h. 92 It is proposed that water deprivation-induced headache is the result of intracranial dehydration and total plasma volume. Although provision of water may be useful in relieving dehydration-related headache, the utility of increasing water intake for the prevention of headache is less well documented.

The folk wisdom that drinking water can stave off headaches has been relatively unchallenged, and has more traction in the popular press than in the medical literature. Recently, one study examined increased water intake and headache symptoms in headache patients. 93 In this randomized trial, patients with a history of different types of headache, including migraine and tension headache, were either assigned to a placebo condition (a nondrug tablet) or the increased water condition. In the water condition, participants were instructed to consume an additional volume of 1.5 L water/day on top of what they already consumed in foods and fluids. Water intake did not affect the number of headache episodes, but it was modestly associated with reduction in headache intensity and reduced duration of headache. The data from this study suggest that the utility of water as prophylaxis is limited in headache sufferers, and the ability of water to reduce or prevent headache in the broader population remains unknown.

One of the more pervasive myths regarding water intake is its relation to improvements of the skin or complexion. By improvement, it is generally understood that individuals are seeking to have a more “moisturized” look to the surface skin, or to minimize acne or other skin conditions. Numerous lay sources such as beauty and health magazines as well as postings on the Internet suggest that drinking 8–10 glasses of water a day will “flush toxins from the skin” and “give a glowing complexion” despite a general lack of evidence 94 , 95 to support these proposals. The skin, however, is important for maintaining body water levels and preventing water loss into the environment.

The skin contains approximately 30% water, which contributes to plumpness, elasticity, and resiliency. The overlapping cellular structure of the stratum corneum and lipid content of the skin serves as “waterproofing” for the body. 96 Loss of water through sweat is not indiscriminate across the total surface of the skin, but is carried out by eccrine sweat glands, which are evenly distributed over most of the body surface. 97 Skin dryness is usually associated with exposure to dry air, prolonged contact with hot water and scrubbing with soap (both strip oils from the skin), medical conditions, and medications. While more serious levels of dehydration can be reflected in reduced skin turgor, 98 , 99 with tenting of the skin acting as a flag for dehydration, overt skin turgor in individuals with adequate hydration is not altered. Water intake, particularly in individuals with low initial water intake, can improve skin thickness and density as measured by sonogram, 100 offsets transepidermal water loss, and can improve skin hydration. 101 Adequate skin hydration, however, is not sufficient to prevent wrinkles or other signs of aging, which are related to genetics and to sun and environmental damage. Of more utility to individuals already consuming adequate fluids is the use of topical emollients; these will improve skin barrier function and improve the look and feel of dry skin. 102 , 103

Many chronic diseases have multifactorial origins. In particular, differences in lifestyle and the impact of environment are known to be involved and constitute risk factors that are still being evaluated. Water is quantitatively the most important nutrient. In the past, scientific interest with regard to water metabolism was mainly directed toward the extremes of severe dehydration and water intoxication. There is evidence, however, that mild dehydration may also account for some morbidities. 4 , 104 There is currently no consensus on a “gold standard” for hydration markers, particularly for mild dehydration. As a consequence, the effects of mild dehydration on the development of several disorders and diseases have not been well documented.

There is strong evidence showing that good hydration reduces the risk of urolithiasis (see Table 2 for evidence categories). Less strong evidence links good hydration with reduced incidence of constipation, exercise asthma, hypertonic dehydration in the infant, and hyperglycemia in diabetic ketoacidosis. Good hydration is associated with a reduction in urinary tract infections, hypertension, fatal coronary heart disease, venous thromboembolism, and cerebral infarct, but all these effects need to be confirmed by clinical trials. For other conditions such as bladder or colon cancer, evidence of a preventive effect of maintaining good hydration is not consistent (see Table 3 ).

Categories of evidence used in evaluating the quality of reports.

Classification .	Description .
Strong	Evidence from a meta-analysis of randomized, controlled trials
Strong	Evidence from at least one randomized, controlled trial
Less strong	Evidence from at least one controlled study without randomization
Less strong	Evidence from at least one other type of quasi-experimental study
Weaker	Evidence from descriptive studies, such as comparative studies, correlation studies, and case control studies
Weaker	Evidence from expert committee reports, opinions or clinical experience of respected authorities, or both
Inconsistent	Evidence from small studies with inconsistent outcomes
Speculative	Posited relationships are based essentially on extrapolation from mechanism

Classification .	Description .
Strong	Evidence from a meta-analysis of randomized, controlled trials
Strong	Evidence from at least one randomized, controlled trial
Less strong	Evidence from at least one controlled study without randomization
Less strong	Evidence from at least one other type of quasi-experimental study
Weaker	Evidence from descriptive studies, such as comparative studies, correlation studies, and case control studies
Weaker	Evidence from expert committee reports, opinions or clinical experience of respected authorities, or both
Inconsistent	Evidence from small studies with inconsistent outcomes
Speculative	Posited relationships are based essentially on extrapolation from mechanism

Data adapted from Manz. 104

Summary of evidence for association of hydration status with chronic diseases.

Chronic disease .	Evidence level .	Findings .
Urolithiasis	Strong	Increased urine volume from increased fluid intake reduces stone recurrence. Favorable associations between increased hydration status and lower stone recurrence rate.
Bronchopulmonary disorders	Strong	Exercise-related asthma is linked with low fluid intake.
Hypertonic dehydration in infants	Less strong	In infants with gastroenteritis, a high urine osmolality due to a high protein and sodium content of formula and weaning foods increases the risk of hypertonic dehydration.
Diabetic hyperglycemia and ketoacidosis	Less strong	In diabetics, experimentally induced dehydration promotes development of hyperglycemia.  Higher serum osmolality at time of hospital admission was the most important predictor of death in children with diabetic ketoacidosis.
Morphological and functional changes in the kidney	Weaker	In patients with polycystic kidney disease and chronic renal failure, sustained high urine volumes with urine osmolalities below plasma osmolality accelerate the decline of glomerular filtration rate.
Hypertension	Weaker	In diabetic patients, lower urine flow and sodium excretion rates are associated with higher blood pressure during the day and a reduced fall in blood pressure at night. In a study of 1,688 healthy men, a low day-to-night ratio for urine production was not associated with hypertension. In one study, eight male hypertensive volunteers and eight controls were exercised in a hot environment with or without water ingestion. In hypertensive men, water ingestion increased exercise-related differences in their systolic and diastolic blood pressure.
Fatal coronary heart disease	Weaker	High water intake is associated with lower risk of fatal heart disease.
Venous thromboembolism	Weaker	High serum osmolality after stroke is associated with increased rate of thromboembolism.
Cerebral infarct (stroke)	Weaker	Increased serum osmolality or hematocrit is associated with increased risk of stroke morbidity/mortality.  Stroke patients with initial midrange hematocrit have better discharge outcomes.
Dental diseases	Weaker	Salivary output decreases with dehydration. Hypohydration may be linked with dental disease.
Urinary tract infection (UTI)	Weaker	Occurrence of UTI is associated with low fluid intake or low urine output. No definitive evidence links susceptibility to UTI to fluid intake.
Bladder and colon cancer	Inconsistent	Generally no association between fluid intake and cancer risk or tumor recurrence.
Gallstone	Speculative	Water intake induces gallbladder emptying, suggesting that a high daily water intake may prevent gallstone formation.
Mitral valve prolapse	Speculative	Mitral valve prolapse developed after dehydration in 1 in 10 healthy men.
Glaucoma	Speculative	Dehydration reduces intraocular pressure and elevated colloid osmotic pressure. Intraocular pressure increases minutes after water ingestion and remains elevated above baseline for up to 45 min post-ingestion.

Chronic disease .	Evidence level .	Findings .
Urolithiasis	Strong	Increased urine volume from increased fluid intake reduces stone recurrence. Favorable associations between increased hydration status and lower stone recurrence rate.
Bronchopulmonary disorders	Strong	Exercise-related asthma is linked with low fluid intake.
Hypertonic dehydration in infants	Less strong	In infants with gastroenteritis, a high urine osmolality due to a high protein and sodium content of formula and weaning foods increases the risk of hypertonic dehydration.
Diabetic hyperglycemia and ketoacidosis	Less strong	In diabetics, experimentally induced dehydration promotes development of hyperglycemia.  Higher serum osmolality at time of hospital admission was the most important predictor of death in children with diabetic ketoacidosis.
Morphological and functional changes in the kidney	Weaker	In patients with polycystic kidney disease and chronic renal failure, sustained high urine volumes with urine osmolalities below plasma osmolality accelerate the decline of glomerular filtration rate.
Hypertension	Weaker	In diabetic patients, lower urine flow and sodium excretion rates are associated with higher blood pressure during the day and a reduced fall in blood pressure at night. In a study of 1,688 healthy men, a low day-to-night ratio for urine production was not associated with hypertension. In one study, eight male hypertensive volunteers and eight controls were exercised in a hot environment with or without water ingestion. In hypertensive men, water ingestion increased exercise-related differences in their systolic and diastolic blood pressure.
Fatal coronary heart disease	Weaker	High water intake is associated with lower risk of fatal heart disease.
Venous thromboembolism	Weaker	High serum osmolality after stroke is associated with increased rate of thromboembolism.
Cerebral infarct (stroke)	Weaker	Increased serum osmolality or hematocrit is associated with increased risk of stroke morbidity/mortality.  Stroke patients with initial midrange hematocrit have better discharge outcomes.
Dental diseases	Weaker	Salivary output decreases with dehydration. Hypohydration may be linked with dental disease.
Urinary tract infection (UTI)	Weaker	Occurrence of UTI is associated with low fluid intake or low urine output. No definitive evidence links susceptibility to UTI to fluid intake.
Bladder and colon cancer	Inconsistent	Generally no association between fluid intake and cancer risk or tumor recurrence.
Gallstone	Speculative	Water intake induces gallbladder emptying, suggesting that a high daily water intake may prevent gallstone formation.
Mitral valve prolapse	Speculative	Mitral valve prolapse developed after dehydration in 1 in 10 healthy men.
Glaucoma	Speculative	Dehydration reduces intraocular pressure and elevated colloid osmotic pressure. Intraocular pressure increases minutes after water ingestion and remains elevated above baseline for up to 45 min post-ingestion.

Categories of evidence: described in Table 2 .

Water consumption, water requirements, and energy intake are linked in fairly complex ways. This is partially because physical activity and energy expenditures affect the need for water but also because a large shift in beverage consumption over the past century or more has led to consumption of a significant proportion of our energy intake from caloric beverages. Nonregulatory beverage intake, as noted earlier, has assumed a much greater role for individuals. 19 This section reviews current patterns of water intake and then refers to a full meta-analysis of the effects of added water on energy intake. This includes adding water to the diet and water replacement for a range of caloric and diet beverages, including sugar-sweetened beverages, juice, milk, and diet beverages. The third component is a discussion of water requirements and suggestions for considering the use of mL water/kcal energy intake as a metric.

Patterns and trends of water consumption

Measurement of total fluid water consumption in free-living individuals is fairly new in focus. As a result, the state of the science is poorly developed, data are most likely fairly incomplete, and adequate validation of the measurement techniques used is not available. Presented here are varying patterns and trends of water intake for the United States over the past three decades followed by a brief review of the work on water intake in Europe.

There is really no existing information to support an assumption that consumption of water alone or beverages containing water affects hydration differentially. 3 , 105 Some epidemiological data suggest water might have different metabolic effects when consumed alone rather than as a component of caffeinated or flavored or sweetened beverages; however, these data are at best suggestive of an issue deserving further exploration. 106 , 107 As shown below, the research of Ershow et al. indicates that beverages not consisting solely of water do contain less than 100% water.

One study in the United States has attempted to examine all the dietary sources of water. 16 , 17 These data are cited in Table 4 as the Ershow study and were based on National Food Consumption Survey food and fluid intake data from 1977–1978. These data are presented in Table 4 for children aged 2–18 years (Panel A) and for adults aged 19 years and older (Panel B). Ershow et al. 16 , 17 spent a great deal of time working out ways to convert USDA dietary data into water intake, including water absorbed during the cooking process, water in food, and all sources of drinking water.

Beverage pattern trends in the United States for children aged 2–18 years and adults aged 19 years and older, (nationally representative).

Beverage type .	1965 .	Ershow .	Original National Food Consumption Survey food and fluid intake data .
.	.	1977–1978 .	1977–1978 .	1988–1994 .	1989–1991 .	1994–1998 .	1999–2002 .	2005–2006 .
Panel A. The amount of beverages consumed in mL per capita for children
Water intrinsic to food		393	393
Water added during cooking		95	95
Water as a beverage		624	624	835	520	531	715	552
Water added to other beverages		186
Water intrinsic to other beverages		594
Water total from all sources		1892
Unsweetened coffee and tea	72		62	27	39	22	18	19
Low-fat milk	20		21	42	52	66	66	70
Diet	5		11	30	31	34	25	41
Nutrients	566		506	427	447	418	425	406
Caloric	212		250	436	311	455	525	442
Total mL of beverages	875		1961	1798	1400	1527	1774	1530
Panel B. The amount of beverages consumed in mL per capita for adults
Water intrinsic to food		488	488
Water added during cooking		95	95
Water as a beverage		736	736	1248	792	856	1352	1127
Water added to other beverages		630
Water intrinsic to other beverages		429
Water total from all sources		2377
Unsweetened coffee and tea	640		544	551	484	464	428	452
Low-fat milk	14		18	54	53	57	68	62
Diet	13		27	117	95	119	159	220
Nutrients	306		328	419	317	340	431	458
Caloric	136		201	418	264	402	549	474
Total mL of beverages	1109		2437	2808	2005	2238	2987	2793

Beverage type .	1965 .	Ershow .	Original National Food Consumption Survey food and fluid intake data .
.	.	1977–1978 .	1977–1978 .	1988–1994 .	1989–1991 .	1994–1998 .	1999–2002 .	2005–2006 .
Panel A. The amount of beverages consumed in mL per capita for children
Water intrinsic to food		393	393
Water added during cooking		95	95
Water as a beverage		624	624	835	520	531	715	552
Water added to other beverages		186
Water intrinsic to other beverages		594
Water total from all sources		1892
Unsweetened coffee and tea	72		62	27	39	22	18	19
Low-fat milk	20		21	42	52	66	66	70
Diet	5		11	30	31	34	25	41
Nutrients	566		506	427	447	418	425	406
Caloric	212		250	436	311	455	525	442
Total mL of beverages	875		1961	1798	1400	1527	1774	1530
Panel B. The amount of beverages consumed in mL per capita for adults
Water intrinsic to food		488	488
Water added during cooking		95	95
Water as a beverage		736	736	1248	792	856	1352	1127
Water added to other beverages		630
Water intrinsic to other beverages		429
Water total from all sources		2377
Unsweetened coffee and tea	640		544	551	484	464	428	452
Low-fat milk	14		18	54	53	57	68	62
Diet	13		27	117	95	119	159	220
Nutrients	306		328	419	317	340	431	458
Caloric	136		201	418	264	402	549	474
Total mL of beverages	1109		2437	2808	2005	2238	2987	2793

Note: The data are age and sex adjusted to 1965.

Values stem from the Ershow calculations. 16

These researchers created a number of categories and used a range of factors measured in other studies to estimate the water categories. The water that is found in food, based on food composition table data, was 393 mL for children. The water that was added as a result of cooking (e.g., rice) was 95 mL. Water consumed as a beverage directly as water was 624 mL. The water found in other fluids, as noted, comprised the remainder of the milliliters, with the highest levels in whole-fat milk and juices (506 mL). There is a small discrepancy between the Ershow data regarding total fluid intake measures for these children and the normal USDA figures. That is because the USDA does not remove milk fats and solids, fiber, and other food constituents found in beverages, particularly juice and milk.

A key point illustrated by these nationally representative US data is the enormous variability between survey waves in the amount of water consumed (see Figure 1 , which highlights the large variation in water intake as measured in these surveys). Although water intake by adults and children increased and decreased at the same time, for reasons that cannot be explained, the variation was greater among children than adults. This is partly because the questions the surveys posed varied over time and there was no detailed probing for water intake, because the focus was on obtaining measures of macro- and micronutrients. Dietary survey methods used in the past have focused on obtaining data on foods and beverages containing nutrient and non-nutritive sweeteners but not on water. Related to this are the huge differences between the the USDA surveys and the National Health and Nutrition Examination Survey (NHANES) performed in 1988–1994 and in 1999 and later. In addition, even the NHANES 1999–2002 and 2003–2006 surveys differ greatly. These differences reflect a shift in the mode of questioning with questions on water intake being included as part of a standard 24-h recall rather than as stand-alone questions. Water intake was not even measured in 1965, and a review of the questionnaires and the data reveals clear differences in the way the questions have been asked and the limitations on probes regarding water intake. Essentially, in the past people were asked how much water they consumed in a day and now they are asked for this information as part of a 24-h recall survey. However, unlike for other caloric and diet beverages, there are limited probes for water alone. The results must thus be viewed as crude approximations of total water intake without any strong research to show if they are over- or underestimated. From several studies of water and two ongoing randomized controlled trials performed by us, it is clear that probes that include consideration of all beverages and include water as a separate item result in the provision of more complete data.

Water consumption trends from USDA and NHANES surveys (mL/day/capita), nationally representative. Note: this includes water from fluids only, excluding water in foods. Sources for 1965, 1977–1978, 1989–1991, and 1994–1998, are USDA. Others are NHANES and 2005–2006 is joint USDA and NHANES.

Water consumption data for Europe are collected far more selectively than even the crude water intake questions from NHANES. A recent report from the European Food Safety Agency provides measures of water consumption from a range of studies in Europe. 4 , – 109 Essentially, what these studies show is that total water intake is lower across Europe than in the United States. As with the US data, none are based on long-term, carefully measured or even repeated 24-h recall measures of water intake from food and beverages. In an unpublished examination of water intake in UK adults in 1986–1987 and in 2001–2002, Popkin and Jebb have found that although intake increased by 226 mL/day over this time period, it was still only 1,787 mL/day in the latter period (unpublished data available from BP); this level is far below the 2,793 mL/day recorded in the United States for 2005–2006 or the earlier US figures for comparably aged adults.

A few studies have been performed in the United States and Europe utilizing 24-h urine and serum osmolality measures to determine total water turnover and hydration status. Results of these studies suggest that US adults consume over 2,100 mL of water per day while adults in Europe consume less than half a liter. 4 , 110 Data on total urine collection would appear to be another useful measure for examining total water intake. Of course, few studies aside from the Donald Study of an adolescent cohort in Germany have collected such data on population levels for large samples. 109

Effects of water consumption on overall energy intake

There is an extensive body of literature that focuses on the impact of sugar-sweetened beverages on weight and the risk of obesity, diabetes, and heart disease; however, the perspective of providing more water and its impact on health has not been examined. The literature on water does not address portion sizes; instead, it focuses mainly on water ad libitum or in selected portions compared with other caloric beverages. A detailed meta-analysis of the effects of water intake alone (i.e., adding additional water) and as a replacement for sugar-sweetened beverages, juice, milk, and diet beverages appears elsewhere. 111

In general, the results of this review suggest that water, when consumed in place of sugar-sweetened beverages, juice, and milk, is linked with reduced energy intake. This finding is mainly derived from clinical feeding studies but also from one very good randomized, controlled school intervention and several other epidemiological and intervention studies. Aside from the issue of portion size, factors such as the timing of beverage and meal intake (i.e., the delay between consumption of the beverage and consumption of the meal) and types of caloric sweeteners remain to be considered. However, when beverages are consumed in normal free-living conditions in which five to eight daily eating occasions are the norm, the delay between beverage and meal consumption may matter less. 112 , – 114

The literature on the water intake of children is extremely limited. However, the excellent German school intervention with water suggests the effects of water on the overall energy intake of children might be comparable to that of adults. 115 In this German study, children were educated on the value of water and provided with special filtered drinking fountains and water bottles in school. The intervention schoolchildren increased their water intake by 1.1 glasses/day ( P < 0.001) and reduced their risk of overweight by 31% (OR = 0.69, P = 0.40).

Classically, water data are examined in terms of milliliters (or some other measure of water volume consumed per capita per day by age group). This measure does not link fluid intake and caloric intake. Disassociation of fluid and calorie intake is difficult for clinicians dealing with older persons with reduced caloric intake. This milliliter water measure assumes some mean body size (or surface area) and a mean level of physical activity – both of which are determinants of not only energy expenditure but also water balance. Children are dependent on adults for access to water, and studies suggest that their larger surface area to volume ratio makes them susceptible to changes in skin temperatures linked with ambient temperature shifts. 116 One option utilized by some scholars is to explore food and beverage intake in milliliters per kilocalorie (mL/kcal), as was done in the 1989 US recommended dietary allowances. 4 , 117 This is an option that is interpretable for clinicians and which incorporates, in some sense, body size or surface area and activity. Its disadvantage is that water consumed with caloric beverages affects both the numerator and the denominator; however, an alternative measure that could be independent of this direct effect on body weight and/or total caloric intake is not presently known.

Despite its critical importance in health and nutrition, the array of available research that serves as a basis for determining requirements for water or fluid intake, or even rational recommendations for populations, is limited in comparison with most other nutrients. While this deficit may be partly explained by the highly sensitive set of neurophysiological adaptations and adjustments that occur over a large range of fluid intakes to protect body hydration and osmolarity, this deficit remains a challenge for the nutrition and public health community. The latest official effort at recommending water intake for different subpopulations occurred as part of the efforts to establish Dietary Reference Intakes in 2005, as reported by the Institute of Medicine of the National Academies of Science. 3 As a graphic acknowledgment of the limited database upon which to express estimated average requirements for water for different population groups, the Committee and the Institute of Medicine stated: “While it might appear useful to estimate an average requirement (an EAR) for water, an EAR based on data is not possible.” Given the extreme variability in water needs that are not solely based on differences in metabolism, but also on environmental conditions and activities, there is not a single level of water intake that would assure adequate hydration and optimum health for half of all apparently healthy persons in all environmental conditions. Thus, an adequate intake (AI) level was established in place of an EAR for water.

The AIs for different population groups were set as the median water intakes for populations, as reported in the National Health and Nutrition Examination Surveys; however, the intake levels reported in these surveys varied greatly based on the survey years (e.g., NHANES 1988–1994 versus NHANES 1999–2002) and were also much higher than those found in the USDA surveys (e.g., 1989–1991, 1994–1998, or 2005–2006). If the AI for adults, as expressed in Table 5 , is taken as a recommended intake, the wisdom of converting an AI into a recommended water or fluid intake seems questionable. The first problem is the almost certain inaccuracy of the fluid intake information from the national surveys, even though that problem may also exist for other nutrients. More importantly, from the standpoint of translating an AI into a recommended fluid intake for individuals or populations, is the decision that was made when setting the AI to add an additional roughly 20% of water intake, which is derived from some foods in addition to water and beverages. While this may have been a legitimate effort to use total water intake as a basis for setting the AI, the recommendations that derive from the IOM report would be better directed at recommendations for water and other fluid intake on the assumption that the water content of foods would be a “passive” addition to total water intake. In this case, the observations of the dietary reference intake committee that it is necessary for water intake to meet needs imposed by metabolism and environmental conditions must be extended to consider three added factors, namely body size, gender, and physical activity. Those are the well-studied factors that allow a rather precise measurement and determination of energy intake requirements. It is, therefore, logical that those same factors might underlie recommendations to meet water intake needs in the same populations and individuals. Consideration should also be given to the possibility that water intake needs would best be expressed relative to the calorie requirements, as is done regularly in the clinical setting, and data should be gathered to this end through experimental and population research.

Water requirements expressed in relation to energy recommendations.

Age group (y) .	Kcal/d estimated energy requirement .	Al for fluid intake (mL/d) .	Ratio AI mL/d: EER Kcal/d .
Children 2–3	1000–1400	1300	0.93
Female
4–8	1400–1600	1700	1.06
9–13	1600–2000	2100	1.05
14–18	2000	2300	1.15
19–30	2000–2200	2700	1.23
31–50	2000	2700	1.35
50+	1800	2700	1.5
Male
4–8	1400–1600	1700	1.06
9–13	1800–2000	2400	1.20
14–18	2400–2800	3300	1.18
19–30	2600–2800	3700	1.32
31–50	2400–2600	3700	1.42
50+	2200–2400	3700	1.54

Age group (y) .	Kcal/d estimated energy requirement .	Al for fluid intake (mL/d) .	Ratio AI mL/d: EER Kcal/d .
Children 2–3	1000–1400	1300	0.93
Female
4–8	1400–1600	1700	1.06
9–13	1600–2000	2100	1.05
14–18	2000	2300	1.15
19–30	2000–2200	2700	1.23
31–50	2000	2700	1.35
50+	1800	2700	1.5
Male
4–8	1400–1600	1700	1.06
9–13	1800–2000	2400	1.20
14–18	2400–2800	3300	1.18
19–30	2600–2800	3700	1.32
31–50	2400–2600	3700	1.42
50+	2200–2400	3700	1.54

AI for total fluids derived from dietary reference intakes for water, potassium, sodium, chloride, and sulphate.

Ratios for water intake based on the AI for water in liters/day calculated using EER for each range of physical activity. EER adapted from the Institute of Medicine Dietary Reference Intakes Macronutrients Report, 2002.

It is important to note that only a few countries include water on their list of nutrients. 118 The European Food Safety Authority is developing a standard for all of Europe. 105 At present, only the United States and Germany provide AI values for water. 3 , 119

Another approach to the estimation of water requirements, beyond the limited usefulness of the AI or estimated mean intake, is to express water intake requirements in relation to energy requirements in mL/kcal. An argument for this approach includes the observation that energy requirements for each age and gender group are strongly evidence-based and supported by extensive research taking into account both body size and activity level, which are crucial determinants of energy expenditure that must be met by dietary energy intake. Such measures of expenditure have used highly accurate methods, such as doubly labeled water; thus, estimated energy requirements have been set based on solid data rather than the compromise inherent in the AIs for water. Those same determinants of energy expenditure and recommended intake are also applicable to water utilization and balance, and this provides an argument for pegging water/fluid intake recommendations to the better-studied energy recommendations. The extent to which water intake and requirements are determined by energy intake and expenditure is understudied, but in the clinical setting it has long been practice to supply 1 mL/kcal administered by tube to patients who are unable to take in food or fluids. Factors such as fever or other drivers of increased metabolism affect both energy expenditure and fluid loss and are thus linked in clinical practice. This concept may well deserve consideration in the setting of population intake goals.

Finally, for decades there has been discussion about expressing nutrient requirements per 1,000 kcal so that a single number would apply reasonably across the spectrum of age groups. This idea, which has never been adopted by the Institute of Medicine and the National Academies of Science, may lend itself to an improved expression of water/fluid intake requirements, which must eventually replace the AIs. Table 5 presents the IOM water requirements and then develops a ratio of mL/kcal based on them. The European Food Safety Agency refers positively to the possibility of expressing water intake recommendations in mL/kcal as a function of energy requirements. 105 Outliers in the adult male categories, which reach ratios as high as 1.5, may well be based on the AI data from the United States, which are above those in the more moderate and likely more accurate European recommendations.

The topic of utilizing mL/kcal to examine water intake and water gaps is explored in Table 6 , which takes the full set of water intake AIs for each age-gender grouping and examines total intake. The data suggest a high level of fluid deficiency. Since a large proportion of fluids in the United States is based on caloric beverages and this proportion has changed markedly over the past 30 years, fluid intake increases both the numerator and the denominator of this mL/kcal relationship. Nevertheless, even using 1 mL/kcal as the AI would leave a gap for all children and adolescents. The NHANES physical activity data were also translated into METS/day to categorize all individuals by physical activity level and thus varying caloric requirements. Use of these measures reveals a fairly large fluid gap, particularly for adult males as well as children ( Table 6 ).

Water intake and water intake gaps based on US Water Adequate Intake Recommendations (based on utilization of water and physical activity data from NHANES 2005–2006).

Age Group .	Total mL .	mL/kcal .	Gap assuming 1 mL/kcal AI (liters) .	Gap based on actual METS activity (liters) .
.	.	.	.	Sedentary .	Moderate .	Active .	Total US .
Children 2–3	1076	0.76	−0.38	−0.82	NA	NA	−0.82
Female
4–8	1085	0.67	−0.63	−1.35	NA	NA	−1.35
9–13	1384	0.80	−0.54	−1.20	−0.98	−0.40	−1.12
14–18	1701	1.03	−0.22	−0.83	−0.36	0.00	−0.63
19–30	2336	1.37	0.32	−0.51	0.27	0.54	−0.37
31–50	2513	1.77	0.64	−0.34	0.47	0.32	−0.24
51+	2219	1.49	0.58	−0.60	0.29	0.11	−0.51
Male
4–8	1214	0.67	−0.67	−1.07	NA	NA	−1.07
9–13	1523	0.74	−0.67	−1.45	−0.75	−0.96	−1.35
14–18	2450	0.94	−0.44	−2.04	−1.2	−0.86	−1.52
19–30	3189	1.21	0.29	−1.30	−0.65	−0.60	−1.09
31–50	3361	1.28	0.48	−1.54	−0.65	−0.09	−1.37
51+	2595	1.20	0.28	−1.65	−1.45	−1.16	−1.61
85%	7%	8%

Age Group .	Total mL .	mL/kcal .	Gap assuming 1 mL/kcal AI (liters) .	Gap based on actual METS activity (liters) .
.	.	.	.	Sedentary .	Moderate .	Active .	Total US .
Children 2–3	1076	0.76	−0.38	−0.82	NA	NA	−0.82
Female
4–8	1085	0.67	−0.63	−1.35	NA	NA	−1.35
9–13	1384	0.80	−0.54	−1.20	−0.98	−0.40	−1.12
14–18	1701	1.03	−0.22	−0.83	−0.36	0.00	−0.63
19–30	2336	1.37	0.32	−0.51	0.27	0.54	−0.37
31–50	2513	1.77	0.64	−0.34	0.47	0.32	−0.24
51+	2219	1.49	0.58	−0.60	0.29	0.11	−0.51
Male
4–8	1214	0.67	−0.67	−1.07	NA	NA	−1.07
9–13	1523	0.74	−0.67	−1.45	−0.75	−0.96	−1.35
14–18	2450	0.94	−0.44	−2.04	−1.2	−0.86	−1.52
19–30	3189	1.21	0.29	−1.30	−0.65	−0.60	−1.09
31–50	3361	1.28	0.48	−1.54	−0.65	−0.09	−1.37
51+	2595	1.20	0.28	−1.65	−1.45	−1.16	−1.61
85%	7%	8%

Note: Recommended water intake for actual activity level is the upper end of the range for moderate and active.

A weighted average for the proportion of individuals in each METS-based activity level.

This review has pointed out a number of issues related to water, hydration, and health. Since water is undoubtedly the most important nutrient and the only one for which an absence will prove lethal within days, understanding of water measurement and water requirements is very important. The effects of water on daily performance and short- and long-term health are quite clear. The existing literature indicates there are few negative effects of water intake while the evidence for positive effects is quite clear.

Little work has been done to measure total fluid intake systematically, and there is no understanding of measurement error and best methods of understanding fluid intake. The most definitive US and European documents on total water requirements are based on these extant intake data. 3 , 105 The absence of validation methods for water consumption intake levels and patterns represents a major gap in knowledge. Even varying the methods of probing in order to collect better water recall data has been little explored.

On the other side of the issue is the need to understand total hydration status. There are presently no acceptable biomarkers of hydration status at the population level, and controversy exists about the current knowledge of hydration status among older Americans. 6 , 120 Thus, while scholars are certainly focused on attempting to create biomarkers for measuring hydration status at the population level, the topic is currently understudied.

As noted, the importance of understanding the role of fluid intake on health has emerged as a topic of increasing interest, partially because of the trend toward rising proportions of fluids being consumed in the form of caloric beverages. The clinical, epidemiological, and intervention literature on the effects of added water on health are covered in a related systematic review. 111 The use of water as a replacement for sugar-sweetened beverages, juice, or whole milk has clear effects in that energy intake is reduced by about 10–13% of total energy intake. However, only a few longer-term systematic interventions have investigated this topic and no randomized, controlled, longer-term trials have been published to date. There is thus very minimal evidence on the effects of just adding water to the diet and of replacing water with diet beverages.

There are many limitations to this review. One certainly is the lack of discussion of potential differences in the metabolic functioning of different types of beverages. 121 Since the literature in this area is sparse, however, there is little basis for delving into it at this point. A discussion of the potential effects of fructose (from all caloric sweeteners when consumed in caloric beverages) on abdominal fat and all of the metabolic conditions directly linked with it (e.g., diabetes) is likewise lacking. 122 , – 125 A further limitation is the lack of detailed review of the array of biomarkers being considered to measure hydration status. Since there is no measurement in the field today that covers more than a very short time period, except for 24-hour total urine collection, such a discussion seems premature.

Some ways to examine water requirements have been suggested in this review as a means to encourage more dialogue on this important topic. Given the significance of water to our health and of caloric beverages to our total energy intake, as well as the potential risks of nutrition-related noncommunicable diseases, understanding both the requirements for water in relation to energy requirements, and the differential effects of water versus other caloric beverages, remain important outstanding issues.

This review has attempted to provide some sense of the importance of water to our health, its role in relationship to the rapidly increasing rates of obesity and other related diseases, and the gaps in present understanding of hydration measurement and requirements. Water is essential to our survival. By highlighting its critical role, it is hoped that the focus on water in human health will sharpen.

The authors wish to thank Ms. Frances L. Dancy for administrative assistance, Mr. Tom Swasey for graphics support, Dr. Melissa Daniels for assistance, and Florence Constant (Nestle's Water Research) for advice and references.

This work was supported by the Nestlé Waters, Issy-les-Moulineaux, France, 5ROI AGI0436 from the National Institute on Aging Physical Frailty Program, and NIH R01-CA109831 and R01-CA121152.

Declaration of interest

The authors have no relevant interests to declare.

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Burge MR Garcia N Qualls CR Schade DS . Differential effects of fasting and dehydration in the pathogenesis of diabetic ketoacidosis . Metabolism. 2001 ; 50 : 171 – 177 .

Jayashree M Singhi S . Diabetic ketoacidosis: predictors of outcome in a pediatric intensive care unit of a developing country . Pediatr Crit Care Med. 2004 ; 5 : 427 – 433 .

Hebert LA Greene T Levey A Falkenhain ME Klahr S . High urine volume and low urine osmolality are risk factors for faster progression of renal disease . Am J Kidney Dis. 2003 ; 41 : 962 – 971 .

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Blanker MH Bernsen RM Ruud Bosch JL , et al . Normal values and determinants of circadian urine production in older men: a population based study . J Urol. 2002 ; 168 : 1453 – 1457 .

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Kelly J Hunt BJ Lewis RR , et al . Dehydration and venous thromboembolism after acute stroke . QJM. 2004 ; 97 : 293 – 296 .

Bhalla A Sankaralingam S Dundas R Swaminathan R Wolfe CD Rudd AG . Influence of raised plasma osmolality on clinical outcome after acute stroke . Stroke. 2000 ; 31 : 2043 – 2048 .

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Mazzola BL von Vigier RO Marchand S Tonz M Bianchetti MG . Behavioral and functional abnormalities linked with recurrent urinary tract infections in girls . J Nephrol. 2003 ; 16 : 133 – 138 .

Wilde MH Carrigan MJ . A chart audit of factors related to urine flow and urinary tract infection . J Adv Nurs. 2003 ; 43 : 254 – 262 .

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Aufderheide S Lax D Goldberg SJ . Gender differences in dehydration-induced mitral valve prolapse . Am Heart J. 1995 ; 129 : 83 – 86 .

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dehydration
energy intake
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fluid intake
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Heat waves: a hot topic in climate change research

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Published: 03 September 2021
Volume 146 , pages 781–800, ( 2021 )

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Werner Marx ORCID: orcid.org/0000-0002-1763-5753 1 ,
Robin Haunschild ORCID: orcid.org/0000-0001-7025-7256 1 &
Lutz Bornmann ORCID: orcid.org/0000-0003-0810-7091 1 , 2

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Research on heat waves (periods of excessively hot weather, which may be accompanied by high humidity) is a newly emerging research topic within the field of climate change research with high relevance for the whole of society. In this study, we analyzed the rapidly growing scientific literature dealing with heat waves. No summarizing overview has been published on this literature hitherto. We developed a suitable search query to retrieve the relevant literature covered by the Web of Science (WoS) as complete as possible and to exclude irrelevant literature ( n = 8,011 papers). The time evolution of the publications shows that research dealing with heat waves is a highly dynamic research topic, doubling within about 5 years. An analysis of the thematic content reveals the most severe heat wave events within the recent decades (1995 and 2003), the cities and countries/regions affected (USA, Europe, and Australia), and the ecological and medical impacts (drought, urban heat islands, excess hospital admissions, and mortality). An alarming finding is that the limit for survivability may be reached at the end of the twenty-first century in many regions of the world due to the fatal combination of rising temperatures and humidity levels measured as “wet-bulb temperature” (WBT). Risk estimation and future strategies for adaptation to hot weather are major political issues. We identified 104 citation classics, which include fundamental early works of research on heat waves and more recent works (which are characterized by a relatively strong connection to climate change).

Heatwaves: does global research reflect the growing threat in the light of climate change?

Climate and human health: a review of publication trends in the International Journal of Biometeorology

Knowledge map and global trends in extreme weather research from 1980 to 2019: a bibliometric analysis

Avoid common mistakes on your manuscript.

1 Introduction

As a consequence of the well-documented phenomenon of global warming, climate change has become a major research field in the natural and medical sciences, and more recently also in the social and political sciences. The scientific community has contributed extensively to a comprehensive understanding of the earth’s climate system, providing various data and projections on the future climate as well as on the effects and risks of anticipated global warming (IPCC 2014; CSSR 2017; NCA4 2018; and the multitude of references cited therein). During recent decades, climate change has also become a major political, economic, and environmental issue and a central theme in political and public debates.

One consequence of global warming is the increase of extreme weather events such as heat waves, droughts, floods, cyclones, and wildfires. Some severe heat waves occurring within the last few decades made heat waves a hot topic in climate change research, with “hot” having a dual meaning: high temperature and high scientific activity. “More intense, more frequent, and longer lasting heat waves in the twenty-first century” is the title of a highly cited paper published 2004 in Science (Meehl and Tebaldi 2004 ). This title summarizes in short what most climate researchers anticipate for the future. But what are heat waves (formerly also referred to as “heatwaves”)? In general, a heat wave is a period of excessively hot weather, which may be accompanied by high humidity. Since heat waves vary according to region, there is no universal definition, but only definitions relative to the usual weather in the area and relative to normal temperatures for the season. The World Meteorological Organization (WMO) defines a heat wave as 5 or more consecutive days of prolonged heat in which the daily maximum temperature is higher than the average maximum temperature by 5 °C (9 °F) or more ( https://www.britannica.com/science/heat-wave-meteorology ).

Europe, for example, has suffered from a series of intense heat waves since the beginning of the twenty-first century. According to the World Health Organization (WHO) and various national reports, the extreme 2003 heat wave caused about 70,000 excess deaths, primarily in France and Italy. The 2010 heat wave in Russia caused extensive crop loss, numerous wildfires, and about 55,000 excess deaths (many in the city of Moscow). Heat waves typically occur when high pressure systems become stationary and the winds on their rear side continuously pump hot and humid air northeastward, resulting in extreme weather conditions. The more intense and more frequently occurring heat waves cannot be explained solely by natural climate variations and without human-made climate change (IPCC 2014; CSSR 2017; NCA4 2018). Scientists discuss a weakening of the polar jet stream caused by global warming as a possible reason for an increasing probability for the occurrence of stationary weather, resulting in heavy rain falls or heat waves (Broennimann et al. 2009 ; Coumou et al. 2015 ; Mann 2019 ). This jet stream is one of the most important factors for the weather in the middle latitude regions of North America, Europe, and Asia.

Until the end of the twentieth century, heat waves were predominantly seen as a recurrent meteorological fact with major attention to drought, being almost independent from human activities and unpredictable like earthquakes. However, since about 1950, distinct changes in extreme climate and weather events have been increasingly observed. Meanwhile, climate change research has revealed that these changes are clearly linked to the human influence on the content of greenhouse gases in the earth’s atmosphere. Climate-related extremes, such as heat waves, droughts, floods, cyclones, and wildfires, reveal significant vulnerability to climate change as a result of global warming.

In recent years, research on heat waves has been established as an emerging research topic within the large field of current climate change research. Bibliometric analyses are very suitable in order to have a systematic and quantitative overview of the literature that can be assigned to an emerging topic such as research dealing with heat waves (e.g., Haunschild et al. 2016 ). No summarizing overview on the entire body of heat wave literature has been published until now. However, a bibliometric analysis of research on urban heat islands as a more specific topic in connection with heat waves has been performed (Huang and Lu 2018 ).

In this study, we analyzed the publications dealing with heat waves using appropriate bibliometric methods and tools. First, we determined the amount and time evolution of the scientific literature dealing with heat waves. The countries contributing the most papers are presented. Second, we analyzed the thematic content of the publications via keywords assigned by the WoS. Third, we identified the most important (influential) publications (and also the historical roots). We identified 104 citation classics, which include fundamental early works and more recent works with a stronger connection to climate change.

2 Heat waves as a research topic

The status of the current knowledge on climate change is summarized in the Synthesis Report of the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) (IPCC 2014, https://www.ipcc.ch/report/ar5/syr/ ). This panel is the United Nations body for assessing the science related to climate change. The Synthesis Report is based on the reports of the three IPCC Working Groups , including relevant Special Reports . In its Summary for Policymakers , it provides an integrated view of climate change as the final part of the Fifth Assessment Report (IPCC 2014, https://www.ipcc.ch/site/assets/uploads/2018/02/AR5_SYR_FINAL_SPM.pdf ).

In the chapter Extreme Events , it is stated that “changes in many extreme weather and climate events have been observed since about 1950. Some of these changes have been linked to human influences, including a decrease in cold temperature extremes, an increase in warm temperature extremes, an increase in extreme high sea levels and an increase in the number of heavy precipitation events in a number of regions … It is very likely that the number of cold days and nights has decreased and the number of warm days and nights has increased on the global scale. It is likely that the frequency of heat waves has increased in large parts of Europe, Asia and Australia. It is very likely that human influence has contributed to the observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-twentieth century. It is likely that human influence has more than doubled the probability of occurrence of heat waves in some locations” (p. 7–8). Under Projected Changes , the document summarizes as follows: “Surface temperature is projected to rise over the twenty-first century under all assessed emission scenarios. It is very likely that heat waves will occur more often and last longer, and that extreme precipitation events will become more intense and frequent in many regions” (p. 10).

With regard to the USA, the Climate Science Special Report of the U.S. Global Change Research Program (CSSR 2017, https://science2017.globalchange.gov/ ) mentions quite similar observations and states unambiguously in its Fourth National Climate Assessment (Volume I) report ( https://science2017.globalchange.gov/downloads/CSSR2017_FullReport.pdf ) under Observed Changes in Extremes that “the frequency of cold waves has decreased since the early 1900s, and the frequency of heat waves has increased since the mid-1960s (very high confidence). The frequency and intensity of extreme heat and heavy precipitation events are increasing in most continental regions of the world (very high confidence). These trends are consistent with expected physical responses to a warming climate [p. 19]. Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901 (high confidence) [p. 20]. There are important regional differences in trends, with the largest increases occurring in the northeastern United States (high confidence). Recent droughts and associated heat waves have reached record intensity in some regions of the United States … (very high confidence) [p. 21]. Confidence in attribution findings of anthropogenic influence is greatest for extreme events that are related to an aspect of temperature” (p. 123).

Among the key findings in the chapter on temperature changes in the USA, the report states that “there have been marked changes in temperature extremes across the contiguous United States. The frequency of cold waves has decreased since the early 1900s, and the frequency of heat waves has increased since the mid-1960s (very high confidence). Extreme temperatures in the contiguous United States are projected to increase even more than average temperatures. The temperatures of extremely cold days and extremely warm days are both expected to increase. Cold waves are projected to become less intense while heat waves will become more intense (very high confidence) [p. 185]. Most of this methodology as applied to extreme weather and climate event attribution, has evolved since the European heat wave study of Stott et al.” (p. 128).

Heat waves are also discussed in the Fourth National Climate Assessment (Volume II) report (NCA4 2018, https://nca2018.globalchange.gov/ ). The Report-in-Brief ( https://nca2018.globalchange.gov/downloads/NCA4_Report-in-Brief.pdf ) for example states: “More frequent and severe heat waves and other extreme events in many parts of the United States are expected [p. 38]. Heat waves and heavy rainfalls are expected to increase in frequency and intensity [p. 93]. The season length of heat waves in many U.S. cities has increased by over 40 days since the 1960s [p. 30]. Cities across the Southeast are experiencing more and longer summer heat waves [p. 123]. Exposure to hotter temperatures and heat waves already leads to heat-associated deaths in Arizona and California. Mortality risk during a heat wave is amplified on days with high levels of ground-level ozone or particulate air pollution” (p. 150).

In summary, climate change research expects more frequent and more severe heat wave events as a consequence of global warming. It is likely that the more frequent and longer lasting heat waves will significantly increase excess mortality, particularly in urban regions with high air pollution. Therefore, research around heat waves will become increasingly important and is much more than a temporary research fashion.

3 Methodology

3.1 dataset used.

This analysis is based on the relevant literature retrieved from the following databases accessible under the Web of Science (WoS) of Clarivate Analytics: Web of Science Core Collection: Citation Indexes, Science Citation Index Expanded (SCI-EXPANDED), Social Sciences Citation Index (SSCI), Arts & Humanities Citation Index (A&HCI), Conference Proceedings Citation Index—Science (CPCI-S), Conference Proceedings Citation Index—Social Science & Humanities (CPCI-SSH), Book Citation Index—Science (BKCI-S), Book Citation Index—Social Sciences & Humanities (BKCI-SSH), Emerging Sources Citation Index (ESCI).

We applied the search query given in Appendix 1 to cover the relevant literature as completely as possible and to exclude irrelevant literature. We practiced an iterative query optimization by identifying and excluding the WoS subject categories with most of the non-relevant papers. For example, heat waves are also mentioned in the field of materials science but have nothing to do with climate and weather phenomena. Unfortunately, WoS obviously assigned some heat wave papers related to climate to materials science-related subject categories. Therefore, these subject categories were not excluded. By excluding the other non-relevant subject categories, 597 out of 8,568 papers have been removed, resulting in a preliminary publication set of 7,971 papers (#2 of the search query). But this is no safe method, since the excluded categories may well include some relevant papers. Therefore, we have combined these 597 papers with search terms related to climate or weather and retrieved 62 relevant papers in addition, which we added to our preliminary paper subset, eventually receiving 8,033 publications (#3 to #5 of the search query).

Commonly, publication sets for bibliometric analyses are limited to articles, reviews, and conference proceedings as the most relevant document types and are restricted to complete publication years. In this study, however, we have included all relevant WoS document types for a better literature coverage of the research topic analyzed. For example, conference meetings and early access papers may well be interesting for the content analysis of the literature under study. Such literature often anticipates important results, which are published later as regular articles. Furthermore, we have included the literature until the date of search for considering the recent rapid growth of the field. Our search retrieved a final publication set of 8,011 papers indexed in WoS until the date of search (July 1, 2021) and dealing with heat waves (#6 of the search query). We have combined this publication set with climate change-related search terms from a well-proven search query (Haunschild et al. 2016 ) resulting in 4,588 papers dealing with heat waves in connection with climate change or global warming (# 11 of the search query). Also, we have selected a subset of 2,373 papers dealing with heat waves and mortality (#13 of the search query). The complete WoS search query is given in Appendix 1.

The final publication set of 8,011 papers dealing with heat waves still contains some non-relevant papers primarily published during the first half of the twentieth century, such as some Nature papers within the WoS category Multidisciplinary Sciences . Since these papers are assigned only to this broad subject category and have no abstracts and no keywords included, they cannot be excluded using the WoS search and refinement functions. We do not expect any bias through these papers, because their keywords do not appear in our maps. Also, they normally contain very few (if any) cited references, which could bias/impact our reference analysis.

3.2 Networks

We used the VOSviewer software (Van Eck and Waltman 2010 ) to map co-authorship with regard to the countries of authors (88 countries considered) of the papers dealing with heat waves ( www.vosviewer.com ). The map of the cooperating countries presented is based on the number of joint publications. The distance between two nodes is proportionate to the number of co-authored papers. Hence, largely cooperating countries are positioned closer to each other. The size of the nodes is proportionate to the number of papers published by authors of the specific countries.

The method that we used for revealing the thematic content of the publication set retrieved from the WoS is based on the analysis of keywords. For better standardization, we chose the keywords allocated by the database producer (keywords plus) rather than the author keywords. We also used the VOSviewer for mapping the thematic content of the 104 key papers selected by reference analysis. This map is also based on keywords plus.

The term maps (keywords plus) are based on co-occurrence for positioning the nodes on the maps. The distance between two nodes is proportionate to the co-occurrence of the terms. The size of the nodes is proportionate to the number of papers with a specific keyword. The nodes on the map are assigned by VOSviewer to clusters based on a specific cluster algorithm (the clusters are highlighted in different colors). These clusters identify closely related (frequently co-occurring) nodes, where each node is assigned to only one cluster.

3.3 Reference Publication Year Spectroscopy

A bibliometric method called “Reference Publication Year Spectroscopy” (RPYS, Marx et al. 2014 ) in combination with the tool CRExplorer ( http://www.crexplorer.net , Thor et al. 2016a , b ) has proven useful for exploring the cited references within a specific publication set, in order to detect the most important publications of the relevant research field (and also the historical roots). In recent years, several studies have been published, in which the RPYS method was basically described and applied (Marx et al. 2014 ; Marx and Bornmann 2016 ; Comins and Hussey 2015 ). In previous studies, Marx et al. have analyzed the roots of research on global warming (Marx et al. 2017a ), the emergence of climate change research in combination with viticulture (Marx et al. 2017b ), and tea production (Marx et al. 2017c ) from a quantitative (bibliometric) perspective. In this study, we determined which references have been most frequently cited by the papers dealing with heat waves.

RPYS is based on the assumption that peers produce a useful database by their publications, in particular by the references cited therein. This database can be analyzed statistically with regard to the works most important for their specific research field. Whereas individual scientists judge their research field more or less subjectively, the overall community can deliver a more objective picture (based on the principle of “the wisdom of the crowds”). The peers effectively “vote” via their cited references on which works turned out to be most important for their research field (Bornmann and Marx 2013 ). RPYS implies a normalization of citation counts (here: reference counts) with regard to the research area and the time of publication, which both impact the probability to be cited frequently. Basically, the citing and cited papers analyzed were published in the same research field and the reference counts are compared with each other only within the same publication year.

RPYS relies on the following observation: the analysis of the publication years of the references cited by all the papers in a specific research topic shows that publication years are not equally represented. Some years occur particularly frequently among the cited references. Such years appear as distinct peaks in the distribution of the reference publication years (i.e., the RPYS spectrogram). The pronounced peaks are frequently based on a few references that are more frequently cited than other references published in the same year. The frequently cited references are—as a rule—of specific significance to the research topic in question (here: heat waves) and the earlier references among them represent its origins and intellectual roots (Marx et al. 2014 ).

The RPYS changes the perspective of citation analysis from a times cited to a cited reference analysis (Marx and Bornmann 2016 ). RPYS does not identify the most highly cited papers of the publication set being studied (as is usually done by bibliometric analyses in research evaluation). RPYS aims to mirror the knowledge base of research (here: on heat waves).

With time, the body of scientific literature of many research fields is growing rapidly, particularly in climate change research (Haunschild et al. 2016 ). The growth rate of highly dynamic research topics such as research related to heat waves is even larger. As a consequence, the number of potentially citable papers is growing substantially. Toward the present, the peaks of individual publications lie over a broad continuum of newer publications and are less numerous and less pronounced. Due to the many publications cited in the more recent years, the proportion of individual highly cited publications in specific reference publication years falls steadily. Therefore, the distinct peaks in an RPYS spectrogram reveal only the most highly cited papers, in particular the earlier references comprising the historical roots. Further inspection and establishing a more entire and representative list of highly cited works requires consulting the reference table provided by the CRExplorer. The most important references within a specific reference publication year can be identified by sorting the cited references according to the reference publication year (RPY) and subsequently according to the number of cited references (N_CR) in a particular publication year.

The selection of important references in RPYS requires the consideration of two opposing trends: (1) the strongly growing number of references per reference publication year and (2) the fall off near present due to the fact that the newest papers had not sufficient time to accumulate higher citation counts. Therefore, we decided to set different limits for the minimum number of cited references for different periods of reference publication years (1950–1999: N_CR ≥ 50, 2000–2014: N_CR ≥ 150, 2015–2020: N_CR ≥ 100). This is somewhat arbitrary, but is helpful in order to adapt and limit the number of cited references to be presented and discussed.

In order to apply RPYS, all cited references ( n = 408,247) of 216,932 unique reference variants have been imported from the papers of our publication set on heat waves ( n = 8,011). The cited reference publication years range from 1473 to 2021. We removed all references (297 different cited reference variants) with reference publication years prior to 1900. Due to the very low output of heat wave-related papers published before 1990, no relevant literature published already in the nineteenth century can be expected. Also, global warming was no issue before 1900 since the Little Ice Age (a medieval cold period) lasted until the nineteenth century. The references were sorted according to RPY and N_CR for further inspection.

The CRExplorer offers the possibility to cluster and merge variants of the same cited reference (Thor et al. 2016a , b ). We clustered and merged the associated reference variants in our dataset (which are mainly caused by misspelled references) using the corresponding CRExplorer module, clustering the reference variants via volume and page numbers and subsequently merging aggregated 374 cited references (for more information on using the CRExplorer see “guide and datasets” at www.crexplorer.net ).

After clustering and merging, we applied a further cutback: to focus the RPYS on the most pronounced peaks, we removed all references ( n = 212,324) with reference counts below 10 (resulting in a final number of 3,937 cited references) for the detection of the most frequently cited works. A minimum reference count of 10 has proved to be reasonable, in particular for early references (Marx et al. 2014 ). The cited reference publication years now range from 1932 to 2020.

In this study, we have considered all relevant WoS document types for a preferably comprehensive coverage of the literature of the research topic analyzed. The vast majority of the papers of our publication set, however, have been assigned to the document types “article” ( n = 6.738, 84.1%), “proceedings paper” ( n = 485, 6.1%), and “review” ( n = 395 papers, 4.9%). Note that some papers belong to more than one document type.

4.1 Time evolution of literature

In Fig. 1 , the time evolution between 1990 and 2020 of the publications dealing with heat waves is shown (there are only 109 pre-1990 publications dealing with heat waves and covered by the WoS).

Time evolution of the overall number of heat wave publications, of heat wave publications in connection with climate change, and of heat wave publications in connection with mortality, each between 1990 and 2020. For comparison, the overall number of publications (scaled down) in the field of climate change research and the total number of publications covered by the WoS database (scaled down, too) are included

According to Fig. 1 , research dealing with heat waves is a highly dynamic research topic, currently doubling within about 5 years. The number of papers published per year shows a strong increase: since around 2000, the publication output increased by a factor of more than thirty, whereas in the same period, the overall number of papers covered by the WoS increased only by a factor of around three. Also, the portion of heat wave papers dealing with climate change increased substantially: from 16.1 in the period 1990–1999 to 25.7% in 2000, reaching 66.9% in 2020. The distinct decrease of the overall number of papers covered by the WoS between 2019 and 2020 might be a result of the Covid-19 pandemic.

With regard to the various impacts of heat waves, excess mortality is one of the most frequently analyzed and discussed issues in the scientific literature (see below). Whereas the subject specific literature on heat waves increased from 2000 to 2020 by a factor of 33.6, literature on heat waves dealing with mortality increased from 2000 to 2020 by a factor of 51.5. The dynamics of the research topic dealing with heat waves is mirrored by the WoS Citation Report , which shows the time evolution of the overall citation impact of the papers of the publication set (not presented). The citation report curve shows no notable citation impact before 2005, corresponding to the increase of the publication rate since about 2003 as shown in Fig. 1 .

4.2 Countries of authors

In Table 1 , the number of papers assigned to the countries of authors with more than 100 publications dealing with heat waves is presented, showing the national part of research activities on this research topic. For comparative purposes, the percentage of overall papers in WoS of each country is shown. As a comparison with the overall WoS, we only considered WoS papers published between 2000 and 2020, because the heat wave literature started to grow substantially around 2000.

The country-specific percentages from Table 1 are visualized in Fig. 2 . Selected countries are labeled. Countries with a higher relative percentage of more than two percentage points in heat wave research than in WoS overall output are marked blue (blue circle). Countries with a relative percentage at least twice as high in heat wave research than in overall WoS output are marked green (green cross), whereas countries with a relative percentage at most half as much in heat wave research than in overall WoS output are marked with a yellow cross. Only Japan has a much lower output in heat wave research than in WoS overall output, as indicated by the red circle and yellow cross. Most countries are clustered around the bisecting line and are marked gray (gray circle). China and the USA are outside of the plot region. Both countries are rather close to the bisecting line. Some European countries show a much larger activity in heat wave research than in overall WoS output. Australia shows the largest difference and ratio in output percentages as shown by the blue circle and green cross.

Publication percentages of countries in Table 1 . Countries with large deviations between heat wave output and overall WoS output are labeled. Countries with an absolute percentage of more than two percentage points higher (lower) in heat wave research than in overall WoS output are marked blue (red). Countries with a relative percentage at least twice as high (at most half as much) in heat wave research than in overall WoS output are marked green (yellow)

The results mainly follow the expectations of such bibliometric analyses, with one distinct exception: Australia increasingly suffers from extreme heat waves and is comparatively active in heat wave research—compared with its proportion of scientific papers in general. The growth factor of the Australian publication output since 2010 is 8.5, compared to 5.3 for the USA and 3.3 for Germany.

Figure 3 shows the co-authorship network with regard to the countries of authors of the papers dealing with heat waves using the VOSviewer software.

Co-authorship overlay map with regard to the countries of authors and their average publication years from the 8,011 papers dealing with heat waves. The minimum number of co-authored publications of a country is 5; papers with more than 25 contributing countries are neglected; of the 135 countries, 89 meet the threshold, and 88 out of 89 countries are connected and are considered (one country, Armenia, that is disconnected from the network has been removed). The co-authorship network of a single country can be depicted by clicking on the corresponding node in the interactive map. Readers interested in an in-depth analysis can use VOSviewer interactively and zoom into the map via the following URL: https://tinyurl.com/3ywkwv8t

According to Fig. 3 and in accordance with Table 1 , the USA is most productive in heat wave research. This is not unexpected, because the US publication output is at the top for most research fields. However, this aside, the USA has been heavily affected by heat wave events and is leading with regard to the emergence of the topic. Australia appears as another major player and is strongly connected with the US publications within the co-authorship network and thus appears as a large node near the US node in the map. Next, the leading European countries England, France, Germany, Italy, and Spain appear.

The overlay version of the map includes the time evolution of the research activity in the form of coloring of the nodes. The map shows the mean publication year of the publications for each specific author country. As a consequence, the time span of the mean publication years ranges only from 2014 to 2018. Nevertheless, the early activity in France and the USA and the comparatively recent activity in Australia and China, with the European countries in between, become clearly visible.

4.3 Topics of the heat wave literature

Figure 4 shows the keywords (keywords plus) map for revealing the thematic content of our publication set using the VOSviewer software. This analysis is based on the complete publication set ( n = 8,011). The minimum number of occurrences of keywords is 10; of the 10,964 keywords, 718 keywords met the threshold. For each of the 718 keywords, the total strength of the co-occurrence links with other keywords was calculated. The keywords with the greatest total link strength were selected for presentation in the map.

Co-occurrence network map of the keywords plus from the 8,011 papers dealing with heat waves for a rough analysis of the thematic content. The minimum number of occurrences of keywords is 10; of the 10,964 keywords, 718 meet the threshold. Readers interested in an in-depth analysis can use VOSviewer interactively and zoom into the map via the following URL: https://tinyurl.com/enrdbw

According to Fig. 4 , the major keywords are the following: climate change, temperature, mortality, impact, heat waves (searched), and variability. The colored clusters identify closely related (frequently co-occurring) nodes. The keywords marked red roughly originate from fundamental climate change research focused on the hydrological cycle (particularly on drought), the keywords of the green cluster are around heat waves and moisture or precipitation, the keywords marked blue result from research concerning impacts of heat waves on health, the keywords marked yellow are focused on the various other impacts of heat waves, and the keywords of the magenta cluster are around adaptation and vulnerability in connection with heat waves.

The clustering by the VOSviewer algorithm provides basic categorizations, but many related keywords also appear in different clusters. For example, severe heat wave events are marked in different colors. For a better overview of the thematic content of the publications dealing with heat waves, we have assigned the keywords of Fig. 4 (with a minimum number of occurrences of 50) to ten subject categories (each arranged in the order of occurrence):

Countries/regions: United-States, Europe, France, China, Pacific, Australia, London, England

Cities: cities, city, US cities, Chicago, communities

Events: 2003 heat-wave, 1995 heat-wave

Impacts: impact, impacts, air-pollution, drought, soil-moisture, exposure, heat-island, urban, islands, photosynthesis, pollution, heat-island, air-quality, environment, precipitation extremes, biodiversity, emissions

Politics: risk, responses, vulnerability, adaptation, management, mitigation, risk-factors, scenarios

Biology: vegetation, forest, diversity, stomatal conductance

Medicine: mortality, health, stress, deaths, morbidity, hospital admissions, public-health, thermal comfort, population, heat, sensitivity, human health, disease, excess mortality, heat-stress, heat-related mortality, comfort, behavior, death, stroke

Climate research: climate change, temperature, climate, model, simulation, energy, projections, simulations, cmip5, ozone, el-nino, parametrization, elevated CO 2 , models, climate variability, carbon, carbon-dioxide

Meteorology: heat waves, variability, precipitation, summer, heat-wave, weather, ambient-temperature, waves, extremes, wave, cold, water, rainfall, circulation, heat, air-temperature, extreme heat, climate extremes, heatwaves, temperature extremes, temperatures, temperature variability, high-temperature, ocean, extreme temperatures, atmospheric circulation, interannual variability, sea-surface temperature, oscillation, surface temperature, surface

Broader terms (multi-meaning): trends, events, patterns, growth, performance, time-series, indexes, system, dynamics, association, index, tolerance, productivity, ensemble, resilience, increase, quality, prediction, frequency, particulate matter, future, framework, 20 th -century, time, reanalysis, systems

Although allocated by the database provider, the keywords are not coherent. For example, the same keyword may appear as singular or plural, and complex keywords are written with and without hyphens.

In order to compare the thematic content of the complete publication set with the earlier literature on heat waves, we have analyzed the pre-2000 publications ( n = 297) separately. Figure 5 shows the keywords (keywords plus) map for revealing the thematic content of the pre-2000 papers.

Co-occurrence network map of the keywords plus from the 297 pre-2000 papers dealing with heat waves for a rough analysis of the thematic content. The minimum number of occurrences of keywords is 1; of the 389 keywords, 277 keywords are connected, and all items are shown. Readers interested in an in-depth analysis can use VOSviewer interactively and zoom into the map via the following URL: https://tinyurl.com/u2zzr399

The major nodes in Fig. 5 are heat waves (searched), temperature, United States, and mortality, with climate change appearing only as a smaller node here. Obviously, the connection between heat waves and climate change was not yet pronounced, which can also be seen from Fig. 1 . Compared with Fig. 4 , the thematic content of the clusters is less clear and the clusters presented in Fig. 5 can hardly be assigned to specific research areas. For a better overview of the thematic content of the early publications dealing with heat waves, we have assigned the connected keywords of Fig. 5 to seven subject categories:

Countries/regions: United-States, Great-Plains

Cities: St-Louis, Athens, Chicago

Events: 1980 heat-wave, 1995 heat-wave

Impacts: impacts, responses, drought, precipitation, comfort, sultriness

Climate research: climate, climate change, model, temperature, variability

Medicine: cardiovascular deaths, mortality, air pollution

Meteorology: atmospheric flow, weather, heat, humidity index

4.4 Important publications

Figures 6 – 8 show the results of the RPYS analysis performed with the CRExplorer and present the distribution of the number of cited references across the reference publication years. Figure 6 shows the RPYS spectrogram of the full range of reference publication years since 1925. Figure 7 presents the spectrogram for the reference publication year period 1950–2000 for better resolving the historical roots. Figure 8 shows the spectrogram for the period 2000–2020, comprising the cited references from the bulk of the publication set analyzed.

Annual distribution of cited references throughout the time period 1925–2020, which have been cited in heat wave-related papers (published between 1964 and 2020). Only references with a minimum reference count of 10 are considered

Annual distribution of cited references throughout the time period 1950–2000, which have been cited in heat wave-related papers (published between 1972 and 2020). Only references with a minimum reference count of 10 are considered

Annual distribution of cited references throughout the time period 2000–2020, which have been cited in heat wave-related papers (published between 2000 and 2020). Only references with a minimum reference count of 10 are considered

The gray bars (Fig. 6 ) and red lines (Figs. 7 – 8 ) in the graphs visualize the number of cited references per reference publication year. In order to identify those publication years with significantly more cited references than other years, the (absolute) deviation of the number of cited references in each year from the median of the number of cited references in the two previous, the current, and the two following years (t − 2; t − 1; t; t + 1; t + 2) is also visualized (blue lines). This deviation from the 5-year median provides a curve smoother than the one in terms of absolute numbers. We inspected both curves for the identification of the peak papers.

Which papers are most important for the scientific community performing research on heat waves? We use the number of cited references (N_CR) as a measure of the citation impact within the topic-specific literature of our publication set. N_CR should not be confused with the overall number of citations of the papers as given by the WoS citation counts (times cited). These citation counts are based on all citing papers covered by the complete database (rather than a topic-specific publication set) and are usually much higher.

Applying the selection criteria mentioned above (minimum number of cited references between 50 and 150 in three different periods), 104 references have been selected as key papers (important papers most frequently referenced within the research topic analyzed) and are presented in Table 2 in Appendix 2. The peak papers corresponding to reference publication years below about 2000 can be seen as the historical roots of the research topic analyzed. Since around 2000, the number of references with the same publication year becomes increasingly numerous, usually with more than one highly referenced (cited) paper at the top. Although there are comparatively fewer distinct peaks visible in the RPYS spectrogram of Fig. 8 , the most frequently referenced papers can easily be identified via the CRE reference listing. Depending on the specific skills and needs (i.e., the expert knowledge and the intended depth of the analysis), the number of top-referenced papers considered key papers can be defined individually.

Table 2 lists the first authors and titles of the 104 key papers selected, their number of cited references (N_CR), and the DOIs for easy access. Some N_CR values are marked by an asterisk, indicating a high value of the N_TOP10 indicator implemented in the CRExplorer. The N_TOP10 indicator value is the number of reference publication years in which a focal cited reference belongs to the 10% most referenced publications. In the case of about half of the cited references in Table 2 ( n = 58), the N_TOP10 value exceeded a value of 9. The three highest values in our dataset are 24, 21, and 20.

Out of the 104 key papers from Table 2 , 101 have a DOI of which we found 101 papers in the WoS. Three papers have no DOI but could be retrieved from WoS. The altogether 104 papers were exported and their keywords (keywords plus) were displayed in Fig. 9 for revealing the thematic content of the key papers from the RPYS analysis at a glance.

Co-occurrence network map of the keywords plus of the 104 key papers dealing with heat waves selected applying RPYS via CRE software and listed in Table 2 . The minimum number of occurrences of keywords is 2; of the 310 keywords, 91 meet the threshold. Readers interested in an in-depth analysis can use VOSviewer interactively and zoom into the map via the following URL: https://tinyurl.com/4vwpc4t2

Overall, the keywords mapped in Fig. 9 are rather similar to the keywords presented in Fig. 4 . Besides climate change, temperature, weather, and air-pollution, the keywords deaths, health, mortality, and United-States appear as the most pronounced terms.

The key papers presented in Table 2 can be categorized as follows: (1) papers dealing with specific heat wave events, (2) the impact of heat waves on human health, (3) heat wave-related excess mortality and implications for prevention, (4) the interaction between air pollution and high temperature, (5) circulation pattern and the meteorological basis, (6) future perspectives and risks, and (7) climate models, indicators, and statistics.

5 Discussion

Today, the hypothesis of a human-induced climate change is no longer abstract but has become a clear fact, at least for the vast majority of the scientific community (IPCC 2014; CSSR 2017; NCA4 2018; and the multitude of references cited therein). The consequences of a warmer climate are already obvious. The rapidly growing knowledge regarding the earth’s climate system has revealed the connection between global warming and extreme weather events. Heat waves impact people directly and tangibly and many people are pushing for political actions. Research on heat waves came up with the occurrence of some severe events in the second half of the twentieth century and was much stimulated by the more numerous, more intense, and longer lasting heat waves that have occurred since the beginning of the twenty-first century.

As already mentioned in Sect. 1 , the more intense and more frequently occurring heat waves cannot be explained solely by natural climate variations but only with human-made climate change. As a consequence, research on heat waves has become embedded into meteorology and climate change research and has aimed to understand the specific connection with global warming. Scientists discuss a weakening of the polar jet stream as a possible reason for an increasing probability for the occurrence of heat waves (e.g., Broennimann et al. 2009 ; Coumou et al. 2015 ; Mann 2019 ). Climate models are used for projections of temperature and rainfall variability in the future, based on various scenarios of greenhouse gas emissions. As a result, the corresponding keywords appear in the maps of Figs. 4 and 9 . Also, the application of statistics plays a major role in the papers of our publication set; some of the most highly referenced (early) papers in Table 2 primarily deal with statistical methods. These methods provide the basis for research on heat waves.

Our analysis shows that research on heat waves has become extremely important in the medical area, since severe heat waves have caused significant excess mortality (e.g., Kalkstein and Davis 1989 ; Fouillet et al. 2006 ; Anderson and Bell 2009 , 2011 ). The most alarming is that the limit for survivability may be reached at the end of the twenty-first century in many regions of the world due to the fatal combination of rising temperatures and humidity levels (e.g., Pal and Eltahir 2016 ; Im et al. 2017 ; Kang and Eltahir 2018 ). The combination of heat and humidity is measured as the “wet-bulb temperature” (WBT), which is the lowest temperature that can be reached under current ambient conditions by the evaporation of water. At 100% relative humidity, the wet-bulb temperature is equal to the air temperature and is different at lower humidity levels. For example, an ambient temperature of 46 °C and a relative humidity of 50% correspond to 35 °C WBT, which is the upper limit that can kill even healthy people within hours. By now, the limit of survivability has almost been reached in some places. However, if global warming is not seriously tackled, deadly heat waves are anticipated for many regions that have contributed little to climate change.

According to high-resolution climate change simulations, North China and South Asia are particularly at risk, because the annual monsoon brings hot and humid air to these regions (Im et al. 2017 ; Kang and Eltahir 2018 ). The fertile plain of North China has experienced vast expansion of irrigated agriculture, which enhances the intensity of heat waves. South Asia, a region inhabited by about one-fifth of the global human population, is likely to approach the critical threshold by the late twenty-first century, if greenhouse gas emissions are not lowered significantly. In particular, the densely populated agricultural regions of the Ganges and Indus river basins are likely to be affected by extreme future heat waves. Also, the Arabic-speaking desert countries of the Gulf Region in the Middle East and the French-speaking parts of Africa are expected to suffer from heat waves beyond the limit of human survival. But to date, only 12 papers have been published on heat waves in connection with wet-bulb temperature (#15 of the search query); no paper was published before 2016. Some papers report excess hospital admissions during heat wave events (e.g., Semenza et al. 1999 ; Knowlton et al. 2009 ), with the danger of a temporary capacity overload of local medical systems in the future. Presumably, this will be an increasingly important issue in the future, when more and larger urban areas are affected by heat waves beyond the limit of human survival indicated by wet-bulb temperatures above 35° C.

The importance of heat waves for the medical area is underlined by the large portion of papers discussing excess hospital admissions and excess mortality during intense heat wave events, particularly in urban areas with a high population density. As was the case during the boom phase of the Covid-19 pandemic, local medical health care systems may become overstressed by long-lasting heat wave events and thus adaptation strategies are presented and discussed. Finally, the analysis of the keywords in this study reveals the connection of heat wave events with air pollution in urban regions. There seems to be evidence of an interaction between air pollution and high temperatures in the causation of excess mortality (e.g., Katsouyanni et al. 1993 ). Two more recent papers discuss the global risk of deadly heat (Mora et al. 2017 ) and the dramatically increasing chance of extremely hot summers since the 2003 European heat wave (Christidis et al. 2015 ).

Another important topic of the heat wave papers is related to the consequences for agriculture and forestry. Reduced precipitation and soil moisture result in crop failure and put food supplies at risk. Unfortunately, large regions of the world that contribute least to the emission of greenhouse gases are affected most by drought, poor harvests, and hunger. Some more recent papers discuss the increasing probability of marine heat waves (Oliver et al. 2018 ) and the consequences for the marine ecosystem (Smale et al. 2019 ).

The results of this study should be interpreted in terms of its limitations:

We tried to include in our bibliometric analyses all relevant heat wave papers covered by the database. Our long-standing experience in professional information retrieval has shown, however, that it is sheer impossible to get complete and clean results by search queries against the backdrop of the search functions provided by literature databases like WoS or others. Also, the transition from relevant to non-relevant literature is blurred and is a question of the specific needs. In this study, we used bibliometric methods that are relatively robust with regard to the completeness and precision of the publication sets analyzed. For example, it is an advantage of RPYS that a comparatively small portion of relevant publications (i.e., an incomplete publication set) contains a large amount of the relevant literature as cited references. The number of cited references is indeed lowered as a consequence of an incomplete publication set. However, this does not significantly affect the results, since the reference counts are only used as a relative measure within specific publication years.

As most literature databases, the WoS does not cover each and every scientific journal but only a carefully selected set of core journals most important for scientific disciplines. The coverage or comprehensiveness of the database can be estimated by comparing the number of all cited references with the number of the linked cited references (i.e., the references, which correspond to papers appearing in publications covered by the database as publication records). Based on the publication years 1990, 1995, 2000, 2005, and 2010, about 70% of all references in the natural sciences are linked references (Marx and Bornmann 2015 ). Thus, about 30% of the cited literature of these disciplines is not covered by the database in the form of paper records, presumably many non-English publications. It may be true that the publication set analyzed is biased toward mid-latitude developed countries, disadvantaging countries with most people suffering from humid heat waves. Parts of the most extreme heat waves occur in the French-speaking parts of Africa and the Arabic-speaking desert countries. Presumably, relevant literature like national reports discussing for example the local impact of extreme heat waves is not included in this analysis. However, if such documents were highly relevant, they should be cited in the literature covered by the WoS. In this case, our RPYS analysis would have discovered them. Therefore, we are confident that at least the highly relevant documents of the heat wave literature are considered in our analysis.

Two other limitations of this study refer to the RPYS of the heat wave paper set:

There are numerous rather highly cited references retrieved by RPYS via CRExplorer but not considered in the listing of Table 2 due to the selection criteria applied. Many of these non-selected papers have N_CR values just below the limits that we have set. Therefore, papers not included in our listing are not per se qualified as much less important or even unimportant.

In the interpretation of cited references counts, one should have in mind that they rely on the “popularity” of a publication being cited in subsequent research. The counts measure impact but not scientific importance or accuracy (Tahamtan and Bornmann 2019 ). Note that there are many reasons why authors cite publications (Tahamtan and Bornmann 2018 ), thus introducing a lot of “noise” in the data (this is why RPYS focuses on the cited reference peaks).

Our suggestions for future empirical analysis refer to the impact of the scientific heat wave discourse on social networks and funding of basic research on heat waves around topics driven by political pressure. Whereas this paper focuses on the scientific discourse around heat waves, it would be interesting if future studies were to address the policy relevance of the heat waves research.

Data availability

Not applicable.

Code availability

Change history, 23 february 2022.

The original version of this paper was updated to add the missing compact agreement Open Access funding note.

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All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Werner Marx, Robin Haunschild, and Lutz Bornmann. The first draft of the manuscript was written by Werner Marx and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

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Appendix 1 1)

WoS search query (date of search: July 1, 2021)

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Marx, W., Haunschild, R. & Bornmann, L. Heat waves: a hot topic in climate change research. Theor Appl Climatol 146 , 781–800 (2021). https://doi.org/10.1007/s00704-021-03758-y

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DOI : https://doi.org/10.1007/s00704-021-03758-y

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15 August 2024

‘Unacceptable’: a staggering 4.4 billion people lack safe drinking water, study finds

Alix Soliman

You can also search for this author in PubMed Google Scholar

Low-angle view of people gathered around a roadside water pipeline collecting drinking water

People gather around a roadside pipeline to collect drinking water in Bangladesh. Credit: Mamunur Rashid/NurPhoto/Getty

Approximately 4.4 billion people drink unsafe water — double the previous estimate — according to a study published today in Science 1 . The finding, which suggests that more than half of the world’s population is without clean and accessible water, puts a spotlight on gaps in basic health data and raises questions about which estimate better reflects reality.

That this many people don’t have access is “unacceptable”, says Esther Greenwood, a water researcher at the Swiss Federal Institute of Aquatic Science and Technology in Dübendorf and an author on the Science paper. “There’s an urgent need for the situation to change.”

The United Nations has been tracking access to safely managed drinking water, recognized as a human right, since 2015. Before this, the UN reported only whether global drinking-water sources were ‘improved’, meaning they were probably protected from outside contamination with infrastructure such as backyard wells, connected pipes and rainwater-collection systems. According to this benchmark, it seemed that 90% of the global population had its drinking water in order. But there was little information on whether the water itself was clean, and, almost a decade later, statisticians are still relying on incomplete data.

“We really lack data on drinking-water quality,” Greenwood says. Today, water-quality data exist for only about half of the global population. That makes calculating the exact scale of the problem difficult, Greenwood adds.

Crunching numbers

In 2015, the UN created its Sustainable Development Goals to improve human welfare . One of them is to “achieve universal and equitable access to safe and affordable drinking water for all” by 2030. The organization updated its criteria for safely managed drinking-water sources: they must be improved, consistently available, accessible where a person lives and free from contamination.

The world faces a water crisis — 4 powerful charts show how

Using this framework, the Joint Monitoring Programme for Water Supply, Sanitation and Hygiene (JMP), a research collaboration between the World Health Organization (WHO) and the UN children’s agency UNICEF, estimated in 2020 that there are 2.2 billion people without access to safe drinking water. To arrive at this figure, the programme aggregated data from national censuses, reports from regulatory agencies and service providers and household surveys.

But it assessed drinking-water availability differently from the method used by Greenwood and her colleagues. The JMP examined at least three of the four criteria in a given location, and then used the lowest value to represent that area’s overall drinking-water quality. For instance, if a city had no data on whether its water-source was consistently available, but 40% of the population had uncontaminated water, 50% had improved water sources and 20% had water access at home, then the JMP estimated that 20% of that city’s population had access to safely managed drinking water. The programme then scaled this figure across a nation’s population using a simple mathematical extrapolation.

By contrast, the Science paper used survey responses about the four criteria from 64,723 households across 27 low- and middle-income countries between 2016 and 2020. If a household failed to meet any of the four criteria, it was categorized as not having safe drinking water. From this, the team trained a machine-learning algorithm and included global geospatial data — including factors such as regional average temperature, hydrology, topography and population density — to estimate that 4.4 billion people lack access to safe drinking water, of which half are accessing sources tainted with the pathogenic bacteria Escherichia coli .

The model also suggested that almost half of the 4.4 billion live in south Asia and sub-Saharan Africa (see ‘Water woes’).

Water Woes. Bar chart. A modelling study has estimated that four billion people are without safe drinking water.

Source: Ref 1.

‘A long way to go’

It’s “difficult” to say which estimate — the JMP’s or the new figure — is more accurate, says Robert Bain, a statistician at UNICEF’s Middle East and North Africa Regional Office, based in Amman, Jordan, who contributed to the calculation of both numbers. The JMP brings together many data sources but has limitations in its aggregation approach, whereas the new estimation takes a small data set and scales it up with a sophisticated model, he says.

The study by Greenwood and colleagues really highlights “the need to pay closer attention to water quality”, says Chengcheng Zhai, a data scientist at the University of Notre Dame in Indiana. Although the machine-learning technique used by the team is “very innovative and clever”, she says, water access is dynamic, so the estimation might still not be quite right. Wells can be clean of E. coli one day and become contaminated the next, and the household surveys don’t capture that, Zhai suggests.

“Whichever number you run with — two billion or four billion — the world has a long way to go” towards ensuring that people’s basic rights are fulfilled, Bain says.

doi: https://doi.org/10.1038/d41586-024-02621-0

Greenwood, E. E. et al. Science 385 , 784–790 (2024).

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Diversity and ecological functions of fungal communities in tangchi hot spring in lujiang (china).

1. Introduction

2. materials and methods, 2.1. sampling, 2.2. fungal its high-throughput sequencing, 2.3. bioinformatics analysis, 2.4. function prediction, 2.5. data accession, 3.1. water sample parameters of tangchi hot spring, 3.2. sequencing results of hot spring samples, 3.3. composition of fungal communities, 3.4. fungal alpha diversity analysis, 3.5. fungal function prediction, 4. discussion, 5. conclusions, author contributions, data availability statement, conflicts of interest.

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Click here to enlarge figure

	Barcode	SeqNum	BaseNum	MeanLen	MinLen	MaxLen
Raw Data	ACGAGTG	56,052	18,071,057	322.4	44	493
Clean Data	ACGAGTG	56,039	15,711,131	280.36	101	451

OTU ID	Taxonomy
OTU1	d__Fungi; p__Chytridiomycota; c__Rhizophydiomycetes; o__Rhizophydiales; f__Rhizophydiaceae; g__Rhizophydium; s__unclassified_Rhizophydium
OTU4	d__Fungi; p__Ascomycota; c__Leotiomycetes; o__Helotiales; f__Sclerotiniaceae; g__Sclerotinia; s__Sclerotinia_sclerotiorum_SH1152486.08FU
OTU6	d__Fungi; p__Ascomycota; c__Dothideomycetes; o__Dothideales; f__Aureobasidiaceae; g__Aureobasidium; s__unclassified_Aureobasidium
OTU5	d__Fungi; p__Basidiomycota; c__Tremellomycetes; o__Cystofilobasidiales; f__Mrakiaceae; g__Tausonia; s__Tausonia_pullulans_SH1650607.08FU
OTU7	d__Fungi; p__Olpidiomycota; c__GS17; o__GS17; f__unclassified_GS17;g__unclassified_GS17; s__unclassified_GS17
OTU8	d__Fungi; p__Basidiomycota; c__Microbotryomycetes; o__Sporidiobolales; f__Sporidiobolaceae; g__Rhodotorula; s__Rhodotorula_babjevae_SH2272266.08FU
OTU9	d__Fungi; p__Ascomycota; c__Dothideomycetes; o__Pleosporales; f__Phaeosphaeriaceae; g__Sclerostagonospora; s__Sclerostagonospora_lathyri_SH1525096.08FU
OTU10	d__Fungi; p__Basidiomycota; c__Microbotryomycetes; o__Sporidiobolales; f__Sporidiobolaceae; g__Rhodotorula; s__Rhodotorula_mucilaginosa_SH1181860.08FU
OTU61	d__Fungi; p__Ascomycota; c__Saccharomycetes; o__Saccharomycetales; f__Saccharomycetaceae; g__Saccharomyces; s__Saccharomyces_cerevisiae_SH1613178.08FU
OTU12	d__Fungi; p__Ascomycota; c__Dothideomycetes; o__Pleosporales; f__Pleosporaceae; g__Alternaria; s__unclassified_Alternaria
OTU11	d__Fungi; p__Ascomycota; c__Dothideomycetes; o__Capnodiales; f__Cladosporiaceae; g__Cladosporium; s__Cladosporium_exasperatum_SH2320217.08FU
OTU13	d__Fungi;p__Basidiomycota; c__Tremellomycetes; o__Cystofilobasidiales; f__Mrakiaceae;g__Mrakia; s__unclassified_Mrakia
OTU33	d__Fungi; p__Basidiomycota; c__Agaricostilbomycetes; o__Agaricostilbales; f__Ruineniaceae; g__Ruinenia; s__Ruinenia_clavata_SH1575142.08FU
OTU28	d__Fungi; p__Ascomycota; c__Dothideomycetes; o__Dothideales; f__Aureobasidiaceae; g__Aureobasidium; s__Aureobasidium_pullulans_SH1515141.08FU
OTU26	d__Fungi; p__Chytridiomycota; c__unclassified_Chytridiomycota; o__unclassified_Chytridiomycota; f__unclassified_Chytridiomycota; g__unclassified_Chytridiomycota; s__unclassified_Chytridiomycota

Sample	Shannon	Chao	Ace	Simpson	Shannoneven	Coverage
Tangchi	4.339195	541.965517	542.70909	0.037157	0.689481	0.99976

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Zhang, F.-Q.; Liu, J.; Chen, X.-J. Diversity and Ecological Functions of Fungal Communities in Tangchi Hot Spring in Lujiang (China). Water 2024 , 16 , 2308. https://doi.org/10.3390/w16162308

Zhang F-Q, Liu J, Chen X-J. Diversity and Ecological Functions of Fungal Communities in Tangchi Hot Spring in Lujiang (China). Water . 2024; 16(16):2308. https://doi.org/10.3390/w16162308

Zhang, Feng-Qin, Jun Liu, and Xiao-Ju Chen. 2024. "Diversity and Ecological Functions of Fungal Communities in Tangchi Hot Spring in Lujiang (China)" Water 16, no. 16: 2308. https://doi.org/10.3390/w16162308

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Warming water temperatures and increased nutrient levels are putting freshwater ecosystems at serious risk

New international research, led by the University of Sheffield found that food webs are becoming less complex in warmer, nutrient-rich waters

The Sylvenstein lake reservoir and river Isar

The effect of rising water temperatures on ecosystems is likely to interact with pre-existing pressures, such as excess nutrients from pollution, resulting in a dangerous combination, according to a new study
New international research, conducted by the University of Sheffield, University of Oxford, University of Savoie Mont Blanc and the French National Research Institute for Agriculture, Food and the Environment, found that food webs are becoming less complex in warmer, nutrient-rich waters
This simplification means shorter food chains and a degraded functioning ecosystem
The urgent global need to reduce river pollution levels was recognised internationally when Olympic open water swimming events, due to take place in the Seine river in Paris, had to be postponed due to harmful bacterial levels

Warming water temperatures and increased nutrient levels are putting freshwater ecosystems at serious risk, new research has revealed.

Scientists previously thought warming temperatures caused by climate change and increased nutrient levels due to pollution might offset each other’s impact on aquatic life. However, a new study has shown warming waters and nutrient overload is a dangerous combination threatening our rivers and lakes.

The research, conducted by an international team of scientists from the University of Sheffield, University of Oxford and University of Savoie Mont Blanc and the French National Research Institute for Agriculture, Food and the Environment, examined hundreds of lakes and streams, analysing the complex relationships between fish species.

Researchers found that food webs, the intricate networks of who eats who, are becoming less complex in warmer, nutrient-rich waters. This simplification means shorter food chains, and an ecosystem with a degraded functioning. Top predators are particularly vulnerable to these changes, but they are essential to the functioning and the stability of ecosystems.

Less than three per cent of the water on our planet is fresh, yet freshwater habitats are home to almost 10 per cent of all known animals and almost half of all known fish species. Freshwater ecosystems not only help to maintain water quality of the land and sea, but also allow wildlife such as eels and salmon to travel vast distances to complete their life cycles, and act as conveyor belts transporting nutrients that make soil good for growing food.

The far reaching impact of the need to reduce pollution levels in our rivers was felt globally when the Olympic open water swimming events such as the triathlon, due to take place in the Seine river in Paris, had to be postponed due to harmful bacterial levels. The study highlights that this need is even more urgent with the rising impacts of warming of climate change, but it also suggests that reducing pollution levels is a promising path to mitigate the impacts of climate change.

Alain Danet, Postdoctoral Research Associate at the University of Sheffield and co-first author of the study, said: “The eyes of the world have been on the Seine River this summer due to the Olympic Games. The challenges faced in Paris demonstrate how important it is to reduce the pollution levels in our rivers and lakes not only for our own health but for the health of our ecosystems.

“Our rivers, lakes, wetlands and underground water supplies store and clean the water that's crucial for people and wildlife. From providing drinking water to providing essential resources to terrestrial and marine ecosystems, healthy freshwater systems are essential.

“These vital ecosystems also protect us from flooding, filter pollution and support countless plants and animals.”

Willem Bonnaffé, Postdoctoral Research Associate at the University of Oxford and co-first author of the study, said: “Lake and river creatures are like the kidneys of our planet. They clean water and prevent blooms of poisonous algae and bacteria. Keeping them safe is keeping us safe.

“Climate change and pollution are not isolated problems for our rivers and lakes, as scientists previously believed. Our study has found they interact in complex ways that threaten the balance of life in our waters.

“By reducing greenhouse gas emissions and improving water quality, we can help safeguard the health of our rivers and lakes for future generations.”

Camille Leclerc, Postdoctoral Research Associate at the French National Research Institute for Agriculture, Food and the Environment and co-first author of the study, said: “This important research has shown what a dangerous combination warming waters caused by climate change and increased nutrient levels caused by pollution are.

“More positively this study shows that if we can reduce the pollution in our lakes and rivers, it will better protect them against the effects of climate change and warmer waters.”

View the full paper published in the journal Ecology Letters

Amy Huxtable

The University's cross-faculty research centres harness our interdisciplinary expertise to solve the world's most pressing challenges.

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Heat waves: a hot topic in climate change research

Werner marx.

1 Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany

Robin Haunschild

Lutz bornmann.

2 Science Policy and Strategy Department, Max Planck Society, Administrative Headquarters, Hofgartenstr. 8, 80539 Munich, Germany

Associated Data

Not applicable.

Introduction

Heat waves as a research topic

Methodology

Dataset used.

Reference Publication Year Spectroscopy

Time evolution of literature

In Fig. 1 , the time evolution between 1990 and 2020 of the publications dealing with heat waves is shown (there are only 109 pre-1990 publications dealing with heat waves and covered by the WoS).

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Countries of authors

In Table Table1, 1 , the number of papers assigned to the countries of authors with more than 100 publications dealing with heat waves is presented, showing the national part of research activities on this research topic. For comparative purposes, the percentage of overall papers in WoS of each country is shown. As a comparison with the overall WoS, we only considered WoS papers published between 2000 and 2020, because the heat wave literature started to grow substantially around 2000.

Top countries of authors with more than 100 papers dealing with heat waves up to the date of the search

Country of authors	#Papers	%Papers heat waves	%Papers overall in WoS
USA	2081	26.0	27.4
Australia	1026	12.8	3.1
Peoples Republic of China	965	12.0	12.1
England	760	9.5	6.7
Germany	737	9.2	6.3
France	638	8.0	4.3
Italy	536	6.7	3.9
Spain	506	6.3	3.1
Switzerland	361	4.5	1.6
Canada	356	4.4	4.0
India	236	2.9	3.3
Netherlands	227	2.8	2.2
South Korea	206	2.6	2.5
Sweden	206	2.6	1.4
Portugal	204	2.5	0.7
Belgium	176	2.2	1.2
Japan	168	2.1	5.2
Greece	163	2.0	0.7
Russia	149	1.9	2.1
Poland	141	1.8	1.4
Austria	137	1.7	0.9
Czech Republic	130	1.6	0.7
Denmark	119	1.5	0.9
South Africa	119	1.5	0.6
Brazil	116	1.4	2.1
Scotland	106	1.3	1.0

The country-specific percentages from Table Table1 1 are visualized in Fig. 2 . Selected countries are labeled. Countries with a higher relative percentage of more than two percentage points in heat wave research than in WoS overall output are marked blue (blue circle). Countries with a relative percentage at least twice as high in heat wave research than in overall WoS output are marked green (green cross), whereas countries with a relative percentage at most half as much in heat wave research than in overall WoS output are marked with a yellow cross. Only Japan has a much lower output in heat wave research than in WoS overall output, as indicated by the red circle and yellow cross. Most countries are clustered around the bisecting line and are marked gray (gray circle). China and the USA are outside of the plot region. Both countries are rather close to the bisecting line. Some European countries show a much larger activity in heat wave research than in overall WoS output. Australia shows the largest difference and ratio in output percentages as shown by the blue circle and green cross.

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Publication percentages of countries in Table Table1. 1 . Countries with large deviations between heat wave output and overall WoS output are labeled. Countries with an absolute percentage of more than two percentage points higher (lower) in heat wave research than in overall WoS output are marked blue (red). Countries with a relative percentage at least twice as high (at most half as much) in heat wave research than in overall WoS output are marked green (yellow)

Figure 3 shows the co-authorship network with regard to the countries of authors of the papers dealing with heat waves using the VOSviewer software.

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According to Fig. 3 and in accordance with Table Table1, 1 , the USA is most productive in heat wave research. This is not unexpected, because the US publication output is at the top for most research fields. However, this aside, the USA has been heavily affected by heat wave events and is leading with regard to the emergence of the topic. Australia appears as another major player and is strongly connected with the US publications within the co-authorship network and thus appears as a large node near the US node in the map. Next, the leading European countries England, France, Germany, Italy, and Spain appear.

Topics of the heat wave literature

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Countries/regions: United-States, Europe, France, China, Pacific, Australia, London, England
Cities: cities, city, US cities, Chicago, communities
Events: 2003 heat-wave, 1995 heat-wave
Impacts: impact, impacts, air-pollution, drought, soil-moisture, exposure, heat-island, urban, islands, photosynthesis, pollution, heat-island, air-quality, environment, precipitation extremes, biodiversity, emissions
Politics: risk, responses, vulnerability, adaptation, management, mitigation, risk-factors, scenarios
Biology: vegetation, forest, diversity, stomatal conductance
Medicine: mortality, health, stress, deaths, morbidity, hospital admissions, public-health, thermal comfort, population, heat, sensitivity, human health, disease, excess mortality, heat-stress, heat-related mortality, comfort, behavior, death, stroke
Climate research: climate change, temperature, climate, model, simulation, energy, projections, simulations, cmip5, ozone, el-nino, parametrization, elevated CO 2 , models, climate variability, carbon, carbon-dioxide
Meteorology: heat waves, variability, precipitation, summer, heat-wave, weather, ambient-temperature, waves, extremes, wave, cold, water, rainfall, circulation, heat, air-temperature, extreme heat, climate extremes, heatwaves, temperature extremes, temperatures, temperature variability, high-temperature, ocean, extreme temperatures, atmospheric circulation, interannual variability, sea-surface temperature, oscillation, surface temperature, surface
Broader terms (multi-meaning): trends, events, patterns, growth, performance, time-series, indexes, system, dynamics, association, index, tolerance, productivity, ensemble, resilience, increase, quality, prediction, frequency, particulate matter, future, framework, 20 th -century, time, reanalysis, systems

Although allocated by the database provider, the keywords are not coherent. For example, the same keyword may appear as singular or plural, and complex keywords are written with and without hyphens.

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Countries/regions: United-States, Great-Plains
Cities: St-Louis, Athens, Chicago
Events: 1980 heat-wave, 1995 heat-wave
Impacts: impacts, responses, drought, precipitation, comfort, sultriness
Climate research: climate, climate change, model, temperature, variability
Medicine: cardiovascular deaths, mortality, air pollution
Meteorology: atmospheric flow, weather, heat, humidity index

Important publications

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The gray bars (Fig. 6 ) and red lines (Figs. (Figs.7 7 – 8 ) in the graphs visualize the number of cited references per reference publication year. In order to identify those publication years with significantly more cited references than other years, the (absolute) deviation of the number of cited references in each year from the median of the number of cited references in the two previous, the current, and the two following years (t − 2; t − 1; t; t + 1; t + 2) is also visualized (blue lines). This deviation from the 5-year median provides a curve smoother than the one in terms of absolute numbers. We inspected both curves for the identification of the peak papers.

Applying the selection criteria mentioned above (minimum number of cited references between 50 and 150 in three different periods), 104 references have been selected as key papers (important papers most frequently referenced within the research topic analyzed) and are presented in Table Table2 2 in Appendix 2. The peak papers corresponding to reference publication years below about 2000 can be seen as the historical roots of the research topic analyzed. Since around 2000, the number of references with the same publication year becomes increasingly numerous, usually with more than one highly referenced (cited) paper at the top. Although there are comparatively fewer distinct peaks visible in the RPYS spectrogram of Fig. 8 , the most frequently referenced papers can easily be identified via the CRE reference listing. Depending on the specific skills and needs (i.e., the expert knowledge and the intended depth of the analysis), the number of top-referenced papers considered key papers can be defined individually.

Listing of the key papers ( n = 104) revealed by RPYS via CRE ( RPY reference publication year, N_CR number of cited references, Title title of the cited reference, DOI allows easily to retrieve the full paper via WoS or Internet)


1945	94	Mann, H.B	Nonparametric tests against trend	Not available
1968	108	Sen, P.K	Estimates of regression coefficient based on Kendalls Tau	Not available
1972	72	Schuman, S.H	Patterns of urban heat-wave deaths and implications for prevention—Data from New York and St-Louis during July, 1966
1973	81*	Oke, T.R	City size and urban heat island
1979	110*	Steadman, R.G	The assessment of sultriness. Part I: A temperature-humidity index based on human physiology and clothing science	<0861:TAOSPI>2.0.CO;2
1980	62	Berry, J	Photosynthetic response and adaptation to temperature in higher-plants
1982	175	Oke, T.R	The energetic basis of the urban heat island
1982	107*	Jones, T.S	Morbidity and mortality associated with the July 1980 heat-wave in St.-Louis and Kansas-City, MO
1982	88	Kilbourne, E.M	Risk-factors for heat-stroke—A case–control study
1984	112*	Steadman, R.G	A universal scale of apparent temperature	<1674:AUSOAT>2.0.CO;2
1984	52	Mearns, L.O	Extreme high-temperature events—Changes in their probabilities with changes in mean temperature	<1601:EHTECI>2.0.CO;2
1986	59	Kalkstein, L.S	An evaluation of summer discomfort in the United-States using a relative climatological index	<0842:AEOSDI>2.0.CO;2
1986	53	Keatinge, W.R	Increased platelet and red-cell counts, blood-viscosity, and plasma-cholesterol levels during heat stress, and mortality from coronary and cerebral thrombosis
1987	71	Mayer, H	Thermal comfort of man in different urban environments
1989	87*	Kalkstein, L.S	Weather and human mortality—An evaluation of demographic and interregional responses in the United-States
1989	65	Dudhia, J	Numerical study of convection observed during the winter monsoon experiment using a mesoscale two-dimensional model	<3077:NSOCOD>2.0.CO;2
1989	53	Joseph, D.D	Heat waves
1992	85*	Katz, R.W	Extreme events in a changing climate—Variability is more important than averages
1993	74*	Kunst, A.E	Outdoor air temperature and mortality in the Netherlands: A time-series analysis
1993	52	Katsouyanni, K	Evidence for interaction between air-pollution and high-temperature in the causation of excess mortality
1995	52	Sartor, F	Temperature, ambient ozone levels, and mortality during summer, 1994, in Belgium
1996	368*	Semenza, J.C	Heat-related deaths during the July 1995 heat wave in Chicago
1996	327*	Kalnay, E	The NCEP/NCAR 40-year reanalysis project	<0437:TNYRP>2.0.CO;2
1996	106*	Changnon, S.A	Impacts and responses to the 1995 heat wave: A call to action	<1497:IARTTH>2.0.CO;2
1996	84	Kunkel, K.E	The July 1995 heat wave in the midwest: A climatic perspective and critical weather factors	<1507:TJHWIT>2.0.CO;2
1996	63	Kalkstein, L.S	The Philadelphia hot weather-health watch warning system: Development and application, summer 1995	<1519:TPHWHW>2.0.CO;2
1997	180*	Whitman, S	Mortality in Chicago attributed to the July 1995 heat wave
1997	116*	Kalkstein, L.S	An evaluation of climate/mortality relationships in large US cities and the possible impacts of a climate change
1997	105*	Karl, T.R	The 1995 Chicago heat wave: How likely is a recurrence?	<1107:TCHWHL>2.0.CO;2
1997	74	Mlawer, E.J	Radiative transfer for inhomogeneous atmospheres: RRTM, a validated correlated-k model for the longwave
1997	72	Keatinge, W.R	Cold exposure and winter mortality from ischaemic heart disease, cerebrovascular disease, respiratory disease, and all causes in warm and cold regions of Europe	Not available
1997	53	Easterling, D.R	Maximum and minimum temperature trends for the globe
1997	53	Mantua, N.J	A Pacific interdecadal climate oscillation with impacts on salmon production	<1069:APICOW>2.0.CO;2
1997	50	Ballester, F	Mortality as a function of temperature. A study in Valencia, Spain, 1991–1993
1998	105*	Rooney, C	Excess mortality in England and Wales, and in Greater London, during the 1995 heatwave
1998	81*	Dematte, J.E	Near-fatal heat stroke during the 1995 heat wave in Chicago
1998	55	Smoyer, K.E	A comparative analysis of heat waves and associated mortality in St. Louis, Missouri—1980 and 1995
1998	53	Gaffen, D.J	Increased summertime heat stress in the US
1999	200*	Semenza, J.C	Excess hospital admissions during the July 1995 heat wave in Chicago
1999	91	Hoppe, P	The physiological equivalent temperature—A universal index for the biometeorological assessment of the thermal environment
1999	56	Matzarakis, A	Applications of a universal thermal index: physiological equivalent temperature
1999	52*	Kunkel, K.E	Temporal fluctuations in weather and climate extremes that cause economic and human health impacts: A review	<1077:TFIWAC>2.0.CO;2
2000	275*	Easterling, D.R	Climate extremes: observations, modelling, and impacts
2001	277*	Robinson, P.J	On the definition of a heat wave	<0762:OTDOAH>2.0.CO;2
2001	203*	Hynen, M.M.T.E	The impact of heat waves and cold spells on mortality rates in the Dutch population
2002	286*	Basu, R	Relation between elevated ambient temperature and mortality: A review of the epidemiologic evidence
2002	279*	Curriero, F.C	Temperature and mortality in 11 cities of the eastern United States
2002	233*	Frich, P	Observed coherent changes in climatic extremes during the second half of the twentieth century
2002	156*	Bouchama, A	Medical progress—Heat stroke
2004	1099*	Meehl, G.A	More intense, more frequent, and longer lasting heat waves in the twenty-first century
2004	562*	Schaer, C	The role of increasing temperature variability in European summer heatwaves
2004	326*	Stott, P.A	Human contribution to the European heatwave of 2003
2004	232*	Beniston, P.M	The 2003 heat wave in Europe: A shape of things to come? An analysis based on Swiss climatological data and model simulations
2004	209*	Black, E	Factors contributing to the summer 2003 European heatwave
2004	180*	Luterbacher, J	European seasonal and annual temperature variability, trends, and extremes since 1500
2004	152	Kovats, R.S	Contrasting patterns of mortality and hospital admissions during hot weather and heat waves in Greater London, UK
2005	313*	Ciais, P	Europe-wide reduction in primary productivity caused by the heat and drought in 2003
2005	161*	Patz, J.A	Impact of regional climate change on human health
2005	157*	Conti, S	Epidemiologic study of mortality during the Summer 2003 heat wave in Italy
2006	298*	Fouillet, A	Excess mortality related to the August 2003 heat wave in France
2006	257*	Alexander, L.V	Global observed changes in daily climate extremes of temperature and precipitation
2006	190*	Seneviratne, S.I	Land–atmosphere coupling and climate change in Europe
2006	180*	Vandentorren, S	August 2003 heat wave in France: Risk factors for death of elderly people living at home
2006	177*	Hajat, S	Impact of high temperatures on mortality—Is there an added heat wave effect?
2006	153*	Kovats, R.S	Heatwaves and public health in Europe
2007	238*	Fischer, E.M	Soil moisture—Atmosphere interactions during the 2003 European summer heat wave
2007	156*	Della-Marta, P.M	Doubled length of western European summer heat waves since 1880
2008	372*	Robine, J.M	Death toll exceeded 70,000 in Europe during the summer of 2003
2008	287*	Kovats, R.S	Heat stress and public health: A critical review
2008	178*	Baccini, M	Heat effects on mortality in 15 European cities
2008	175*	Luber, G	Climate change and extreme heat events
2008	160*	Fouillet, A	Has the impact of heat waves on mortality changed in France since the European heat wave of summer 2003? A study of the 2006 heat wave
2009	339*	Anderson, Brooke G	Weather-related mortality: How heat, cold, and heat waves affect mortality in the United States
2009	212*	Basu, R	High ambient temperature and mortality: A review of epidemiologic studies from 2001 to 2008
2009	205*	Knowlton, K	The 2006 California heat wave: Impacts on hospitalizations and emergency department visits
2010	282*	Fischer, E.M	Consistent geographical patterns of changes in high-impact European heatwaves
2010	208*	D’Ippoliti, D	The impact of heat waves on mortality in 9 European cities: results from the EuroHEAT project
2010	208*	Seneviratne, S.I	Investigating soil moisture-climate interactions in a changing climate: A review
2010	196*	Garcia-Herrera, R	A review of the European summer heat wave of 2003
2011	381*	Dee, D.P	The ERA-Interim reanalysis: configuration and performance of the data assimilation system
2011	355*	Barriopedro, D	The hot summer of 2010: Redrawing the temperature record map of Europe
2011	303*	Anderson, G.B	Heat waves in the United States: Mortality risk during heat waves and effect modification by heat wave characteristics in 43 U.S. communities
2011	198*	Dole, R	Was there a basis for anticipating the 2010 Russian heat wave?
2011	154*	Gasparrini, A	The Impact of Heat Waves on Mortality
2012	243	Taylor, K.E	An overview of CMIP5 and the experiment design
2012	236	Perkins, S.E	Increasing frequency, intensity and duration of observed global heatwaves and warm spells
2012	204	Coumou, D	A decade of weather extremes
2013	289	Perkins, S.E	On the measurement of heat waves
2013	159	Wernberg, T	An extreme climatic event alters marine ecosystem structure in a global biodiversity hotspot
2014	197	Miralles, D.G	Mega-heatwave temperatures due to combined soil desiccation and atmospheric heat accumulation
2014	171	Russo, S	Magnitude of extreme heat waves in present climate and their projection in a warming world
2015	182	Gasparrini, A	Mortality risk attributable to high and low ambient temperature: A multicountry observational study
2015	168	Perkins, S.E	A review on the scientific understanding of heatwaves—Their measurement, driving mechanisms, and changes at the global scale
2015	159	Russo, S	Top ten European heatwaves since 1950 and their occurrence in the coming decades
2015	131	Bond, N.A	Causes and impacts of the 2014 warm anomaly in the NE Pacific
2015	111	Christidis, N	Dramatically increasing chance of extremely hot summers since the 2003 European heatwave
2016	197	Hobday, A.J	A hierarchical approach to defining marine heatwaves
2016	121	Wernberg, T	Climate-driven regime shift of a temperate marine ecosystem
2016	113	Di Lorenzo, E.	Multi-year persistence of the 2014/15 North Pacific marine heatwave
2017	125	Mora, C	Global risk of deadly heat
2017	112	Hughes, T.P	Global warming and recurrent mass bleaching of corals
2018	209	Oliver, E.C.J	Longer and more frequent marine heatwaves over the past century
2018	124	Frolicher, T.L	Marine heatwaves under global warming
2019	126	Smale, D.A	Marine heatwaves threaten global biodiversity and the provision of ecosystem services

*N_TOP10 > 9; the N_TOP10 indicator is the number of reference publication years in which a focal cited reference belongs to the 10% most referenced publications.

Table Table2 2 lists the first authors and titles of the 104 key papers selected, their number of cited references (N_CR), and the DOIs for easy access. Some N_CR values are marked by an asterisk, indicating a high value of the N_TOP10 indicator implemented in the CRExplorer. The N_TOP10 indicator value is the number of reference publication years in which a focal cited reference belongs to the 10% most referenced publications. In the case of about half of the cited references in Table Table2 2 ( n = 58), the N_TOP10 value exceeded a value of 9. The three highest values in our dataset are 24, 21, and 20.

Out of the 104 key papers from Table Table2, 2 , 101 have a DOI of which we found 101 papers in the WoS. Three papers have no DOI but could be retrieved from WoS. The altogether 104 papers were exported and their keywords (keywords plus) were displayed in Fig. 9 for revealing the thematic content of the key papers from the RPYS analysis at a glance.

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Co-occurrence network map of the keywords plus of the 104 key papers dealing with heat waves selected applying RPYS via CRE software and listed in Table Table2. 2 . The minimum number of occurrences of keywords is 2; of the 310 keywords, 91 meet the threshold. Readers interested in an in-depth analysis can use VOSviewer interactively and zoom into the map via the following URL: https://tinyurl.com/4vwpc4t2

The key papers presented in Table Table2 2 can be categorized as follows: (1) papers dealing with specific heat wave events, (2) the impact of heat waves on human health, (3) heat wave-related excess mortality and implications for prevention, (4) the interaction between air pollution and high temperature, (5) circulation pattern and the meteorological basis, (6) future perspectives and risks, and (7) climate models, indicators, and statistics.

As already mentioned in Sect. 1 , the more intense and more frequently occurring heat waves cannot be explained solely by natural climate variations but only with human-made climate change. As a consequence, research on heat waves has become embedded into meteorology and climate change research and has aimed to understand the specific connection with global warming. Scientists discuss a weakening of the polar jet stream as a possible reason for an increasing probability for the occurrence of heat waves (e.g., Broennimann et al. 2009 ; Coumou et al. 2015 ; Mann 2019 ). Climate models are used for projections of temperature and rainfall variability in the future, based on various scenarios of greenhouse gas emissions. As a result, the corresponding keywords appear in the maps of Figs. Figs.4 4 and and9. 9 . Also, the application of statistics plays a major role in the papers of our publication set; some of the most highly referenced (early) papers in Table Table2 2 primarily deal with statistical methods. These methods provide the basis for research on heat waves.

The results of this study should be interpreted in terms of its limitations:

We tried to include in our bibliometric analyses all relevant heat wave papers covered by the database. Our long-standing experience in professional information retrieval has shown, however, that it is sheer impossible to get complete and clean results by search queries against the backdrop of the search functions provided by literature databases like WoS or others. Also, the transition from relevant to non-relevant literature is blurred and is a question of the specific needs. In this study, we used bibliometric methods that are relatively robust with regard to the completeness and precision of the publication sets analyzed. For example, it is an advantage of RPYS that a comparatively small portion of relevant publications (i.e., an incomplete publication set) contains a large amount of the relevant literature as cited references. The number of cited references is indeed lowered as a consequence of an incomplete publication set. However, this does not significantly affect the results, since the reference counts are only used as a relative measure within specific publication years.

Two other limitations of this study refer to the RPYS of the heat wave paper set:

There are numerous rather highly cited references retrieved by RPYS via CRExplorer but not considered in the listing of Table Table2 2 due to the selection criteria applied. Many of these non-selected papers have N_CR values just below the limits that we have set. Therefore, papers not included in our listing are not per se qualified as much less important or even unimportant.
In the interpretation of cited references counts, one should have in mind that they rely on the “popularity” of a publication being cited in subsequent research. The counts measure impact but not scientific importance or accuracy (Tahamtan and Bornmann 2019 ). Note that there are many reasons why authors cite publications (Tahamtan and Bornmann 2018 ), thus introducing a lot of “noise” in the data (this is why RPYS focuses on the cited reference peaks).

Appendix 1 1)

WoS search query (date of search: July 1, 2021)

# 15		#14 AND #6 = =
# 14		("wet bulb temperature*" OR WBT) = =
# 13		#6 AND TS = mortality = =
# 12		#2 OR #4 ( ARTICLE OR MEETING ABSTRACT OR CORRECTION OR PROCEEDINGS PAPER OR LETTER OR REVIEW OR NEWS ITEM OR BOOK CHAPTER OR EARLY ACCESS OR EDITORIAL MATERIAL OR BOOK REVIEW) AND ( 1984 OR 1967 OR 1983 OR 1966 OR 1982 OR 1965 OR 1999 OR 1981 OR 1964 OR 1998 OR 1980 OR 1963 OR 1997 OR 1979 OR 1962 OR 1996 OR 1978 OR 1961 OR 1995 OR 1977 OR 1959 OR 1994 OR 1976 OR 1954 OR 1993 OR 1975 OR 1949 OR 1992 OR 1974 OR 1940 OR 1991 OR 1973 OR 1938 OR 1990 OR 1972 OR 1930 OR 1989 OR 1971 OR 1926 OR 1988 OR 1970 OR 1914 OR 1987 OR 1969 OR 1912 OR 1986 OR 1968 OR 1906 OR 1985) = =
# 11		#10 AND #6 = =
# 10		#9 OR #8 OR #7 = =
# 9		(climat* OR palaeoclimat* OR paleoclimat*) = =
# 8		("global temperature" OR "global warm" OR "greenhouse effect" OR "greenhouse gas" OR "greenhouse warm") = =
# 7		("climate chang" OR "climatic chang" OR "climate varia" OR "climatic varia" OR "climate warm" OR "climatic warm") = =
# 6		#2 OR #4 ( ARTICLE OR MEETING ABSTRACT OR CORRECTION OR PROCEEDINGS PAPER OR LETTER OR REVIEW OR NEWS ITEM OR BOOK CHAPTER OR EARLY ACCESS OR EDITORIAL MATERIAL OR BOOK REVIEW) = =
# 5		#2 OR #4 = =
# 4		#3 AND TS = (climat* OR greenhouse OR warming OR atmospher* OR tropospher* OR weather) = =
# 3		#1 NOT #2 = =
# 2		("heat wave" OR "heat waves" OR heatwave OR heatwaves OR "hot spell" OR "hot spells") [excluding] ( NANOSCIENCE NANOTECHNOLOGY OR ASTRONOMY ASTROPHYSICS OR NUCLEAR SCIENCE TECHNOLOGY OR PHYSICS APPLIED OR PHYSICS ATOMIC MOLECULAR CHEMICAL OR PHYSICS CONDENSED MATTER OR PHYSICS FLUIDS PLASMAS OR PHYSICS MATHEMATICAL OR PHYSICS MULTIDISCIPLINARY OR LITERARY REVIEWS OR MECHANICS) = =
# 1		("heat wave" OR "heat waves" OR heatwave OR heatwaves OR "hot spell" OR "hot spells") = =

Table Table2 2

Author contribution

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Data availability

Code availability, declarations.

The authors declare no competing interests.

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Change history

The original version of this paper was updated to add the missing compact agreement Open Access funding note.

Contributor Information

Werner Marx, Email: [email protected] .

Robin Haunschild, Email: [email protected] .

Lutz Bornmann, Email: [email protected] , Email: ed.gpm.vg@nnamnrob .

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Regulation of fluid intake

Fluid balance of the two compartments

Regulatory drinking

Nonregulatory drinking

Effects of aging on fluid intake regulation

Thermoregulation

Physical performance

Cognitive performance

Dehydration and delirium

Gastrointestinal function

Kidney function

Heart function and hemodynamic response

Patterns and trends of water consumption

Effects of water consumption on overall energy intake

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Heatwaves: does global research reflect the growing threat in the light of climate change?

Climate and human health: a review of publication trends in the International Journal of Biometeorology

Knowledge map and global trends in extreme weather research from 1980 to 2019: a bibliometric analysis

1 Introduction

2 Heat waves as a research topic

3 Methodology

3.2 Networks

3.3 Reference Publication Year Spectroscopy

4.1 Time evolution of literature

4.2 Countries of authors

4.3 Topics of the heat wave literature

4.4 Important publications

5 Discussion

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