Heat: what really cools your body – Knowledge

1. What this article is about

Heat is the natural hazard that causes the most deaths in Switzerland. In the summer of 2025, 410 people died from the effects of heat, and 84 percent of them were over 75 [1]. More than half of these deaths fell on days that were only moderately hot, with a daily mean temperature below 25 °C [1]. There are many such days. The advice in this article therefore applies from the first warm week on, and not only once MeteoSwiss issues a heat warning.

This article answers four questions. What does heat do to the body, and who is most at risk? Do cold drinks cool you, or do they warm you, as was claimed on television? What cools for real, and what only seems to? And when must you act instead of waiting?

2. What heat does to your body

Your body produces heat continuously, about 80 to 100 watts at rest, as much as an old light bulb [7]. During exertion it is 500 watts and more. This heat has to leave the body, otherwise core temperature rises. As long as the air is cooler than the skin, the skin passes heat directly to the air. From about 35 °C air temperature that no longer works. Then only one route remains: sweat that evaporates on the skin. One gram of evaporated sweat carries away 2.43 kilojoules of heat [7]. Sweat that drips off does not cool at all. That is why humid heat is harder to bear than dry heat: less evaporates in humid air.

With age this system works less well. Older people sweat less per sweat gland, their skin vessels widen less, and their heart pumps less blood into the skin [4]. In one experiment, 20 young and 39 older adults sat for nine hours in 40 °C dry heat [5]. After six hours the core temperature of the older group was 37.9 °C, that of the younger group 37.6 °C. The older adults stored 88 kilojoules more heat in the first three hours. Add to this a weaker sense of thirst, see section 4.

Most at risk are people who cannot leave their bed, who do not leave the house daily, who live alone or have a mental illness, and people with cardiovascular disease [3]. In Switzerland the risk of dying on days with a maximum of 33 °C was 21 percent higher than on days with a comfortable temperature, and 67 percent higher for people with dementia [2]. Protected in every investigation were people who spent a few hours a day in a cool place and who had contact with others [3], [48]. The first heat wave of the summer is the most dangerous: in 43 American cities it raised mortality by about 5 percent, later heat waves by just under 3 percent [6]. The body had not yet adapted, see section 6.

3. Cold drinks: do they cool you, or warm you?

3.1 What is claimed

In the summer of 2026, Swiss television reported that cooling down too much switches on "the body's own heating". Anyone who drinks or showers ice-cold ends up sweating even more. Hand-warm drinks were recommended. An older version of the same idea says that the body has to spend energy to warm a cold drink up to 37 °C. Both statements sound plausible. We looked for studies that support them. We found the opposite.

3.2 How much heat a cold drink absorbs

Let us begin with the arithmetic. Half a litre of water from the fridge is at 4 °C. By the time it reaches body temperature it has absorbed 69 kilojoules of heat from the body. That heat is gone. It equals the heat the body produces at rest in about 13 minutes, or the cooling from 28 grams of evaporated sweat. This is calculated with the physical constants from a review by the University of Sydney [7]. So the body does not have to "generate this heat in addition". It gives it to the drink. That is precisely the cooling.

The arithmetic also shows the limit: 69 kilojoules is not much. Spread over a body of 70 kilograms, it lowers mean body temperature by about 0.3 °C. A drink cools less than it feels.

3.3 What the stomach does to sweating

Now the part from which the claim arose. There are temperature sensors in the stomach and abdomen. When cold water reaches them, the body dampens sweating within a minute. Warm water at 50 °C increases it. A research group in Ottawa measured this in twelve men [10]. They also had the water swilled in the mouth only, or delivered straight into the stomach through a tube. Only the stomach reacted, the mouth did not. The effect lasts 10 to 15 minutes.

Less sweat means less cooling, but only if that sweat would have evaporated. The same group had nine men cycle for 75 minutes at 24 °C with a fan and gave them water at 1.5, 10, 37 or 50 °C [9]. Under these conditions every drop of sweat evaporates. With the cold water, the damped sweating cancelled the cooling from the drink. With the 50 °C water the men sweated 575 instead of 465 grams, and the heat balance was the most favourable. This is where the headline "hot tea cools you" came from. In a second experiment nine men cycled at 34 °C in dry air with a fan and were given ice slurry [11]. The slurry absorbed 200 kilojoules but reduced evaporation by 381 kilojoules. On balance the body lost 131 kilojoules less heat than with water at 37 °C.

Two things stand out in these studies. First: even there, core temperature never rose with cold water or ice slurry. The researchers calculated a less favourable heat balance. Measured in the rectum, they found no difference [9], [11]. Second: these were cyclists, half-naked, with wind in their faces, in dry air, exerting themselves. A hot day at home does not look like that. The research group itself writes: as soon as sweat drips instead of evaporating, cold drinks cool on balance [7]. That applies in humid air, in still air, under clothing and at high sweat rates. An experiment from Singapore confirmed it: at 35 °C and 80 percent humidity, core temperature at the end of exercise was 0.5 °C lower with ice slurry than with warm water; in dry air it was 0.3 °C lower [12].

3.4 What was measured at rest

For hot days, what matters is what happens at rest. There are studies on that, all with small groups.

• Seven men sat at 35 °C and ate about 150 grams of crushed ice over 30 minutes [13]. Their core temperature fell by 0.55 °C and kept falling for 20 minutes after eating. With water at 4 °C it fell by 0.11 °C.
• Eight men sat at 30 °C and 80 percent humidity [14]. Ice slurry lowered core temperature and forehead temperature. In this study, water at 4 °C did not differ from water at 37 °C.
• Eleven adults drank about 1.8 litres of water at 4 °C within one hour at rest [15]. The temperature in the gastrointestinal tract fell by 0.76 °C.
• Twelve adults spent three hours at 40 °C and 40 percent humidity doing light activity, a simulated heat wave [16]. With water at 4 °C, core temperature rose by 0.76 °C; with water at 20 °C by 0.72 °C. Without water it rose by 1.05 °C. The temperature of the drink made no difference. Whether they drank made a difference.
• Ten young men aged 22 and ten men aged 60 alternated between light work and sitting in 40 °C dry heat [17]. With ice slurry, the bodies of the young men stored 69 instead of 216 kilojoules of heat, those of the older men 90 instead of 254 kilojoules. Evaporation from the skin did not change, measured in a chamber that records the entire heat exchange. This is the only study with older participants.
• In twelve adults who were heated at rest, water at 1.5 °C did not measurably change sweating that was already under way [18].

A summary of 47 studies with 486 participants reaches the same conclusion: cold drinks lower core temperature by a small amount and sweating by a small amount [19]. We found no study in which a cold drink at rest raised body temperature. The only piece of work pointing in that direction is a two-page note from 1967 with an unknown number of participants. No later study has confirmed it.

3.5 Does the body have to "heat up" the drink?

That leaves the question of the "body's own heating". Does the body produce extra heat after a cold drink? This has been measured. A Berlin group found in 2003 that energy expenditure rose by 30 percent after half a litre of water at 22 °C [20]. That figure still appears in many guides. Three later studies could not reproduce it. After half a litre at 3 °C, expenditure rose by 4.5 percent for one hour [21], in another study by 2.9 percent [22], and in a third with water at 22 °C by 2.7 percent compared with 1.5 percent after a sham drink, with no reliable difference [23]. A 4.5 percent rise for one hour is about 12 to 15 kilojoules. The drink had absorbed 69 kilojoules. So the "heating" replaces at most a fifth of the cooling. The rest comes from the heat the body wants to get rid of anyway. Part of it is covered by a brief narrowing of the skin vessels [22]. That was measured at 22 °C room temperature at rest. In the heat, the skin vessels are wide open anyway.

3.6 And hot tea?

The advice to drink hot in the heat rests on the cycling study from section 3.3 [9]. In that study and its follow-up, about 20 young men in total were tested with drinks above body temperature, all exercising with a fan [9], [10]. In a British study of ten people in dry heat, a 49 °C drink did not change thermoregulation, a 5 °C drink did [24]. For people at rest on a hot day there is no study on hot drinks. The head of the Sydney group, from whose laboratory the cycling study comes, does not himself recommend hot drinks on hot days.

3.7 What follows from this

In the heat, drink as cold or as warm as you like. A cold drink cools a little, but it never warms. It improves how you feel, and in a study of racing drivers in race suits the participants drank considerably more of the cold water [25]. In the simulated heat wave, the participants drank less of the cold water [16]. The amount decides, not the temperature. If cold water makes it easier for you to drink, drink cold. If it gives you stomach ache, drink lukewarm. If you really want to cool down in great heat, wet your skin, see section 5. A quarter litre of water that evaporates on the skin removes 607 kilojoules. Drunk, it removes 39 kilojoules [8].

4. Drinking: how much, and what

4.1 How much

The European Food Safety Authority gives 2.0 litres of water a day for women and 2.5 litres for men at moderate temperatures [26]. About a fifth of that is in food, mainly fruit and vegetables. That leaves 1.6 to 2 litres as drinks. The Swiss Federal Office of Public Health gives at least 1.5 litres of drinks a day for hot days [38]. In the heat, the need rises with sweating. Anyone who sweats a lot needs more.

The sense of thirst weakens with age. Seven healthy men between 67 and 75 and seven young men were given nothing to drink for 24 hours [27]. Afterwards the blood of the older men was more concentrated. Yet they were less thirsty and drank less. In British care homes, a fifth of the 188 residents examined were dehydrated, and thirst had nothing to do with the lack of water [28]. Anyone over 65 therefore does better to drink by the clock than by thirst: one glass an hour during the day, the glass visible on the table [50]. Urine colour helps in younger people: pale like straw is good, dark like apple juice means too little drunk [30]. In older people this sign is unreliable [29].

4.2 What

Water, tea, diluted juice and coffee all count. The fear that coffee dehydrates has been refuted. Fifty men drank two decilitres of coffee four times a day for three days, and the same amount of water on other days [31]. Urine volume was the same: 2.4 litres a day in both cases. In a comparison of 13 drinks, the body retained milk and oral rehydration solution better than water [32]. Coffee, tea, cola and beer at 4 percent alcohol acted like water.

Alcohol dehydrates less than many think, but it remains unwise in the heat. In 20 men between 65 and 75, wine and spirits raised urine volume slightly over four hours, beer not at all; over 24 hours there was no difference [33]. Whether alcohol disturbs thermoregulation has only been studied in 93 young men, and there it did not [34]. For older people there are no data. Alcohol makes you drowsy, worsens sleep, raises the risk of falling and goes badly with many medicines. So: little, and a glass of water with every glass.

4.3 Too much water exists too

Anyone who drinks large amounts of plain water over hours dilutes the salt in the blood. Too little sodium in the blood (hyponatraemia) causes nausea, headache and confusion and can become life-threatening. In Sweden, hospital admissions for low sodium rose from a daily mean temperature of about 15 °C [35]. At 25 °C they were seven times as frequent as on cool days. In people over 80 there were 35 admissions per million people and day on such days, 15 times as many as on cool days. In the emergency department of Geneva University Hospital, 13 in 1,000 patients had a severely lowered sodium level in summer, 5 in 1,000 in winter [36]. Those taking a thiazide diuretic, a common blood-pressure drug, were affected particularly often.

The rule is therefore: drink regularly, but not litres in reserve. During long exertion, drink to thirst, as the expert conference on this disorder in athletes recommends [37]. Replace the salt from sweat through food: a soup, bread with cheese, salted vegetables. Salt tablets and electrolyte drinks are for people who sweat heavily for hours. Anyone with heart failure or kidney disease who has been prescribed a fluid limit discusses the amount for hot days with their doctor [38].

5. Cooling the body: what works

The following figures come from climate chambers in which older people spent three to eight hours in the heat. What was usually studied was the strain on the heart, measured as the product of heart rate and blood pressure, and core temperature. None of these studies has deaths as an outcome. What works over days we know from the investigations after the heat waves of Chicago 1995 and Europe 2003 [3], [48]. A review in The Lancet names wet skin and fans as the measures that achieve the most with little electricity, and air conditioning as necessary for the most vulnerable people [51].

5.1 Wet skin: the most effective measure

Water on the skin cools because it evaporates, just like sweat. Fifty-eight people around 70, 27 of them with coronary artery disease, each spent three hours at 38 °C and 60 percent humidity and at 45 °C dry heat [39]. Those who regularly wetted their skin with water had a lower cardiac strain in both climates. In nine people around 68, a T-shirt soaked with half a litre of water at 42 °C lowered the rise in core temperature, the heart rate and sweat loss [40]. In 23 people between 66 and 84 in 47 °C dry heat, core temperature rose 0.24 °C less with water spraying than without [41]. Wet skin is the only measure that helped older people in every heat tested. It costs nothing: a wet cloth on the neck and forearms, a spray bottle, a damp T-shirt, a lukewarm shower without towelling off.

5.2 Fans: up to what temperature

A fan cools as long as the air is cooler than the skin and as long as there is sweat for it to evaporate. If the air is hotter than the skin, it blows like a hair dryer. Where the limit lies is disputed. A modelling study from Sydney gives 39 °C for healthy young adults, 38 °C for healthy people over 65 and 37 °C for older people on medicines that dampen sweating [42]. A research group from Ottawa considers 35 °C the safe limit, because above that a fan barely lowers core temperature any more [45]. The World Health Organization gives 40 °C [50].

Above these limits the fan does harm. Nine people around 68 sat at 42 °C with and without a fan [43]. With the fan, heart rate and core temperature were higher. At 45 °C dry heat, the fan arm of the study with the 58 heart patients had to be stopped for safety reasons [39]. At 47 °C, core temperature rose 0.6 °C more with a fan than without, and heart rate by 15 beats [41]. Below the limits a fan helps little, but it does no harm: 18 people around 72 spent eight hours at 36 °C and 45 percent humidity [44]. Core temperature reached 38.3 °C with and without a fan. Heart rate was 4 beats lower with the fan, and the participants felt more comfortable. A fan on wet skin is better in humid heat than either alone [39]. In dry heat the fan cancels the benefit of the wet T-shirt [40]. And a fan when dehydrated raises heart rate instead of lowering it [46]. For Swiss homes this means: at 30 to 35 °C room temperature a fan makes sense, best on wet skin and with a glass of water beside you. At room temperatures above 37 °C, switch it off.

5.3 Foot bath, shower, neck cloth

A cool foot bath feels good. Over a long day, though, it does not measurably lower core temperature. Seventeen people around 72 spent six hours at 38 °C [47]. Those who put their feet in 20 °C water for 40 minutes every hour ended with the same core temperature as without the foot bath, a heart rate 4 beats lower and less sweat loss. A wet cloth around the neck in addition changed nothing. As a supplement, the foot bath is fine. As the only measure it is not enough. There is no study of cool showers in older people. After the 2003 heat wave, people who took extra showers or baths had a lower risk of dying, without the figure being statistically reliable [3]. Physically, a lukewarm shower is the same as wet skin. Those who do not towel off afterwards cool for longer.

5.4 Keeping the home cool, finding a cool place

The strongest protection in all investigations after heat waves was a cool place: a cooled home, or a few hours a day in a cooled building [3], [48]. That also applies to people without air conditioning. A shopping centre, a church, a library, a cellar or a forest in the morning are cool places. The World Health Organization recommends two to three hours a day in a cool place if the home stays hot [50].

At home the rule is: windows closed by day and shaded from outside, with shutters or blinds. Ventilate at night and in the early morning, as soon as it is cooler outside than inside [38], [50]. Switch off appliances that produce heat. Sleep in the coolest room, even if that is the living room. Heat at night steals sleep: wristband data from 47,000 people in 68 countries show that nights above 30 °C shorten sleep by an average of 14 minutes, mainly through falling asleep later and more so in older people [49]. A thermometer in the bedroom shows whether the ventilation is working.

6. Exercise and sport in the heat

Exercise remains right in the heat, with three adjustments. First, the time of day: the Federal Office of Public Health advises avoiding strenuous activity between 11 a.m. and 5 p.m. [38]. Early morning and late evening are the times for walks, training and gardening. Second, intensity: at the same effort the body produces the same heat but sheds it less well. Go slower, take longer breaks in the shade, and shorten the session. The American College of Sports Medicine advises non-acclimatised people against sport at a wet-bulb globe temperature above 28 °C; this measure combines air temperature, humidity and sun [52]. Third, drinking: a large glass before exercise, half a litre during an hour of exercise, and to thirst afterwards.

The body adapts to heat. After one to two weeks of daily heat exposure it starts sweating earlier, blood volume rises, and heart rate and core temperature stay lower at the same load [52], [53]. The largest adaptation comes after more than 14 days. Older people adapt too. Before the summer, people between 55 and 72 stored 222 kilojoules more heat in a three-hour heat test than young people; after the summer this difference had disappeared [54]. It is therefore worth walking half an hour in the warmth every day in early summer rather than spending the first hot days entirely indoors. The first heat wave hits the unadapted body hardest [6].

On clothing: loose and airy counts more than colour. Black Bedouin robes absorbed more solar heat at the surface in the desert than white ones, but passed on the same amount to the body, because the loose clothing created a draught [55]. A tight black shirt in the sun is still warmer than a loose light one. A hat protects the head. Sunscreen does not hinder cooling: in nine young adults, two sunscreens changed neither sweating nor the temperature limit at which core temperature began to rise [56]. Sunburn, on the other hand, disturbs sweating.

7. Medicines and conditions that tolerate heat less well

Many medicines interfere with thermoregulation [57]. Diuretics and blood-pressure drugs from the ACE-inhibitor and sartan groups increase water loss. Bladder medicines, older antidepressants, some antihistamines and antipsychotics dampen sweating. Beta-blockers reduce blood flow to the skin. Painkillers from the NSAID group, such as ibuprofen, strain the kidneys together with diuretics. In 353 mostly young participants in 35 studies, core temperature in the heat was 0.42 °C higher with strongly anticholinergic drugs than without [59]. Among American insurees over 65 with chronic conditions, heat waves raised heat-related hospital admissions by 21 to 33 percent; those taking diuretics, antipsychotics or anticholinergic drugs had a higher risk all summer long [58]. Do not stop any medicine on your own. Before the summer, ask at your doctor's practice or pharmacy whether a dose should be adjusted for hot days [38].

Some conditions react to heat in particular. In multiple sclerosis, symptoms worsen temporarily in 60 to 80 percent of those affected as soon as core temperature rises by 0.2 to 0.5 °C (Uhthoff's phenomenon); with cooling this reverses [60]. Of 247 people with Parkinson's disease, 79 percent said they had become more heat-sensitive since diagnosis [61]. In cardiovascular disease, mortality rises by about 2 percent per degree of heat and by about 12 percent in heat waves [62]. People with diabetes sweat less when nerves are damaged, people with kidney disease tolerate water loss less well, and in dementia the sense of thirst and the knowledge of what to do are often missing [4]. Anyone who has one of these conditions, or cares for someone who does, plans hot days in advance.

8. Warning signs: when you must act

In older people, heat rarely shows itself as heat stroke and more often as dehydration, kidney weakness, confusion or a urinary tract infection. In American hospital data on about 24 million people over 65, admissions on hot days rose two-and-a-half-fold for heat stroke, by 18 percent for fluid and salt disorders and by 14 percent for kidney failure [65]. Watch for these signs in yourself and in others [38], [63]:

• weakness, tiredness, dizziness, headache
• muscle cramps, dry mouth, nausea, vomiting
• confusion, drowsiness, unusual behaviour
• fast pulse, hot skin

With weakness, dizziness, cramps or nausea (heat exhaustion): go to a cool place, lie down, wet your skin, drink in small sips. If this does not improve within 30 minutes, call your doctor's practice or 144 [38].

With confusion, drowsiness, unconsciousness or a seizure (heat stroke): call 144 at once. Heat stroke is an emergency in which core temperature is above 40 °C and the brain fails [63]. How long the temperature stays that high decides about lasting damage. So cool before the ambulance arrives: open the clothing, pour water over the whole body and fan air over it, ice packs on the neck, armpits and groin. Where there is a pool, a fountain or a bathtub, immersion up to the neck is the fastest [64]. Fever medicines do not help. Do not give an unconscious person anything to drink.

9. What you should know

• Heat claims hundreds of lives in Switzerland every year, more than half of them on only moderately hot days [1]. Most at risk are people over 75 who live alone, are bedridden or have cardiovascular disease [3].
• Cold drinks cool a little and never warm. Half a litre at 4 °C absorbs 69 kilojoules, as much as 13 minutes of resting heat [7]. The stomach dampens sweating for a few minutes [10]. That cancelled the cooling only in cyclists in dry wind, and even there core temperature did not rise [9], [11]. At rest it fell in every study or stayed the same [13], [16], [17].
• The body hardly "heats up" a cold drink. Expenditure rises by 3 to 5 percent for an hour, which covers at most a fifth of the heat absorbed [21], [22].
• Hot drinks cool only in the cycling laboratory with a fan and dry air [9]. For people at rest there is no study.
• Drink 1.5 to 2 litres a day, more in the heat, from 65 on by the clock rather than by thirst [27], [38]. Coffee and tea count [31]. Litres of plain water in reserve do harm [35].
• Wet skin is the most effective cooling and helped older people in every heat tested [39], [41].
• Fan up to about 37 °C room temperature, best on wet skin [42]. Above that it does harm [43], [41].
• A cool place for a few hours a day is the strongest protection [3]. Shade by day, ventilate at night [38].
• Exercise early in the morning or in the evening, slower and shorter [38], [52]. The body adapts in one to two weeks, also in older age [54].
• Discuss medicines before the summer, never stop them on your own [57].
• Confusion or unconsciousness in the heat: call 144 at once and cool at once [63], [64].

References

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Transparency

• Author: Roger Hilfiker
• AI assistance: the literature search and the draft text were produced with Claude (Anthropic). Roger Hilfiker checked all statements, figures and sources and revised the text.
• Created: 26 September 2026
• Last updated: 26 September 2026
• Sources: the 65 works in the reference list. All DOIs were checked against the Crossref register.
• How the sources were found: searches of the Europe PMC and Crossref databases and of publishers' pages in September 2026 on drink temperature and heat balance, fans, skin wetting, foot baths, fluid intake, low sodium, medicines and heat mortality, supplemented by the federal reports on heat-related deaths. Where this article says that no study exists on a question, that refers to this search.
• Occasion: a report by SRF Wissen of 4 September 2026 in which an expert from Bern University of Applied Sciences warned against cold drinks in the heat. This article tests that claim against the studies.
• Conflicts of interest: our practice earns nothing from this topic. We sell neither drinks nor fans nor cooling products and offer no treatment of heat illness.
• Funding: Physiotherapie Tschopp & Hilfiker, 3902 Glis. The article was produced from the practice's own funds.
• Next review: 1 May 2027, before the next summer.