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Exercising in the Heat · prepare, protect, cool
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In one pass When you exercise, your muscles work like an engine: about four-fifths of the energy never becomes movement and turns into heat instead. Not this — Sweating from exercise equals detox — Sweat is cooling, not detox. Sweating more mostly means you are hot and losing water and salt — not getting cleaner.
Educational content, not medical advice — consult a clinician.
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Chapter 1
How the body dumps heat
When you exercise, your muscles work like an engine: about four-fifths of the energy never becomes movement and turns into heat instead. If that heat is not shed, core temperature keeps climbing.
The main way out is sweat evaporating. Blood flowing through the skin carries heat to the surface, and sweat evaporating from the skin takes it away. When humidity is high, sweat cannot evaporate, and that main route is blocked. That is why a humid 30°C is far more dangerous than a dry 30°C.
The second cost is blood flow. The skin needs blood to shed heat, and the working muscles need blood too. The heart has to supply both, so it pushes heart rate up. At the same pace, your heart rate runs higher in the heat and the effort feels harder.
If someone becomes confused, behaves strangely, or collapses during or after exercise in the heat, treat it as an emergency: start cooling them on the spot right away, and call emergency services at the same time. Headache, vomiting, or confusion after a long endurance event can also mean that drinking too much water has diluted blood sodium; get medical care immediately in that case too.
The main way out is sweat evaporating. Blood flowing through the skin carries heat to the surface, and sweat evaporating from the skin takes it away. When humidity is high, sweat cannot evaporate, and that main route is blocked. That is why a humid 30°C is far more dangerous than a dry 30°C.
The second cost is blood flow. The skin needs blood to shed heat, and the working muscles need blood too. The heart has to supply both, so it pushes heart rate up. At the same pace, your heart rate runs higher in the heat and the effort feels harder.
If someone becomes confused, behaves strangely, or collapses during or after exercise in the heat, treat it as an emergency: start cooling them on the spot right away, and call emergency services at the same time. Headache, vomiting, or confusion after a long endurance event can also mean that drinking too much water has diluted blood sodium; get medical care immediately in that case too.
Mechanism · What evaporation and blood flow each do
The two routes work together, but they do not carry equal weight. Skin blood flow moves heat from deep in the body to the surface, which only relocates it. What actually takes heat out of the body is sweat evaporating on the skin. Evaporating 1 liter of sweat removes about 580 kcal. Once the air is close to saturated, sweat just sits on the skin and that bill cannot be paid. A humid 30°C is more dangerous than a dry 30°C not because the air is hotter, but because the main route, evaporation, is blocked.Blood flow is the second cost. The amount of blood the heart can pump each minute — cardiac output — has a ceiling. The skin wants more blood to carry heat to the surface, and the working muscles want blood too. When both claim a share, the heart copes by driving heart rate up (González-Alonso 2008). The pace has not changed, yet heart rate is already drifting upward: that is where the extra strain comes from. Sawka's 2011 review treats heat production, skin blood flow, and evaporation as one physiology. The three pull on one another; they are not three separate jobs.
Chapter 2
From cramps to heatstroke
Heat illness is not an on-off switch. It is a spectrum from mild to severe: heat cramps (muscle cramps after heavy sweating), heat exhaustion, and heatstroke.
Heat exhaustion means the body can no longer keep up: dizziness, nausea, heavy sweating, and weakness. The person is usually still clear-headed, and core temperature is usually still below 40°C. Stopping, cooling down, and taking fluids brings most people around.
Heatstroke is the most dangerous stage. The sports-medicine definition needs two things at once: core temperature above 40°C, plus a change in mental state (confusion, talking nonsense, seizures, collapse). Mouth, ear, and armpit readings are all unreliable during heavy sweating and can miss it, so do not use a home thermometer to rule it in or out. Once mental state changes, treat it as heatstroke: cool first, and call emergency services at the same time.
Heat exhaustion means the body can no longer keep up: dizziness, nausea, heavy sweating, and weakness. The person is usually still clear-headed, and core temperature is usually still below 40°C. Stopping, cooling down, and taking fluids brings most people around.
Heatstroke is the most dangerous stage. The sports-medicine definition needs two things at once: core temperature above 40°C, plus a change in mental state (confusion, talking nonsense, seizures, collapse). Mouth, ear, and armpit readings are all unreliable during heavy sweating and can miss it, so do not use a home thermometer to rule it in or out. Once mental state changes, treat it as heatstroke: cool first, and call emergency services at the same time.
Clinical · How the three stages differ
Two things separate the three stages: whether the person is clear-headed, and how high core temperature is. The definition of exertional heatstroke is the same in the position statements of the American College of Sports Medicine (ACSM, 2007) and the National Athletic Trainers' Association (NATA, 2015), and in a 2019 NEJM review: rectal core temperature above 40°C (104°F) together with a failure of central nervous system function (confusion, talking nonsense, abnormal behavior, seizures, collapse). Both parts have to be present; it is not one number alone.Core temperature has to be measured rectally. Casa 2007 compared several devices during outdoor exercise in the heat: mouth, ear, armpit, and forehead readings were all unreliable during hard exercise with heavy sweating, and would miss a heatstroke. Do not use these home readings to diagnose yourself or anyone else. If someone is already confused, behaving strangely, and alarmingly hot, treat it as an emergency. The rule for handling it is cool first, transport second.
Chapter 3
Prepare by getting used to heat
If you do only one thing to protect yourself in the heat, acclimatize: exercise in hot conditions, building up gradually, for about 7-14 days, so the body remodels itself in advance. An international sports-medicine consensus rates it the most effective protection.
The body changes three things. Plasma volume expands. Sweating starts earlier, runs heavier, and gets more dilute. At the same effort, core temperature and heart rate both run lower.
The flip side: training hard in serious heat before you have adapted is one of the most common backdrops to heatstroke. The first hard session of summer, arriving somewhere hot, and a sudden heat wave are all high-risk moments.
The body changes three things. Plasma volume expands. Sweating starts earlier, runs heavier, and gets more dilute. At the same effort, core temperature and heart rate both run lower.
The flip side: training hard in serious heat before you have adapted is one of the most common backdrops to heatstroke. The first hard session of summer, arriving somewhere hot, and a sudden heat wave are all high-risk moments.
Mechanism · What changes over one to two weeks
Acclimatization turns coping with heat from gritting your teeth into a body that has genuinely changed. Périard's 2015 review lays out the changes that stack up over those 7-14 days, and each one lands on one of the two cooling routes:Plasma volume expands: there is more blood to go around, the skin and muscles compete less for it, and the heart works less hard.Sweating starts earlier, runs heavier, and gets more dilute: sweat carries less sodium, which helps you hold on to salt.At the same effort, core temperature and heart rate both run lower.
The Racinais 2015 consensus calls acclimatization the most effective protection. That is not because willpower got stronger; it is because the evaporation and blood-flow routes were remodeled in advance. The reverse also holds: training hard in serious heat before those 1-2 weeks are done is one of the most common backdrops to heatstroke.
Chapter 4
Fluids, weather, and who is at risk
Beyond acclimatization, fluids and weather can lower the risk further.
With fluids, guard against both extremes: do not run dry, and do not overdrink. Losing more than 2% of body weight to dehydration hurts performance and adds to heat strain. Drinking more than you sweat is just as dangerous: blood sodium gets diluted, and in severe cases brain cells take on water and swell, which can be fatal. This is called exercise-associated hyponatremia. The safest rule is to drink when you are thirsty, not on a schedule; extra salt cannot offset drinking too much. If headache, vomiting, or confusion appears during or after a long endurance event, get medical care immediately.
For weather, check the wet-bulb globe temperature (WBGT, one number that combines air temperature, humidity, and sun), not just the air temperature. Cooling down before or during exercise (iced drinks, ice slurry, cooling vests) reduces heat strain, and a found it also improves performance in the heat.
With fluids, guard against both extremes: do not run dry, and do not overdrink. Losing more than 2% of body weight to dehydration hurts performance and adds to heat strain. Drinking more than you sweat is just as dangerous: blood sodium gets diluted, and in severe cases brain cells take on water and swell, which can be fatal. This is called exercise-associated hyponatremia. The safest rule is to drink when you are thirsty, not on a schedule; extra salt cannot offset drinking too much. If headache, vomiting, or confusion appears during or after a long endurance event, get medical care immediately.
For weather, check the wet-bulb globe temperature (WBGT, one number that combines air temperature, humidity, and sun), not just the air temperature. Cooling down before or during exercise (iced drinks, ice slurry, cooling vests) reduces heat strain, and a found it also improves performance in the heat.
Safety · Who is at higher risk
The more risk factors stack up, the greater the danger: high intensity or long duration, a high wet-bulb globe temperature (WBGT), dehydration, no acclimatization, low fitness, obesity, a recent illness or fever, lack of sleep, and certain medications (ACSM 2007; NATA 2015).Older adults really are more vulnerable. With age, sweating and skin blood flow respond more sluggishly, and even the sense of thirst dulls, so heat waves hit them harder (Kenney 2003).
Children call for correcting a widespread old claim. It used to be taught that children are innately worse at regulating body temperature, but later research overturned that: as long as they drink enough, children shed heat in hot conditions no worse than adults do (AAP 2011; Falk 2008). A child's real risk comes from things that can be changed: pushing on and refusing to stop, protective gear that traps heat, and too little rest or fluid. So for children the key is good planning (fluids, rotating rest breaks, avoiding the hottest hours), not the belief that they simply cannot take the heat.
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Chapter 5
Emergency: cool first, then hospital
If it does go wrong, remember one line: cool first, transport second.
Heatstroke is an emergency where minutes count. What decides survival is often not how fast the person reaches a hospital but how fast their core temperature comes down. The accepted best method — the gold standard — is cold-water immersion: putting the person in cold water to cool them fast. If you can, immerse them on site and keep cooling while you wait for help; do not rush to move the person first.
Whatever the stage, calling emergency services is never optional.
Heatstroke is an emergency where minutes count. What decides survival is often not how fast the person reaches a hospital but how fast their core temperature comes down. The accepted best method — the gold standard — is cold-water immersion: putting the person in cold water to cool them fast. If you can, immerse them on site and keep cooling while you wait for help; do not rush to move the person first.
Whatever the stage, calling emergency services is never optional.
Clinical · How fast cold water works; heat exhaustion
The case for cold-water immersion comes from field records. Over many years, the finish-line medical tent at the Falmouth Road Race in the US logged 274 cases of heatstroke, all treated with cold-water immersion: the average cooling rate was about 0.22°C per minute, and survival was 100% (DeMartini 2015). A consensus on prehospital care (Belval 2018) says that when cooling is delayed beyond 30 minutes after collapse, organ damage and death rise sharply — and 30 minutes is shorter than a typical ambulance transport plus emergency-department assessment. So if you can, immerse the person on site and keep cooling while help arrives.Heat exhaustion (not yet heatstroke) is handled far more gently: stop at once, move to shade, take off extra clothing and gear, cool down, raise the legs, give fluids if the person can drink, and watch closely. The moment mental state changes, treat it as heatstroke immediately (NATA 2015). This is what the emergency consensus says; it does not replace the on-scene judgment of emergency responders.
Chapter 6
Common beliefs that don't hold up
A few old claims about exercising in the heat do not hold up.
Sweating more is not healthier, and it is not detox. Sweating is just the body shedding heat; a lot of sweat mostly means you are hot and losing water and salt.
Salt tablets are not supported by evidence for most exercisers and can even backfire. Food plus electrolyte drinks usually cover sodium. Only people who sweat heavily, have especially salty sweat, and train at high intensity need a personal sodium plan.
Pushing through when the heat feels miserable is exactly how heatstroke happens. Heat illness is overwhelmingly preventable.
Sweating more is not healthier, and it is not detox. Sweating is just the body shedding heat; a lot of sweat mostly means you are hot and losing water and salt.
Salt tablets are not supported by evidence for most exercisers and can even backfire. Food plus electrolyte drinks usually cover sodium. Only people who sweat heavily, have especially salty sweat, and train at high intensity need a personal sodium plan.
Pushing through when the heat feels miserable is exactly how heatstroke happens. Heat illness is overwhelmingly preventable.
In practice · How to train safely in the heat
Training in the heat comes down to five habits: acclimatize first, check the wet-bulb globe temperature (WBGT), drink when thirsty, pre-cool when needed, and pay attention to your body's signals. Acclimatization plus sensible planning plus prompt cooling brings the risk right down (Racinais 2015). The point about salt tablets comes from the fluid-replacement position statements (NATA 2017; ACSM 2007): most people do not need them, and those who do should plan around their own sweating.But the moment someone is confused, behaving strangely, or alarmingly hot, do not hesitate: cool them on the spot and call emergency services immediately. This story is education to help you understand why, not medical advice. For the other side — heat used as a tool (sauna, heat conditioning) — see Sauna & heat exposure; for how to replace fluids and electrolytes, see Water & Electrolytes.
References · 16
- Sawka, M. N., Leon, L. R., Montain, S. J., & Sonna, L. A. (2011). Integrated physiological mechanisms of exercise performance, adaptation, and maladaptation to heat stress. Comprehensive Physiology, 1(4), 1883-1928. 10.1002/cphy.c100082
- González-Alonso, J., Crandall, C. G., & Johnson, J. M. (2008). The cardiovascular challenge of exercising in the heat. The Journal of Physiology, 586(1), 45-53. 10.1113/jphysiol.2007.142158
- Armstrong, L. E., Casa, D. J., Millard-Stafford, M., Moran, D. S., Pyne, S. W., & Roberts, W. O. (2007). American College of Sports Medicine position stand: Exertional heat illness during training and competition. Medicine & Science in Sports & Exercise, 39(3), 556-572. 10.1249/MSS.0b013e31802fa199
- Casa, D. J., DeMartini, J. K., Bergeron, M. F., Csillan, D., Eichner, E. R., Lopez, R. M., Ferrara, M. S., Miller, K. C., O'Connor, F., Sawka, M. N., & Yeargin, S. W. (2015). National Athletic Trainers' Association position statement: Exertional heat illnesses. Journal of Athletic Training, 50(9), 986-1000. 10.4085/1062-6050-50.9.07
- Epstein, Y., & Yanovich, R. (2019). Heatstroke. New England Journal of Medicine, 380(25), 2449-2459. 10.1056/NEJMra1810762
- Casa, D. J., Becker, S. M., Ganio, M. S., Brown, C. M., Yeargin, S. W., Roti, M. W., Siegler, J., Blowers, J. A., Glaviano, N. R., Huggins, R. A., Armstrong, L. E., & Maresh, C. M. (2007). Validity of devices that assess body temperature during outdoor exercise in the heat. Journal of Athletic Training, 42(3), 333-342. www.ncbi.nlm.nih.gov/pmc/articles/PMC1978469
- Racinais, S., Alonso, J. M., Coutts, A. J., et al. (2015). Consensus recommendations on training and competing in the heat. British Journal of Sports Medicine, 49(18), 1164-1173. 10.1136/bjsports-2015-094915
- Périard, J. D., Racinais, S., & Sawka, M. N. (2015). Adaptations and mechanisms of human heat acclimation: applications for competitive athletes and sports. Scandinavian Journal of Medicine & Science in Sports, 25(S1), 20-38. 10.1111/sms.12408
- Sawka, M. N., Burke, L. M., Eichner, E. R., Maughan, R. J., Montain, S. J., & Stachenfeld, N. S. (2007). American College of Sports Medicine position stand: Exercise and fluid replacement. Medicine & Science in Sports & Exercise, 39(2), 377-390. Abstract content: drink to prevent excessive dehydration (more than 2% body-weight loss from water deficit) and excessive electrolyte change; sweat rates and sweat electrolyte content vary considerably between individuals, so programmes should be customised, e.g. by weighing before and after exercise. The abstract gives no sweat sodium or potassium figures (abstract, PMID 17277604). 10.1249/mss.0b013e31802ca597
- Hew-Butler, T., Rosner, M. H., Fowkes-Godek, S., Dugas, J. P., Hoffman, M. D., Lewis, D. P., Maughan, R. J., Miller, K. C., Montain, S. J., Rehrer, N. J., Roberts, W. O., Rogers, I. R., Siegel, A. J., Stuempfle, K. J., Winger, J. M., & Verbalis, J. G. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine, 25(4), 303–320. Full text read 2026-09-24 (simultaneous BJSM publication, 49(22), 1432, via a Wayback snapshot of 10 October 2024; PubMed has no abstract, PMID 26102445): the single most important risk factor is sustained excessive intake of water, sports drinks or other hypotonic fluids beyond sweat, respiratory and renal losses; all sports drinks are hypotonic (sodium about 10-38 mmol/L), so overdrinking them is not protective. Using thirst to guide drinking should limit overdrinking while preventing excessive dehydration (Grade 1C); earlier advice to drink before thirst was meant for high sweat rates; fluid deficits up to about 3% of body mass are tolerated in cool to temperate conditions. Sodium supplements cannot prevent EAH when fluid intake is excessive (Grade 1C). Treatment: symptom severity, not the sodium value, guides therapy (Grade 1A); restrict hypotonic and isotonic fluids until urinating; severe EAH with encephalopathy gets an immediate 100 mL bolus of 3% NaCl, repeated up to twice (about 10-min intervals) if there is no improvement, without waiting for a lab value (Grade 1B); IV hypotonic fluids, lactated Ringer's or normal saline are contraindicated in confirmed dilutional EAH. Travel and meals for the panel were funded by CrossFit, Inc. 10.1097/JSM.0000000000000221
- Hew-Butler, T., Verbalis, J. G., & Noakes, T. D. (2006). Updated fluid recommendation: position statement from the International Marathon Medical Directors Association (IMMDA). Clinical Journal of Sport Medicine, 16(4), 283-292. 10.1097/00042752-200607000-00001
- Bongers, C. C. W. G., Thijssen, D. H. J., Veltmeijer, M. T. W., Hopman, M. T. E., & Eijsvogels, T. M. H. (2015). Precooling and percooling (cooling during exercise) both improve performance in the heat: a meta-analytical review. British Journal of Sports Medicine, 49(6), 377-384. 10.1136/bjsports-2013-092928
- Belval, L. N., Casa, D. J., Adams, W. M., Chiampas, G. T., Holschen, J. C., Hosokawa, Y., Jardine, J., Kane, S. F., Labotz, M., Lemieux, R. S., McClaine, K. B., Nye, N. S., O'Connor, F. G., Prine, B., Raukar, N. P., Smith, M. S., & Stearns, R. L. (2018). Consensus statement: prehospital care of exertional heat stroke. Prehospital Emergency Care, 22(3), 392-397. 10.1080/10903127.2017.1392666
- Casa, D. J., McDermott, B. P., Lee, E. C., Yeargin, S. W., Armstrong, L. E., & Maresh, C. M. (2007). Cold water immersion: the gold standard for exertional heatstroke treatment. Exercise and Sport Sciences Reviews, 35(3), 141-149. 10.1097/jes.0b013e3180a02bec
- DeMartini, J. K., Casa, D. J., Stearns, R., Belval, L., Crago, A., Davis, R., & Jardine, J. (2015). Effectiveness of cold water immersion in the treatment of exertional heat stroke at the Falmouth Road Race. Medicine & Science in Sports & Exercise, 47(2), 240-245. 10.1249/MSS.0000000000000409
- McDermott, B. P., Anderson, S. A., Armstrong, L. E., Casa, D. J., Cheuvront, S. N., Cooper, L., Kenney, W. L., O'Connor, F. G., & Roberts, W. O. (2017). National Athletic Trainers' Association position statement: Fluid replacement for the physically active. Journal of Athletic Training, 52(9), 877-895. 10.4085/1062-6050-52.9.02