Place · Level 3
Exercising in the Heat · prepare, protect, cool
运动产热靠汗蒸发散掉, 湿度一高就失灵 · 热射病是急症 · 热适应是最强防护 · 补水别过量 · 降温先于转运
Story path
- 1How the body dumps heatHow the body dumps heat
- 2From cramps to heatstrokeFrom cramps to heatstroke
- 3Prepare · acclimatizationPrepare · acclimatization
- 4Protect · fluids, WBGT, who's at riskProtect · fluids, WBGT, who's at risk
- 5Rescue · cool first, transport secondRescue · cool first, transport second
- 6Myths + the honest closeMyths + the honest close
Chapter 1
How the body dumps heat
How the body dumps heat
During exercise, muscle works like an engine: most of the energy doesn't become movement — it becomes heat (roughly 80%). That heat has to be shed fast, or core temperature climbs.
The body sheds it two ways: skin blood flow carries heat to the surface, and sweat evaporation carries it away (evaporating 1 liter of sweat removes about 580 kcal). The key point — evaporation is the workhorse, and when humidity is high, sweat can't evaporate, so cooling collapses. A humid 30°C is far more dangerous than a dry 30°C.
There's a second problem: skin and muscle compete for blood flow. Skin needs blood at the surface to dump heat; muscle needs blood to do the work; the heart has to supply both, and it copes by driving heart rate up (González-Alonso 2008). That's why on a hot day the same pace feels much harder.
The body sheds it two ways: skin blood flow carries heat to the surface, and sweat evaporation carries it away (evaporating 1 liter of sweat removes about 580 kcal). The key point — evaporation is the workhorse, and when humidity is high, sweat can't evaporate, so cooling collapses. A humid 30°C is far more dangerous than a dry 30°C.
There's a second problem: skin and muscle compete for blood flow. Skin needs blood at the surface to dump heat; muscle needs blood to do the work; the heart has to supply both, and it copes by driving heart rate up (González-Alonso 2008). That's why on a hot day the same pace feels much harder.
Chapter 2
From cramps to heatstroke
From cramps to heatstroke
Heat trouble isn't a switch — it's a spectrum from mild to severe:
Heat cramps: muscle cramps after heavy sweating, the mildest.Heat exhaustion: you can't go on — dizziness, nausea, heavy sweating, weakness; core temperature is usually still below 40°C, and crucially there is no major change in mental status. Stopping, cooling, and fluids usually turn it around.Exertional heatstroke: the severe end, and its definition is hard — rectal core temperature above 40°C (104°F) together with central-nervous-system dysfunction (confusion, incoherence, abnormal behavior, seizures, collapse) (ACSM 2007; NATA 2015; NEJM 2019). This is a life-threatening emergency, measured in minutes.
What separates them is core temperature — and it has to be measured rectally. Oral, ear, armpit, and forehead readings are all unreliable during hard exercise with heavy sweating, and will miss a heatstroke (Casa 2007).
Heat cramps: muscle cramps after heavy sweating, the mildest.Heat exhaustion: you can't go on — dizziness, nausea, heavy sweating, weakness; core temperature is usually still below 40°C, and crucially there is no major change in mental status. Stopping, cooling, and fluids usually turn it around.Exertional heatstroke: the severe end, and its definition is hard — rectal core temperature above 40°C (104°F) together with central-nervous-system dysfunction (confusion, incoherence, abnormal behavior, seizures, collapse) (ACSM 2007; NATA 2015; NEJM 2019). This is a life-threatening emergency, measured in minutes.
What separates them is core temperature — and it has to be measured rectally. Oral, ear, armpit, and forehead readings are all unreliable during hard exercise with heavy sweating, and will miss a heatstroke (Casa 2007).
Chapter 3
Prepare · acclimatization
Prepare · acclimatization
If a hot-weather workout lets you do just one protective thing, make it heat acclimatization — widely regarded as the single most effective measure (Racinais 2015 consensus).
The method isn't mysterious: exercise in the heat progressively over 1-2 weeks (about 7-14 days), and the body reshapes itself step by step:
Plasma volume expands — more blood to go around, less strain on the heart.Sweating starts earlier, runs higher, and gets more dilute — less sodium in the sweat, so you conserve salt.At a given workload, core temperature and heart rate both drop (Périard 2015).
In other words, acclimatization turns tolerating heat from gritting through it into the body genuinely adapting. Conversely, hammering a workout in the heat without acclimatizing is one of the most common backdrops to heatstroke — the first summer session, arriving somewhere hot, a sudden heat wave: all high-risk moments.
The method isn't mysterious: exercise in the heat progressively over 1-2 weeks (about 7-14 days), and the body reshapes itself step by step:
Plasma volume expands — more blood to go around, less strain on the heart.Sweating starts earlier, runs higher, and gets more dilute — less sodium in the sweat, so you conserve salt.At a given workload, core temperature and heart rate both drop (Périard 2015).
In other words, acclimatization turns tolerating heat from gritting through it into the body genuinely adapting. Conversely, hammering a workout in the heat without acclimatizing is one of the most common backdrops to heatstroke — the first summer session, arriving somewhere hot, a sudden heat wave: all high-risk moments.
Chapter 4
Protect · fluids, WBGT, who's at risk
Protect · fluids, WBGT, who's at risk
Beyond acclimatization, a few things push risk down further:
Hydration: don't run dry, don't overdo it. Dehydration beyond 2% of body mass clearly hurts performance and adds thermal strain (ACSM 2007). But overdrinking is dangerous too — taking in more than you sweat out dilutes the blood's sodium and can cause exercise-associated hyponatremia, which can be fatal. The simplest general rule is drink to thirst, which prevents virtually all hyponatremia (Hew-Butler 2015; IMMDA).Watch WBGT, not just air temperature. Real heat risk is gauged by wet-bulb globe temperature (WBGT) — it folds in temperature, humidity, solar radiation, and wind. The same 30°C is far more dangerous when it's humid (NATA 2015).Pre-cooling helps. Cold drinks, an ice slurry, or a cooling vest before or during exercise lowers thermal strain and improves performance in the heat (a meta-analysis found about a 6.7% gain, Bongers 2015).
Hydration: don't run dry, don't overdo it. Dehydration beyond 2% of body mass clearly hurts performance and adds thermal strain (ACSM 2007). But overdrinking is dangerous too — taking in more than you sweat out dilutes the blood's sodium and can cause exercise-associated hyponatremia, which can be fatal. The simplest general rule is drink to thirst, which prevents virtually all hyponatremia (Hew-Butler 2015; IMMDA).Watch WBGT, not just air temperature. Real heat risk is gauged by wet-bulb globe temperature (WBGT) — it folds in temperature, humidity, solar radiation, and wind. The same 30°C is far more dangerous when it's humid (NATA 2015).Pre-cooling helps. Cold drinks, an ice slurry, or a cooling vest before or during exercise lowers thermal strain and improves performance in the heat (a meta-analysis found about a 6.7% gain, Bongers 2015).
Who's at risk (one myth to correct)
Risk factors stack up: high intensity or long duration, high WBGT, dehydration, no acclimatization, low fitness, obesity, recent illness or fever, sleep deprivation, and certain medications (ACSM 2007; NATA 2015).Older adults are genuinely more vulnerable: with age, sweating and skin blood flow blunt, and even the sense of thirst dulls — so heat waves hit them harder (Kenney 2003).
Children deserve a correction of a widespread old claim. It used to be taught that children are innately worse at temperature regulation, but newer evidence overturned that: as long as they stay hydrated, children shed heat in the warmth no worse than adults do (AAP 2011; Falk 2008). A child's real risk comes from modifiable factors — pushing too hard, heat-trapping gear, too little rest or fluid. So for kids the key is good planning (hydration, rotation, avoiding the hottest hours), not the belief that they simply can't take it.
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Chapter 5
Rescue · cool first, transport second
Rescue · cool first, transport second
If it does go wrong, remember one line: cool first, transport second.
Exertional heatstroke is an emergency measured in minutes, and what decides survival is often not how fast you reach a hospital, but how fast the core temperature comes down. The gold standard is cold-water immersion — putting the person in cold water to cool rapidly (Casa 2007). In the largest treated series to date (274 heatstroke cases at the Falmouth Road Race), the average cooling rate was about 0.22°C/min, with 100% survival (DeMartini 2015). The goal is to get core temperature below 40.5°C (104.5°F) within about 30 minutes of collapse (Belval 2018 consensus). So if you can, immerse on site and cool while help arrives — don't rush to move the person first.
Heat exhaustion (not yet heatstroke) is handled far more gently: stop immediately, move to shade, remove excess clothing and gear, cool, elevate the legs, give fluids if tolerated, and watch closely — the moment mental status changes, treat it as heatstroke (NATA 2015).
Whatever the tier, calling emergency services is a step you never skip.
Exertional heatstroke is an emergency measured in minutes, and what decides survival is often not how fast you reach a hospital, but how fast the core temperature comes down. The gold standard is cold-water immersion — putting the person in cold water to cool rapidly (Casa 2007). In the largest treated series to date (274 heatstroke cases at the Falmouth Road Race), the average cooling rate was about 0.22°C/min, with 100% survival (DeMartini 2015). The goal is to get core temperature below 40.5°C (104.5°F) within about 30 minutes of collapse (Belval 2018 consensus). So if you can, immerse on site and cool while help arrives — don't rush to move the person first.
Heat exhaustion (not yet heatstroke) is handled far more gently: stop immediately, move to shade, remove excess clothing and gear, cool, elevate the legs, give fluids if tolerated, and watch closely — the moment mental status changes, treat it as heatstroke (NATA 2015).
Whatever the tier, calling emergency services is a step you never skip.
Chapter 6
Myths + the honest close
Myths + the honest close
A few hot-weather workout claims worth dismantling:
More sweat = healthier / detox? Sweat is just the body cooling itself, not detoxing; sweating a lot mostly means you're hot and losing water and salt — not getting stronger.Load up on salt tablets? For most exercisers, routine salt tablets are not supported by evidence and can be counterproductive; sodium is usually covered by food plus electrolyte drinks, and only very heavy salty sweaters at high intensity need individualized sodium (NATA 2017; ACSM 2007).Grit through the misery? That is exactly what breeds heatstroke. The right way to train in heat is: acclimatize first, watch WBGT, drink to thirst, pre-cool, and heed the body's signals.
The honest close: heat illness is overwhelmingly preventable — acclimatization plus sensible planning plus prompt cooling drives the risk right down (Racinais 2015). But the moment someone is confused, behaving strangely, or alarmingly hot, don't hesitate: cool them on the spot and call emergency services immediately. This page is education to understand the why, not medical advice. For the other side — heat used as a tool (sauna, heat conditioning) — see the sauna & heat exposure story; for how to handle fluids and electrolytes, see the water & electrolytes story.
More sweat = healthier / detox? Sweat is just the body cooling itself, not detoxing; sweating a lot mostly means you're hot and losing water and salt — not getting stronger.Load up on salt tablets? For most exercisers, routine salt tablets are not supported by evidence and can be counterproductive; sodium is usually covered by food plus electrolyte drinks, and only very heavy salty sweaters at high intensity need individualized sodium (NATA 2017; ACSM 2007).Grit through the misery? That is exactly what breeds heatstroke. The right way to train in heat is: acclimatize first, watch WBGT, drink to thirst, pre-cool, and heed the body's signals.
The honest close: heat illness is overwhelmingly preventable — acclimatization plus sensible planning plus prompt cooling drives the risk right down (Racinais 2015). But the moment someone is confused, behaving strangely, or alarmingly hot, don't hesitate: cool them on the spot and call emergency services immediately. This page is education to understand the why, not medical advice. For the other side — heat used as a tool (sauna, heat conditioning) — see the sauna & heat exposure story; for how to handle fluids and electrolytes, see the water & electrolytes story.