What happens to your body during heat stroke?
During heat stroke, body temperature rises above 40 °C (104 °F) and the brain stops working right: the person becomes confused or collapses (Bouchama, 2022). It is a life-threatening emergency: cool them down at once; cold water lowers their temperature by about 0.2 °C (0.36 °F) per minute (Casa, 2015).
Simplified diagram, not to scale. Dotted curves are estimates drawn between measured values; the numbers in the notes are measured.
What happens to your body during heat stroke?
A person exercising in the heat, greatly simplified. Choose a situation, then tap a label on the drawing to learn what it is.
Sources: definition and deaths (Bouchama et al., Nat Rev Dis Primers, 2022); landmarks, exhaustion and cold bath (Casa et al., J Athl Train, 2015); temperature at exhaustion, heart and skin (González-Alonso et al., J Appl Physiol, 1999); acclimatization (Pandolf, 1998; Racinais et al., 2015); sweat (Baker et al., 2016; Barnes et al., 2019); steps without a tub (NHS). Drawing not to scale. Pulse next to the heart: 197 bpm at exhaustion according to González-Alonso (1999); the other values, from 105 to 177 bpm, are our own rough estimate, to be checked.
Keyboard: 1 to 3 situations, Space pause, L slow motion, R restart, ← → previous or next step.
Frequently asked questions about heat stroke
What are the signs of heat stroke?
The warning sign is a brain that stops working right in someone who is very hot: confusion, unusual behavior, loss of balance, collapse or passing out (Casa et al., 2015). Core temperature is then above 40 °C (104 °F) (Bouchama et al., 2022), and above 40.5 °C (105 °F) during exercise (Casa et al., 2015). The skin is hot; during exercise it often stays wet with sweat, but it can also be dry and red (NHS).
What is the difference between heat exhaustion and heat stroke?
With heat exhaustion, you can no longer keep going but your mind stays clear; with heat stroke, the brain is affected and life is in danger (Casa et al., 2015). Heat exhaustion usually happens below a core temperature of 40.5 °C (105 °F) and passes with rest somewhere cool and water; if it is no better within 30 minutes, call for emergency help (NHS).
What should you do for someone with heat stroke?
Call 911 and cool the person down right away, without waiting for help to arrive. The most effective way is to put them in cold water up to the neck: they lose about 0.2 °C (0.36 °F) per minute, and the goal is to get below 38.9 °C (102 °F) within 30 minutes (Casa et al., 2015). Without a tub, spray them with cool water, fan them and put cold packs wrapped in a cloth under the armpits or on the neck (NHS).
Can you die from heat stroke?
Yes, heat stroke can be fatal, especially if cooling is delayed. Among people treated in intensive care, 26.5% die after exertional heat stroke and 63.2% after classic heat stroke during a heat wave, which mostly affects older or frail people (Bouchama et al., 2022). Lasting damage can remain, and no treatment replaces fast cooling (Bouchama et al., 2022).
How long does it take to get used to the heat?
It takes about 7 to 10 days of exercise in the heat for the body to adapt almost fully (Pandolf, 1998). Two-thirds to 75% of the changes show up within 4 to 6 days: you sweat earlier and more, blood volume rises, and heart rate and temperature climb less (Pandolf, 1998; Racinais et al., 2015). Most of these benefits last 2 to 4 weeks (Racinais et al., 2015).
True or false?
During exertional heat stroke, you stop sweating.
False. In exertional heat stroke, the skin often stays wet with sweat: sweaty skin should not reassure you if the person is confused (Casa et al., 2015).
You should cool the person down even before help arrives.
True. After calling 911, start cooling right away: in cold water, the person loses about 0.2 °C (0.36 °F) per minute, and the goal is to get below 38.9 °C (102 °F) within 30 minutes (Casa et al., 2015).
It takes about 10 days of exercise in the heat to get used to it.
True. Heat acclimatization is almost complete after 7 to 10 days of exercise in the heat, and two-thirds of it happens within 4 to 6 days (Pandolf, 1998).
This page as text
The same information as the charts, to read without animation: for each chart, the values at the main moments, then the notes with their level of evidence.
At what temperature does the body give out?
Around 40 °C (104 °F): trained cyclists in a 40 °C room all stopped, exhausted, at 40.1 to 40.2 °C, whatever their starting temperature (González-Alonso, 1999). Scale: 36 °C at the bottom, 41 °C at the top; curves estimated between measured values.
| Minutes of exercise, days of acclimatization or hours of exercise | Started at 38.2 °C, stopped at 28 min (dotted) | Started at 37.4 °C, stopped at 46 min (dotted) | Started at 35.9 °C, stopped at 63 min (dotted) | What happens |
|---|---|---|---|---|
| Start | 44 | 28 | 0 | The start of exercise |
| 10 min | 61 | 42 | 13 | of exercise in the heat |
| 20 min | 77 | 57 | 30 | of exercise in the heat |
| 30 min | 82 | 71 | 48 | of exercise in the heat |
| 40 min | 82 | 80 | 64 | of exercise in the heat |
| 50 min | 82 | 82 | 76 | of exercise in the heat |
| 60 min | 82 | 82 | 82 | of exercise in the heat |
| 70 min | 82 | 82 | 82 | of exercise in the heat |
Key points
- The skin gets more blood (The first 14 minutes, Shown in humans)
To get rid of heat, blood rushes to the skin. This flow stops rising at a core temperature of about 38 °C: beyond that, heat builds up faster. Source: González-Alonso et al., "Influence of body temperature on the development of fatigue during prolonged exercise in the heat", J Appl Physiol, 1999 - Starting out warm shortens exercise (From 14 to 28 min of exercise, Shown in humans)
In a 40 °C room, cyclists who started at 38.2 °C lasted 28 minutes; those starting at 37.4 °C, 46 minutes; those starting at 35.9 °C, 63 minutes. Source: González-Alonso et al., "Influence of body temperature on the development of fatigue during prolonged exercise in the heat", J Appl Physiol, 1999 - Everyone stops around 40 °C (From 28 to 46 min of exercise, Shown in humans)
Whatever their starting temperature, they stopped, exhausted, at 40.1 to 40.2 °C, with a heart rate of 196 to 198 beats per minute. Source: González-Alonso et al., "Influence of body temperature on the development of fatigue during prolonged exercise in the heat", J Appl Physiol, 1999 - Exhaustion: below 40.5 °C, clear mind (From 46 to 58 min of exercise, Shown in humans)
Heat exhaustion usually stays below 40.5 °C (105 °F): tiredness, weakness, dizziness, headache, nausea, but the person stays clear-headed. Source: Casa et al., "National Athletic Trainers’ Association Position Statement: Exertional Heat Illnesses", J Athl Train, 2015 - Heat stroke: above 40 °C and confusion (After 58 min of exercise, Shown in humans)
Heat stroke means a temperature above 40 °C (104 °F) with a brain that stops working right. In intensive care, 26.5% of exertional heat strokes and 63.2% of heat-wave heat strokes are fatal. Source: Bouchama et al., "Classic and exertional heatstroke", Nat Rev Dis Primers, 2022 - Estimated curves (Throughout exercise, Hypothesis)
Starting and ending temperatures and durations are measured; the rise is drawn as steady between the two. After stopping, the curve stays flat for readability.
How many days does it take to get used to the heat?
One to two weeks of exercise in the heat: two-thirds to 75% of the changes happen in 4 to 6 days, and nearly all of them in 7 to 10 days (Pandolf, 1998). The curve is estimated from these values.
| Minutes of exercise, days of acclimatization or hours of exercise | Acclimatization gained (%) (dotted) | What happens |
|---|---|---|
| Day 0 | 0 | Before acclimatization |
| 2 j | 35 | 2 days of exercise in the heat |
| 4 j | 62 | 4 days of exercise in the heat |
| 6 j | 80 | 6 days of exercise in the heat |
| 8 j | 91 | 8 days of exercise in the heat |
| 10 j | 97 | 10 days of exercise in the heat |
| 12 j | 98 | 12 days of exercise in the heat |
| 14 j | 100 | 14 days of exercise in the heat |
Key points
- What changes (The first 4 days, Shown in humans)
You sweat more and sooner, the skin gets more blood, blood volume rises, the heart beats slower and core temperature rises less for the same effort. Source: Racinais et al., "Consensus recommendations on training and competing in the heat", Br J Sports Med, 2015 - Two-thirds in 4 to 6 days (From day 4 to 7, Shown in humans)
About two-thirds to 75% of the body’s changes and performance gains appear within 4 to 6 days of exercise in the heat. Source: Pandolf, "Time course of heat acclimation and its decay", Int J Sports Med, 1998 - Nearly complete in 7 to 10 days (From day 7 to 10, Shown in humans)
Whether the heat is dry or humid, acclimatization is nearly complete after 7 to 10 days. Being fit is not enough on its own: it only gets you partly acclimatized. Source: Pandolf, "Time course of heat acclimation and its decay", Int J Sports Med, 1998 - What you keep afterward (After 10 days, Shown in humans)
The changes fade more slowly than they build: most of the benefits last 2 to 4 weeks. Source: Racinais et al., "Consensus recommendations on training and competing in the heat", Br J Sports Med, 2015 - Estimated curve (Over the 2 weeks, Hypothesis)
Only the 4-to-6-day and 7-to-10-day milestones are published; the shape of the curve between them is estimated.
How much water do you lose sweating in the heat?
On average 1.2 liters per hour in athletes (Baker, 2016). A 154 lb (70 kg) person holds about 42 liters of water (StatPearls); without drinking, they fall below the 2% of body weight threshold (1.4 liters lost) in about 1 h 10, or in 42 minutes at 2 liters per hour. Scale: 36 liters at the bottom, 42 liters at the top.
| Minutes of exercise, days of acclimatization or hours of exercise | Body water, sweating 1.2 L per hour (dotted) | Body water, sweating 2 L per hour (dotted) | Threshold: 2% of body weight lost (40.6 L) | What happens |
|---|---|---|---|---|
| Start | 100 | 100 | 77 | The start |
| 0.5 h | 90 | 83 | 77 | of exercise without drinking |
| 1 h | 80 | 67 | 77 | of exercise without drinking |
| 1.5 h | 70 | 50 | 77 | of exercise without drinking |
| 2 h | 60 | 33 | 77 | of exercise without drinking |
| 2.5 h | 50 | 17 | 77 | of exercise without drinking |
| 3 h | 40 | 0 | 77 | of exercise without drinking |
Key points
- 42 liters of water in the body (The first 0.3 hours, Shown in humans)
In a 154 lb (70 kg) man, water makes up about 60% of body weight, or 42 liters: two thirds inside the cells, the rest in the blood and around the cells. A little less in women and older people. Source: Tobias et al., “Physiology, Water Balance,” StatPearls, 2022 - 1.2 liters an hour on average (From 0.3 to 0.6 h, Shown in humans)
In 506 athletes, sweat averaged 1.21 liters an hour, ranging from 0.26 to 5.73 liters depending on the person, the sport and the heat. Source: Baker et al., "Normative data for regional sweat sodium concentration and whole-body sweating rate in athletes", J Sports Sci, 2016 - By sport (From 0.6 to 1.2 h, Shown in humans)
In 1,303 athletes: 1.51 L an hour in American football, 1.28 L in endurance sports, 0.95 L in basketball, 0.94 L in soccer and 0.83 L in baseball. Source: Barnes et al., "Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: An update and analysis by sport", J Sports Sci, 2019 - Up to 2 liters an hour (From 1.2 to 1.9 h, Shown in humans)
In the heat and during exercise, sweat can reach about 2 liters an hour. Source: Casa et al., "National Athletic Trainers’ Association Position Statement: Exertional Heat Illnesses", J Athl Train, 2015 - Beyond 2% of body weight (From 1.9 to 2.5 h, Shown in humans)
Losing more than 2% of your weight in water hurts performance; drinking during exercise helps you stay under that threshold, without drinking more than you lose. Source: Sawka et al., "American College of Sports Medicine position stand. Exercise and fluid replacement", Med Sci Sports Exerc, 2007 - Drinking is not always enough (After 2.5 h, Shown in humans)
Exertional heat stroke mostly strikes young, healthy people during intense exercise. Drinking helps you keep going, but the only treatment is to cool down fast. Source: Bouchama et al., "Classic and exertional heatstroke", Nat Rev Dis Primers, 2022 - Calculated curves (Throughout exercise, Hypothesis)
Simple calculation for 154 lb (70 kg) and 42 liters of water, drinking nothing, with 1 liter of sweat weighing about 1 kg (2.2 lb); water lost through breathing and water made by the muscles are ignored. The real loss varies with heat, wind and the person.
Cite this page
To cite this page in a paper, an article or a presentation:
Metabolive. "What happens to your body during heat stroke?" Updated October 29, 2026. https://metabolive.com/en/effort-stress/heat-stroke.
Embed this chart
To show this animated chart on your site, paste this code into your page. The link to Metabolive stays visible under the chart.
<iframe src="https://metabolive.com/en/embed/heat-stroke" width="100%" height="720" style="border:0" title="What happens to your body during heat stroke?" loading="lazy"></iframe>