Heat Stroke During Exercise
Exertional heat stroke can turn a hard summer session into a life-threatening emergency within minutes. Our editorial team explains how to spot it, what to do before the ambulance arrives and how to train safely when temperatures rise.
What heat stroke is
Exertional heat stroke is a medical emergency in which body temperature rises to a dangerous level, typically above 40 °C, during physical effort, and the brain begins to malfunction. It is different from heat exhaustion, which is more common and less severe.
The body normally sheds heat through sweating and increased blood flow to the skin. During hard exercise in hot or humid conditions, heat production can outpace heat loss. Once core temperature climbs too far, proteins and cell membranes are damaged and organs such as the brain, kidneys, liver and muscles can fail.
Heat stroke can occur even in the UK. Summer races, outdoor bootcamps, military training and indoor sessions in poorly ventilated gyms have all been settings for cases. A heatwave is not required; a warm, humid day combined with high intensity can be enough.
The good news is that with rapid, aggressive cooling, survival rates are very high. The key is recognising the problem early.
Recognising the warning signs
The most important sign that separates heat stroke from heat exhaustion is a change in mental state. The person may become confused, irritable, unsteady, behave oddly, collapse or lose consciousness.
| Feature | Heat exhaustion | Heat stroke |
|---|---|---|
| Core temperature | Raised, usually below 40 °C | Usually above 40 °C |
| Mental state | Alert, may feel faint | Confusion, aggression, collapse, seizures |
| Skin | Sweaty, pale, clammy | Often still sweating in exertional cases; may be hot and dry |
| Other signs | Headache, nausea, cramps, weakness | Vomiting, rapid heart rate, fast breathing |
| Response to rest and cooling | Improves within about 30 minutes | Does not resolve; emergency |
A common misconception is that people with heat stroke always stop sweating. In exertional heat stroke, the skin is often still wet, so continued sweating should not reassure you.
Oral or forehead thermometers are unreliable in exercising people. In practice, anyone who becomes confused or collapses during exercise in the heat should be treated as having heat stroke until proven otherwise.
What to do: cool first, then transport
Position statements from the US National Athletic Trainers' Association and international consensus groups emphasise the principle of "cool first, transport second" when on-site medical staff are available. The longer the body stays overheated, the greater the damage.
- Call 999 immediately.
- Stop exercise and move the person to shade or a cooler area.
- Remove excess clothing and equipment.
- Cool aggressively: immersion in cold water is the most effective method; if unavailable, continually rotate ice-water-soaked towels over the body and use fans with water spray.
- Do not give fluids by mouth to someone who is confused or not fully conscious.
- Monitor breathing and be ready to start CPR.
Ice packs to the neck, armpits and groin help, but on their own they cool far more slowly than immersion. Event organisers should plan a cooling station for warm-weather races.
Risk factors you can control
Several factors increase the risk of heat illness. Some cannot be changed, but many can.
Lack of heat acclimatisation is one of the most important. The body adapts to heat over roughly one to two weeks of gradual exposure, improving sweating and blood volume. Consensus recommendations by Racinais and colleagues describe how athletes can acclimatise safely.
Dehydration, recent illness with fever or diarrhoea, poor sleep, high body fat, heavy protective clothing and pushing through warning signs all add to risk. So do certain substances: stimulants such as high-dose caffeine, amphetamines and ephedrine-type compounds raise heat production, while some medicines, including antihistamines, diuretics and some antidepressants, can impair heat loss.
Particular warning applies to 2,4-dinitrophenol (DNP), sold illegally as a fat burner. DNP causes uncontrolled heat production, has led to deaths in the UK and is extremely dangerous, especially with exercise or hot weather. There is no safe way to use it.
Training safely in warm weather
Practical steps reduce the risk significantly. Check the forecast and plan intense sessions for cooler times of day. Reduce intensity and volume during the first warm days of the year, before acclimatisation.
Drink to thirst and know your typical sweat losses; weighing yourself before and after a session gives a rough idea. Excessive water intake is also a risk, causing low sodium, so more is not always better.
Wear light, breathable clothing and build in rest breaks in shade. In group settings, coaches should watch for anyone who looks unwell and make it acceptable to stop.
After a heat stroke, return to training must be gradual and supervised by a doctor, often over several weeks, with checks of kidney, liver and muscle markers.
Editorial conclusions
Heat stroke during training is rare but potentially fatal, and it can happen in the British climate. Confusion or collapse during exercise in warm conditions should trigger an immediate 999 call and aggressive cooling, ideally cold-water immersion.
Gradual acclimatisation, sensible scheduling, hydration and avoiding stimulants greatly reduce risk. DNP and similar illicit fat burners can cause fatal overheating and must never be used.
Organisers of summer events should have a cooling plan in place.
Related articles: hydration and electrolytes for endurance training, heat acclimatisation before a summer race, and the dangers of illegal fat burners.
References
- Casa DJ, DeMartini JK, Bergeron MF, et al. National Athletic Trainers' Association position statement: exertional heat illnesses. J Athl Train. 2015;50(9):986–1000.
- Epstein Y, Yanovich R. Heatstroke. N Engl J Med. 2019;380(25):2449–2459.
- Bouchama A, Knochel JP. Heat stroke. N Engl J Med. 2002;346(25):1978–1988.
- Racinais S, Alonso JM, Coutts AJ, et al. Consensus recommendations on training and competing in the heat. Br J Sports Med. 2015;49(18):1164–1173.
- Leon LR, Bouchama A. Heat stroke. Compr Physiol. 2015;5(2):611–647.
