Exactly what happens to your body with heat stroke

The human body suffers in extreme temperatures; heat stroke is a killer that we need to know how to protect ourselves from, particularly as the world warms up due to climate change. A mild case of heat exhaustion can escalate to organ failure in less than two hours. With temperatures in European holiday destinations already reaching highs of 40C, heat awareness is the order of the summer.

What shocks doctors and relations of those affected by heat stroke is how quickly it can be fatal. As it stands, about 3,000 deaths each year in Britain are linked to heat stroke, while across southern Europe the figures are higher. In 2022, more than 60,000 people died because of extreme heat.

“Heat stroke is life-threatening and needs life-saving treatment,” explains Dr Ann Nainan, a GP specialising in urgent care. “We all need to be able to recognise the early symptoms and take corrective action immediately.”

Heat stroke occurs when the body’s cooling mechanism stops functioning and core temperature soars over 40C; symptoms include dizziness and confusion, vomiting and seizures. There are two types of heat stroke: exertional heat stroke comes on quickly as a result of strenuous activity and classic heat stroke comes on more slowly from passive exposure to extreme temperatures. Both can be fatal, although exertional heat stroke has a mortality rate of 3-5 per cent compared with 10-65 per cent for classic heat stroke, according to research.

Children under two and the elderly are more at risk of heat stroke and other heat-related conditions as their cooling mechanisms are less efficient and their immune systems may be weaker. You’re also more likely to contract heat stroke if you drink alcohol, take diuretics or have a pre-existing medical condition such as cystic fibrosis, which affects your ability to sweat. Research suggests that men are more likely to suffer from heat stroke than women. While heat stroke is most common in hot, humid conditions, marathon runners contract it in relatively temperate weather if they run so hard that their body temperature rises uncontrollably.

The NHS advises anyone with heat exhaustion to follow four steps to stop it turning into heat stroke: move to a cool place, remove unnecessary clothing, drink a glass of water and cool the skin with water or ice packs. If someone is still unwell after 30 minutes, call 999.

Yet Dr Rowley Cottingham, a director of the British Association for Immediate Care and a consultant in emergency medicine, believes aggressive cooling measures should be implemented immediately – ideally by throwing the victim into a bath of water. The Boston Marathon has a series of water-filled tubs at the finish line for just this purpose.

Where no preventative measures are taken, symptoms progress rapidly, as outlined below, although Dr Nainan, the founder of the medical blog The Travelling Doc, points out that these are rough estimates as people handle heat differently depending on their age and overall health. “The most important thing to recognise is that heat stroke is always a medical emergency,” she says. “The sooner you cool down, the less damage will be done.”

What happens to your body when it’s exposed to heat – minute by minute

After 5 minutes

Even after just five minutes in temperatures of 35C or more, your body will begin to work harder than usual, says Dr Nainan. Your heart rate increases, your skin will start producing sweat; the vessels near the skin will widen to radiate as much heat as possible. You might start looking hot and flushed. “Our bodies do everything possible to maintain a core temperature of 37.3 degrees,” she explains. “Now is the time to put on appropriate UV clothing and rehydrate.”

After 5-10 minutes

If no action is taken to cool down, initial discomfort sets in. The body’s cooling mechanism kicks into a higher gear, Dr Nainan explains, and you sweat more heavily. If fluids are not replaced now, you will begin to feel dehydrated and could even start experiencing the onset of a headache, although if you were to retreat into the shade for a cool drink, you would most probably start to feel comfortable again.

After 10-20 minutes

By now, the body will be using a lot of energy to cool down and will be sweating profusely. You’ll probably feel thirsty and lacking in energy and if you haven’t applied SPF, your skin will start to burn.

After 40 minutes

You’ve been exposed to the heat for a while now and will most probably be experiencing mild heat exhaustion brought on by dehydration, Dr Nainan says. You might start to experience stomach cramps and muscle spasms in your back and arms due to a lack of electrolytes. Other warning signs that the body is struggling in the heat include nausea, headache, dizziness and excessive sweating, Dr Nainan continues. If you don’t take steps to cool down now, you’re putting yourself at risk of heat stroke.

After 60 minutes

Your body temperature has now reached about 38.5C and you are suffering from heat exhaustion, a less severe form of heat stroke, which sometimes (although not always) precedes it. While it isn’t life-threatening and does not cause neurological problems, it must be taken seriously as it’s the earliest stage of hyperthermia (the inability to regulate your temperature). You might start to feel light-headed and heavy-limbed; the body is exhausted after attempting to cool itself down for so long. Some people report feeling tingly skin, as if ants are crawling over it. “You feel incredibly tired and have an overwhelming desire to stop everything,” says Dr Cottingham.

It’s now essential to cool down: find a spot in the shade or ideally inside with air conditioning; lie down with your legs elevated above your head or take a cool shower or bath. If you have no shelter, soak your clothes with water and drink small sips to rehydrate. Sports drinks containing electrolytes are good for heat stroke, as are bananas, watermelon and salty snacks.

If these measures do not alleviate your symptoms, or if you’re vomiting or feeling confused, seek medical advice immediately. At this stage, full recovery could take 24-48 hours and doctors advise waiting for a few days before returning to exercise.

After 90-120 minutes

If no intervention is taken, your body will lose its ability to regulate temperature and you will contract heat stroke. Once core body temperature has reached 40 degrees or higher, you can be dead in just a few minutes. According to Dr Cottingham, true heat stroke is the end stage after hyperthermia.

The body has failed to cool itself through sweating and radiating heat through and now organ systems including the brain, heart and liver can malfunction. “It’s incredibly dangerous: it kills very quickly,” says Dr Cottingham. “You’re literally cooking like an egg. Body proteins suffer heat damage and this happens to every organ.” It’s the brain that is the first to be affected, swelling in the heat, which is why confusion is one of the biggest indicators of heat stroke.

It’s the brain that is the first to be affected, swelling in the heat, which is why confusion is one of the most certain signs of heat stroke. Soldiers suffering from heat stroke on exercise will lose their way or start weaving as they walk. This isn’t them being hot and bothered, it’s their body failing in the heat, Dr Cottingham continues. Other symptoms include hot dry skin, with an absence of sweating, rapid heart rate (reduced blood to the heart and other circulatory problems and metabolic dysfunction), poor balance, nerve damage, fainting, loss of consciousness and, in severe cases, seizures, shock and coma.

How is severe heat stroke treated?

If no treatment is given, you are unlikely to survive more than half an hour. According to Luke Inman, a medical officer who treated the New Zealand-Australian National Rugby League player Lloyd Perrett when he collapsed from heat stroke, it’s nearly impossible to recover after this point. Your cells are damaged by the heat; the only chance of survival is if your body temperature is brought down rapidly. Inman placed ice packs on Perrett’s main arteries, which helped save his life; medical professionals also mist patients with water and cover them in cooling blankets and encourage them to drink small sips of salty water.

In hospital, heat stroke sufferers will be given medication to prevent seizures as well as oxygen. Then, they’ll be put on an intravenous drip of cool fluids. In extreme cases doctors resort to cold-water lavage, where catheters are used to fill the body with cooled water via the throat or rectum. Some hospitals also offer ice baths.

Medical professionals stop using cooling treatments once the body reaches 38.9C and will then turn their attention to recovery. Prognosis after serious exertional heat stroke is variable and outcomes are probably related to the duration of severe hyperthermia. Even with treatment, though, a person might still suffer from brain damage or organ failure.

The long-term effects of heat stroke

The effects of heat stroke can take months to recover from fully. According to Dr Nainan, one can expect body temperature fluctuations for several weeks and, in severe cases, kidney and liver function will need to be monitored.

Once you’ve experienced true heat stroke, you are more likely to have it again in the future. “Look after your body in the heat,” she urges. “Allow it to acclimatise gradually and drink water regularly, even if you don’t feel thirsty. Stay safe by staying indoors during the hottest parts of the day.”