Exercising in hot weather carries risks of heat exhaustion and heat stroke, with temperatures across the UK expected to reach or exceed 30°C in coming days — a rare occurrence for May that has triggered amber heat health alerts from the UK Health Security Agency (UKHSA) for several regions, including the East Midlands, West Midlands, East of England, London, and the South East. The alerts warn of a potential risk to life, particularly for vulnerable individuals, and indicate that health impacts may be felt across the entire health service. The Met Office has also warned of high UV levels, and climate change is cited as a significant factor behind the increased likelihood and intensity of such extreme hot days in the UK.
The Dangers of Heat Stroke
Heat stroke, also known as severe heat illness, is a medical emergency that occurs when the body’s temperature regulation system fails and core temperature rises above 40°C. Normal body temperature is about 37.5°C. When it climbs to between 38.5°C and 40°C, this is classified as heat exhaustion, or mild heat illness. Heat exhaustion is an early warning sign that the body is overheating and losing essential salts and water through excessive sweating. It can develop gradually and is generally treatable with rest and fluids if caught promptly. Symptoms include feeling excessively hot or tired, nausea, vomiting, headache, dizziness, tunnel vision, excessive sweating, clammy skin, muscle cramps, and rapid breathing or heartbeat.
If heat exhaustion progresses into heat stroke, the person starts to become confused, irritable, and behave oddly, and may even turn aggressive with those trying to help them. The skin becomes hot and flushed and may not be sweating. The pulse becomes rapid, and seizures or loss of consciousness can occur. Heat stroke can lead to liver, kidney and brain damage — and can be deadly. Immediate medical attention by dialling 999 is critical.
Susceptibility to heat exhaustion is variable: two people could do the same level of exercise under the same conditions, and only one would succumb. It seems that some people are just more efficient at getting rid of heat, especially if they are fit, or better acclimatised to warmer temperatures. Because of this varying susceptibility, organisers of endurance events like long-distance runs should be aware that exertional heat illness can strike some people at any time of the year, even when temperatures are cool, according to the UK’s national guidelines on Pre-hospital Management of Exertional Heat Illness. The Ministry of Defence (MOD) has developed extensive guidelines and technologies to combat exertional heat illness, especially within military training. The Commando Training Centre Royal Marines (CTCRM) has pioneered a system using body monitors (the Open Body Area Network, or OBAN) to track trainees’ physiological data, including estimated core temperature and Heat Strain Index. Developed in collaboration with the Institute of Naval Medicine and the US Army Research Institute of Environmental Medicine, this system aims to predict and intervene in cases of heat illness and has already saved lives and prevented serious casualties.
How the Body Regulates Heat — and What Can Go Wrong
Even at rest, the body is constantly generating heat as a byproduct of metabolism, when energy is released from food. Normally we can get rid of that heat by sweating and, if necessary, sending more blood to the surface of the skin. When we exercise, muscles release much more heat than usual — and this is why heat illness usually develops in people undergoing physical activity. Exertional heat illness can occur even in temperate climates like the UK, not just in hot and humid environments.
A person who was subtly ill before the activity may also have a greater chance of developing heat stroke. People should be very cautious about exercising in hot weather if they have even a minor illness, such as a cold or diarrhoea, says Dr Ross Hemingway, a sports GP and advisor to the Ministry of Defence Commando Training Centre Royal Marines. “If you have a runny nose and have a fever with that, then we know that if you do hard intense exercise, then you’re four times more likely to get heat stroke,” he says.
Vulnerability can also be influenced by whether you have been drinking the night before. Alcohol directly impairs the body’s ability to regulate temperature. It causes vasodilation, leading to increased blood flow to the skin and a temporary feeling of warmth, but this actually lowers core body temperature and increases heat loss. Alcohol also reduces shivering responses and dulls awareness of cold. It has a dehydrating effect and reduces sweating, both critical for cooling. Research shows alcohol can disrupt normal circadian body temperature rhythms, increasing nocturnal core temperature and lowering daytime temperature. So people should also avoid intense exercise if they feel hungover, Hemingway adds.
Staying Safe in the Heat
Making sure we are not dehydrated is a key defence against heat stroke. But drinking too much fluid is also a danger, as it can lead to sodium concentration in the blood falling too low — a condition known as Exercise-Associated Hyponatremia (EAH). EAH can be as serious as dehydration, with symptoms including confusion and seizures, and in severe cases can be fatal. The symptoms of hyponatremia and dehydration can be very similar, making correct diagnosis and treatment crucial. Some studies suggest that even electrolyte drinks might not prevent EAH if over-hydration occurs, because they can still dilute blood sodium levels.
People are generally advised now to “drink to thirst” — making sure they drink enough that they do not feel at all thirsty — but no more. Those doing exercise should aim to drink to thirst before, during and after their activity, says Hemingway. “Don’t forget to hydrate before — that’s really important.” Another guide is to look at the colour of urine. It should be a pale yellow — not darker and not completely clear either.
And do not forget the most obvious strategies of avoiding exercise in the heat if possible, Hemingway adds. Consider whether outdoor activity can be done in the early morning or late evening, when temperatures have fallen — the peak heat hours between 11 am and 3 pm should be avoided, a guideline sometimes called the “11 AM rule”. Alternatively, go to an air-conditioned gym. Wear lightweight, loose-fitting, light-coloured clothing made from moisture-wicking fabrics, along with a wide-brimmed hat and sunglasses. Allow for a longer cool-down period after exercise to help the body temperature return to normal gradually.
For athletes preparing for events in hot conditions, heat acclimatisation training (HAT) — repeated exposure to heat during exercise over several days or weeks — allows physiological adaptations that improve heat tolerance and reduce the risk of heat illness.
Certain groups are at higher risk of heat-related illness, including older people, babies, young children, and individuals with pre-existing health conditions such as respiratory and cardiovascular diseases, heart disease, lung disease, or diabetes. Some medications, including certain antidepressants, can also increase the risk. It is crucial to check on these individuals during hot weather and ensure they are aware of the necessary precautions.
In cases of heat stroke, rapid cooling is essential. This can involve submerging the person in ice-cold water, pouring cool water over them, or using cool damp cloths, while continuously monitoring their temperature. Employers have a responsibility to ensure employees can work safely in hot weather, including conducting risk assessments and implementing control measures — although there is no legal maximum working temperature in the UK. Water safety warnings have also been issued during this heatwave, highlighting the dangers of strong currents, underwater hazards, and cold water shock even in warmer weather.