I will never forget my first experience of heat illness. On my first day in Africa, after walking in the heat and taking a long, overcrowded minibus-taxi journey, I became suddenly unwell: feverish, nauseated, vomiting, with a racing heart and a sense that something was badly wrong. As a doctor, I should perhaps have recognised the signs sooner — but I was not thinking clearly. I had overheated, and I have never forgotten how frightening it felt.
Hot weather can feel like an inconvenience rather than a medical risk. But extreme heat kills, and the UK is still poorly adapted to prolonged high temperatures.
Heat illness matters because it can progress quickly. Early symptoms are often easy to dismiss, but if the body’s cooling systems are overwhelmed, heat exhaustion can become heat stroke — a life-threatening emergency.
The body normally keeps its core temperature around 36–37.5°C by sweating, increasing blood flow to the skin and through learned behaviours such as seeking shade, drinking fluids and slowing down.
Problems begin when these mechanisms cannot keep up — for example during heatwaves, heavy exertion, dehydration, illness, or when medicines affect sweating, blood pressure or kidney function.
Heat illness is a spectrum. Early signs include cramps, dizziness, fainting, thirst, headache, nausea, weakness and heavy sweating. These symptoms matter, especially in older adults, young children, people with long-term conditions, outdoor workers and anyone exercising in the heat.
Heat exhaustion occurs when the body is struggling to cool itself – core temperature is likely rising at this point – but brain function is still broadly normal. Symptoms can include tiredness, dizziness, nausea, clammy or sweaty skin, headache, thirst, cramps, a fast pulse or a high temperature. Move the person somewhere cooler, remove unnecessary clothing, give fluids if they can drink safely, and cool the skin with water, fans or cold packs. They should start to improve within about 30 minutes. If not, seek urgent medical help.
Heat stroke is a medical emergency. The key warning sign is impaired brain function: confusion, agitation, poor coordination, slurred speech, seizures or loss of consciousness. The skin may be dry and very hot but do not rely on dry skin alone; some people, particularly with exertional heat stroke, may still be sweating. Call 999 immediately. While waiting, move them somewhere cooler, remove excess clothing, apply cool water to the skin, fan them and, if available, use cold packs around the neck, armpits and groin.
As the body’s core temperature rises, so should the level of concern. A temperature approaching or above 40°C is particularly worrying. However, do not be falsely reassured by a lower reading: if someone is hot and has an altered mental state — such as confusion, agitation, poor coordination, seizures or reduced consciousness — suspect heat stroke and seek urgent medical help.
Not all of us are equally at risk. The following groups deserve particular attention:
Common medicines can make hot weather riskier. Diuretics can increase dehydration risk; beta-blockers may reduce the cardiovascular system’s ability to compensate; anticholinergics and some antipsychotics can impair sweating or temperature regulation; and ACE inhibitors or NSAIDs can increase kidney risk during dehydration. People should not stop medicines without medical advice, but should seek help early if unwell.
This is not hypothetical. During the 2022 UK heatwave, when temperatures exceeded 40°C for the first time on record, around 3,000 heat-associated deaths occurred in England, mostly in older adults. As UK summers become hotter, recognising heat illness early will become increasingly important. Acting quickly can prevent deterioration and save lives.
Call 999 if someone is confused, collapses, loses consciousness, has a seizure, has very fast breathing or heartbeat, or remains unwell after 30 minutes of cooling, rest and fluids. Keep cooling them while help is on the way.
Heat illness is preventable. The first step is recognising it — and acting early.
For practical, everyday advice on how to stay safe during hot weather, read my companion post: Beat the Heat: A GP’s Practical Guide to Staying Safe This Summer.
Helpful links:
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