Heat Exhaustion and Heatstroke: Signs and What to Do

Hot weather illness comes in two stages, and what you do depends on which one you are looking at. Heat exhaustion is the earlier stage: the body has overheated and lost too much water, and it is not usually serious if the person can cool down within 30 minutes¹. Heatstroke is the emergency that follows when the body is no longer able to cool down and the body temperature becomes dangerously high¹. If you suspect someone has heatstroke, call 999 immediately¹.

The practical difference comes down to signs and response. Heat exhaustion shows itself with heavy sweating, pale or clammy skin, dizziness, headache, cramps and intense thirst, and it responds to cooling, fluids and rest¹. Heatstroke shows itself with a very high temperature, hot skin that is not sweating, confusion, a fast heartbeat, fast breathing, seizures and loss of consciousness, and it does not respond to self-care alone: it needs an ambulance¹. Heat exhaustion can develop quickly over a few minutes, or gradually over several hours or days, and heatstroke can also occur suddenly and without any warning².

This page sets out the NHS signs of each condition, the four cooling steps for heat exhaustion, what to do while waiting for an ambulance, and who in your household is most at risk. For the wider picture of preparing your home for hot weather, see our heatwaves and drought guide.

Heat exhaustion: the signs to look for

Heat exhaustion happens when the body overheats and cannot cool down, and when you have lost too much water through sweating³. It is the more common of the two conditions and, treated early, most people recover quickly with self-care⁶. But it is not a condition to sit out in the sun with: if you do not take action to cool down, heat exhaustion can lead to heatstroke, which is more dangerous³.

The common symptoms, as set out in official guidance, are:

• tiredness and weakness
• feeling faint
• headache
• muscle cramps
• feeling or being sick
• heavy sweating and intense thirst
• a fast pulse, faster than 100 beats per minute
• pale, clammy skin
• a core temperature between 38°C and 40°C⁴

Other official summaries describe the same picture in slightly different words: headache, dizziness, nausea, excessive sweating, pale skin and cramps⁷, or dizziness, headache, excessive sweating, feeling sick, tiredness or weakness⁶. The signs overlap because they all come from the same cause: the body is losing water and salt faster than they are being replaced, and its cooling system is beginning to fail.

Heat exhaustion can develop quickly over a few minutes, or gradually over several hours or days². The gradual form matters for households: an older relative who has been quietly sweating through a hot afternoon indoors may be well along this path before anyone notices, which is why checking on people who live alone during hot weather is part of official advice⁸. There is a high risk of heat exhaustion or heatstroke during hot weather or exercise, so the signs deserve active watching on the hottest days and during any strenuous activity³.

Heatstroke: confusion, hot dry skin and a temperature over 40°C

Heatstroke is less common, but more serious². It happens when the body can no longer cool down and the core temperature gets higher than 40°C despite resting in a cool place and drinking fluids for 30 minutes or longer⁹. It can put a strain on the brain, heart, lungs, liver and kidneys, and can be life-threatening².

The common symptoms of heatstroke are:

• a very high temperature, usually more than 40 degrees Celsius⁵
• hot skin that is not sweating, and which might look red
• a fast heartbeat
• fast breathing or shortness of breath
• confusion and lack of co-ordination
• a seizure or fit
• loss of consciousness⁴

The redness of the skin can be harder to see on darker skin shades, so rely on the other signs too: the skin is hot and dry rather than sweaty, and the person's behaviour and breathing have changed³. Heatstroke can also occur suddenly and without any warning¹⁰, and it can develop very quickly into unconsciousness¹⁰. Heat stroke can kill⁶.

If you see these signs, do not wait to see whether they improve. A high temperature, confusion and breathing fast can be symptoms of heat stroke, which is a medical emergency; other serious symptoms include skin that is no longer sweating and seizures¹¹. Call 999, put them in the recovery position if they are unconscious, and keep trying to cool the person down¹¹.

Heat exhaustion or heatstroke: how to tell them apart

The two conditions sit on the same spectrum, and heat exhaustion can lead to heatstroke if it is not treated³. The clearest way to tell them apart is to look at three things together: the skin, the mind and the temperature. In heat exhaustion the skin is pale, clammy and sweating heavily; in heatstroke it is hot, dry and not sweating. In heat exhaustion the person is uncomfortable but aware; in heatstroke they are confused, unco-ordinated or losing consciousness. In heat exhaustion the core temperature sits between 38°C and 40°C⁴; in heatstroke it is usually more than 40 degrees Celsius⁵.

One further test from official guidance is useful: heatstroke is defined by a core temperature that stays higher than 40°C despite resting in a cool place and drinking fluids for 30 minutes or longer⁹. So the 30-minute mark is not just a recovery target for heat exhaustion, it is also the point at which the picture changes and you should be thinking about emergency help. If someone is still unwell after 30 minutes of resting in a cool place, being cooled and drinking fluids, that is a signal to escalate¹².

Heatstroke is treated in hospital, where healthcare professionals will try to cool the body down and may also give fluids through a vein; it can take a few days to recover⁵. If you have suffered heat exhaustion or heatstroke, take time to rest and recover for as long as your GP advises, avoiding hot weather and exercise¹³.

Cooling someone down: four steps for heat exhaustion

If someone is showing signs of heat exhaustion, they need to be cooled down and given fluids¹. The NHS sets out four steps¹:

1. Move them to a cool place. Get them out of the sun and into shade or a cool room.
2. Remove all unnecessary clothing. The NHS example is clothing like a jacket or socks¹.
3. Give them plenty of water to drink. A sports drink or an oral rehydration solution mixed with water also works, replacing salts lost through sweating¹. Sports or rehydration drinks are OK, official guidance confirms¹⁴.
4. Cool their skin. Spray or sponge them with cool water and fan them, or place cold packs wrapped in a cloth under the armpits or on the neck¹.

The same steps appear across official guidance in the four nations. NHS 111 Wales lists the identical four steps³, and nidirect in Northern Ireland adds that you can give water, a fizzy drink or other fluid, and spray or sponge with cool water¹⁵. Broadcast scripts prepared by the UK Chief Medical Officers for use during a national power outage give the same sequence, including the 30-minute check, which shows how standard this first aid is¹⁶.

One instruction to hold firmly: do not give them aspirin or paracetamol. It will not help the raised temperature and may be harmful². The same instruction applies if you suspect heatstroke¹⁷.

They should feel better within 30 minutes

The person should start to cool down and feel better within 30 minutes³. That half hour is the test of whether your first aid is working. If a person is cooling down and feeling better within 30 minutes, this suggests they are improving³.

If they do not improve, or their symptoms get worse, escalate:

• If there is no improvement within 30 minutes, or their symptoms worsen, try to get medical help if you can¹⁶.
• If you are concerned about symptoms, or they are worsening, seek medical advice by contacting NHS 111¹.
• If you are still unwell after 30 minutes of resting in a cool place, being cooled and drinking fluids, contact 111¹².
• If you suspect heat exhaustion has turned into heatstroke, call 999¹³.

You can call 111 or get help from 111 online¹². If you need medical help for a heat-related health problem, contact NHS 111; in an emergency, or when you think someone has heatstroke, call 999¹⁸. Our page on when to call 999 or NHS 111 for heat illness goes into this decision in more detail.

Most people with heat exhaustion recover quickly with self-care⁶. After the illness, rest for a few hours, keep cool and drink water¹⁰, and take time to rest and recover for as long as your GP advises, avoiding hot weather and exercise¹³.

What to do while you wait for the ambulance

If you suspect heatstroke and have called 999, your job is to keep cooling the person and to follow the call handler. While waiting for the ambulance, listen carefully to the call handler and follow the instructions given to you⁷. Stay calm¹⁹.

The cooling actions official guidance sets out for suspected heatstroke while you wait are:

• Move them somewhere cooler, if possible.
• Increase ventilation by opening windows or using a fan.
• Cool them down as quickly as possible: loosen their clothes, sprinkle them with cold water, or wrap them in a damp sheet²⁰.
• If they are conscious, encourage them to drink fluids²⁰.
• Get them to lie down and raise their feet slightly, keep them as cool as possible by spraying or sponging with cool water, and encourage them to keep drinking water; sports or rehydration drinks are OK¹⁴.
• Do not give aspirin or paracetamol²¹.

If they lose consciousness while you are waiting for help, put them in the recovery position and tell the ambulance service¹. Check their breathing and pulse, and carry out CPR if they show no sign of life¹. Continue efforts to cool them down while you wait for assistance³.

Who is most at risk in hot weather

Children, older people and people with long-term health conditions, such as diabetes or heart problems, are more at risk of heat exhaustion or heatstroke³. That short list is the core of the official risk picture across the UK, but the fuller lists go further. NHS Inform in Scotland adds people aged 65 or over, children aged 5 and under, babies, pregnant people, people with underlying health conditions, people with a serious mental health condition, people already ill and dehydrated, people taking alcohol or drugs, active people spending a lot of time outside, people working in a manual job outdoors, homeless people, people who live alone and are unable to care for themselves, people with mobility problems, and people on certain medications⁸.

The UK government's heatwave guidance adds people who live in top floor flats or homes with restricted or limited windows to the list of groups most at risk²². nidirect in Northern Ireland lists elderly people, babies and young children, people with a long-term health condition such as diabetes or a heart or lung condition, people who are already ill and dehydrated, and people doing strenuous exercise for long periods, such as athletes and manual workers².

You are more likely to experience problems if you are dehydrated, there is little breeze or ventilation, or you are wearing tight, restrictive clothing². Look out for those who may struggle to keep themselves cool and hydrated: older people, those with underlying conditions and those who live alone are particularly at risk²³. Our pages on who is most at risk in hot weather and how to check on older and vulnerable neighbours in the heat cover this in depth.

Medicines that raise the risk

Certain medications can increase your risk of developing heat exhaustion or heatstroke². This is not a reason to stop taking any medicine; it is a reason to be more careful in hot weather and to seek advice. The medicines named in official guidance include:

• diuretics
• antihistamines
• beta-blockers
• antipsychotics
• recreational drugs, such as amphetamines and ecstasy²

Social care guidance for hot weather adds anticholinergics, vasoconstrictors and drugs that reduce renal function to the list of medications that may increase a person's risk of developing heat related illness⁴. NHS guidance for Berkshire lists further categories: drugs with levels affected by dehydration, such as lithium, digoxin and antiepileptics; drugs that increase the basal metabolic rate, such as thyroxin; and medications which interfere with cardiovascular responses, such as antihypertensives and antianginal drugs⁹. Somerset Council likewise notes that some medications, including diuretics, antihypertensives and antipsychotics, can affect how the body copes with heat²⁴.

Mental health medication deserves particular attention. Some mental health medications increase sensitivity to hot weather²⁵. People taking psychotropic medications may be at increased risk of death and other physical health impacts during heatwaves compared to those with other chronic conditions such as cardiovascular conditions²⁵. The risks include heatstroke, dehydration, nervous system changes and altered neurological function, such as confusion, agitation or disorientation²⁵. Our page on medicines that raise the risk of illness in the heat lists these in full.

Preventing heat illness during a heatwave

Heat exhaustion and heatstroke can happen when you are exposed to high temperatures, for example if you exercise in hot places or spend a lot of time outside in the heat⁵. Prevention is therefore mostly about limiting that exposure and keeping water levels up. The NHS advice to help prevent heat exhaustion or heatstroke is:

• drink more cold drinks, especially if you are active or exercising
• wear light-coloured, loose clothing
• avoid the sun between 11am and 3pm
• avoid drinking too much alcohol
• avoid extreme exercise
• if inside on a very hot day, close curtains, close windows if it is hotter outside than in your home, and turn off electrical equipment and lights that get hot¹

The 11am to 3pm rule appears across official guidance, including nidirect's advice to keep out of the sun between 11.00 am and 3.00 pm². High temperatures can lead to respiratory problems, dehydration, heat exhaustion and sunburn, often resulting in emergency medical treatment and hospitalisation, so prevention is worth the effort¹³. Not having enough water, dehydration, is one of the main health risks of hot weather alongside overheating itself²³.

Around the home, our guides cover the practical side: how to keep your home cool in hot weather, how to stop the sun heating your rooms through the windows, how to sleep and cool a bedroom on hot nights, and how much water to drink in hot weather. If you want to prepare before the hot weather arrives, our hot weather preparation checklist sets out what to do in advance, and you can sign up for official warnings through our page on heat-health alerts.