Heat Exhaustion vs Heatstroke: How to Tell the Difference

Heat exhaustion is manageable at home. Heatstroke is a 999 emergency. The line between them is crossed in under 30 minutes. Here's how to tell which is which — and exactly what to do.

During a UK heatwave, the two conditions most likely to require medical attention are heat exhaustion and heatstroke. They sound similar. They are not. Heat exhaustion is where you become very hot and start to lose water or salt from your body. Heatstroke is where the body is no longer able to cool itself and a person's body temperature becomes dangerously high.

The distinction matters because the response is completely different. Heat exhaustion does not usually need emergency medical help if you can cool down within 30 minutes. If it turns into heatstroke, it needs to be treated as an emergency.

Understanding the symptoms of each — and the single most important warning sign that one has become the other — is the most practically useful health information available during a heatwave.

TL;DR

• Heat exhaustion occurs when the body becomes too hot and struggles to cool itself — typically when core temperature rises between 37°C and 40°C. It is not usually a medical emergency if the person can be cooled down and starts to feel better within about 30 minutes.
• Heatstroke is a life-threatening condition that occurs when your core body temperature reaches 40°C or higher and must be treated immediately.
• The critical warning sign that heat exhaustion has become heatstroke: the person stops sweating despite feeling hot. This indicates the body's cooling system has failed.
• If someone has signs of heatstroke — still unwell after 30 minutes of resting in a cool place, being cooled and drinking fluids — call 999. Do not drive them to A&E.
• Children, older people, and people with long-term health conditions including diabetes and heart problems are most at risk.
• If left untreated, heatstroke can lead to complications such as organ failure and brain damage. Some people die from heatstroke.

Heat Exhaustion: What It Is and What It Feels Like

Heat exhaustion can occur when the temperature inside the body rises to anything between the normal 37°C up to 40°C. At this temperature, the levels of water and salt in the body begin to fall, which can cause a person to feel sick, feel faint and sweat heavily.

Heat exhaustion can feel like suddenly hitting a wall. Someone may have been coping with the heat for a while, then become weak, dizzy or nauseous. In children, it can sometimes show as unusual tiredness, irritability or sleepiness.

The NHS symptoms of heat exhaustion are:

• Headache
• Dizziness and confusion
• Loss of appetite and feeling sick
• Excessive sweating and skin becoming pale and clammy — though a change in skin colour can be harder to see on brown and black skin
• Cramps in the arms, legs, and stomach
• Fast breathing or pulse
• High temperature of 38°C or above
• Being very thirsty

The symptoms of heat exhaustion are often the same in adults and children, although children may become irritable too.

Heat exhaustion can happen indoors as well as outdoors — in poorly ventilated rooms, during long journeys, or in crowded spaces. It does not require direct sun exposure.

What to Do for Heat Exhaustion

If someone is showing signs of heat exhaustion, follow these four steps: Move them to a cool place. Remove all unnecessary clothing like a jacket or socks. Get them to drink a sports or rehydration drink, or cool water. Cool their skin — spray or sponge them with cool water and fan them. Cold packs, wrapped in a cloth and put under the armpits or on the neck are good too.

The 30-minute window is the key measure. If a person with heat exhaustion is taken quickly to a cool place, given water to drink, and has their excess clothing removed, they should begin to feel better within half an hour and have no long-term complications.

Heat exhaustion will usually get better 30 to 60 minutes after being cooled down. However, the person may continue to feel unwell or tired for up to 24 hours. Symptoms should not get worse during this time. If they do get worse, this is the signal that heat exhaustion has progressed to heatstroke.

Do not give aspirin or paracetamol — they do not help with heat-related illness and are not the correct treatment here.

Heatstroke: The Emergency

Heatstroke is far more serious than heat exhaustion. It occurs when the body can no longer cool itself and starts to overheat. When the core temperature rises above 40°C, the cells inside the body begin to break down and important parts of the body stop working.

The single most important warning sign distinguishing heatstroke from heat exhaustion is the absence of sweating. If someone is not sweating even while feeling too hot, has a high temperature of 40°C or above, or feels confused — call 111 or 999 immediately.

Heatstroke symptoms include:

• Not sweating despite being very hot
• High temperature of 40°C or above
• Confusion, disorientation, or unusual behaviour
• Seizures
• Loss of consciousness
• Hot, dry skin — rather than the clammy skin of heat exhaustion
• Rapid, shallow breathing
• Rapid heartbeat

Severe difficulty breathing — for example, struggling to speak without pausing, gasping or choking — is also a sign requiring immediate emergency response.

When to Call 999 vs 111

Call 999 immediately if:

• The person loses consciousness
• They have a seizure
• They are confused and cannot be roused
• They are breathing rapidly and have hot, dry skin with no sweating
• They do not improve after 30 minutes of cooling and hydration

Call 111 if:

The person still feels unwell after drinking plenty of water, is not sweating even while feeling too hot, has a high temperature of 40°C or above, or feels confused.

Do not drive yourself to A&E. The person you speak to at 999 will give you advice about what to do.

While waiting for emergency services: make sure the person is as cool as possible. Move them to a cool area as quickly as possible, remove excess clothing and try to cool them by fanning them. If they're conscious, give them cool — not cold — water to drink. If someone loses consciousness while you're waiting for help, put them in the recovery position.

Who Is Most at Risk

Children, older people and people with long-term health conditions such as diabetes or heart problems are more at risk of heat exhaustion or heatstroke.

Older adults — the thermoregulatory system becomes less efficient with age. Older adults may not feel hot even as their core temperature rises dangerously. Regular checking on elderly relatives and neighbours during a heatwave is a direct and potentially life-saving action.

Children and babies — children's thermoregulatory systems are less developed. Children might behave strangely or become floppy and sleepy — symptoms that parents may initially attribute to tiredness rather than heat illness. Never leave a child in a parked car.

People on certain medications — diuretics, antihistamines, antidepressants, antipsychotics, and certain heart and blood pressure medications can impair the body's ability to regulate temperature or increase dehydration risk. If you or someone you care for is on regular medication and showing heat illness symptoms, seek medical advice promptly.

People doing physical activity — exercise in hot weather significantly accelerates core temperature rise. Exertional heatstroke — occurring during exercise rather than passive heat exposure — can develop rapidly even in fit, young adults. Training intensity should be significantly reduced during a heatwave and exercise ideally avoided between 11am and 3pm.

Those with certain health conditions — heart disease, kidney disease, and conditions requiring fluid restriction can make both heat illness and treatment more complex. If you have a chronic condition and are experiencing heat illness symptoms, call 111 rather than waiting.

Preventing Heat Exhaustion and Heatstroke

The NHS advice for avoiding heat illness during a heatwave is direct:

If you're inside on a very hot day: close curtains, close windows if it's hotter outside than in your home, and turn off electrical equipment and lights that get hot. This will also prevent dehydration and help your body keep itself cool.

Stay hydrated — drink water regularly throughout the day, before feeling thirsty. Dark yellow urine, infrequent urination, dry mouth, and unusual tiredness are signs of dehydration that increase heat illness risk.

Stay out of direct sun between 11am and 3pm — this is when UV and heat intensity peak in the UK. If outdoor activity is necessary, ensure shade, appropriate clothing, and regular water intake.

Wear appropriate clothing — loose, light-coloured, breathable clothing reduces heat absorption and supports the body's cooling mechanisms.

Check on vulnerable people — neighbours, relatives, and colleagues who are elderly, live alone, or have long-term health conditions are most at risk of heat illness progressing unnoticed. A daily check-in during a heatwave is a meaningful preventive action.

Frequently Asked Questions

What is the difference between heat exhaustion and heatstroke?

Heat exhaustion occurs when the body overheats and struggles to cool itself — typically when core temperature rises between 37°C and 40°C. It causes heavy sweating, pale clammy skin, dizziness, nausea, and weakness. It is manageable at home if the person cools down within 30 minutes. Heatstroke occurs when the body can no longer cool itself and core temperature rises above 40°C — at which point cells begin to break down and organ damage can occur. It is a 999 medical emergency.

How do you know if heat exhaustion has turned into heatstroke?

The key warning signs are stopping sweating despite still being very hot, confusion or disorientation, a temperature of 40°C or above, and not improving after 30 minutes of cooling and hydration. If any of these are present, call 999 immediately. Do not drive the person to A&E.

What should you do for heat exhaustion?

Move the person to a cool place immediately. Remove unnecessary clothing. Give them cool water or a rehydration drink. Cool their skin with cool — not cold — water, a damp cloth, or an ice pack wrapped in cloth applied to the neck and armpits. Fan them. Monitor for improvement. If they do not improve within 30 minutes, call 999.

When should you call 999 for heat illness?

Call 999 if the person loses consciousness, has a seizure, is confused and cannot be roused, has hot dry skin and is not sweating, or does not improve after 30 minutes of cooling and hydration. Call 111 if symptoms persist after 30 minutes but are not yet at emergency level. Do not drive the person to A&E — call 999 and follow the advice given.

Who is most at risk of heatstroke during a heatwave?

Children, older adults, and people with long-term health conditions including diabetes and heart disease are most at risk. People taking diuretics, antihistamines, antidepressants, or blood pressure medications have increased risk. People doing physical activity in the heat — even fit young adults — can develop exertional heatstroke rapidly.

Can heatstroke happen indoors?

Yes — heat exhaustion and heatstroke can occur indoors in poorly ventilated rooms, particularly in upper-floor rooms or rooms with south or west-facing windows during sustained heat. Elderly people who live alone in hot homes are among the most at risk of developing heat illness without anyone noticing. Checking on vulnerable neighbours and relatives during a heatwave can be life-saving.

The Bottom Line

Heat exhaustion is common during a UK heatwave and manageable at home — cool the person down, give them water, and monitor for improvement within 30 minutes. If they improve, no emergency help is needed. If they do not — or if they stop sweating, become confused, lose consciousness, or have a temperature of 40°C or above — call 999 immediately.

The line between heat exhaustion and heatstroke is crossed faster than most people expect. The 30-minute window, the absence of sweating, and the onset of confusion are the three signals that require immediate emergency response rather than continued first aid.

For the sleep disruption that hot nights produce alongside heat illness risk, read our full heatwave sleep guide: How to Sleep in a Heatwave: What Actually Works. Pair it with the Sleep Reset guide and your Reset Companion for personalised support through the hottest nights.

Related reading: How to Sleep in a Heatwave: What Actually Works · UK Melanoma Cases Hit a Record High. Here's What to Do About It. · Sun Protection: What SPF Numbers Actually Mean

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