Heat exhaustion causes heavy sweating, nausea and weakness but usually resolves with cooling and fluids within about 30 minutes. Heat stroke is a medical emergency marked by a very high body temperature, hot skin and confusion, and it requires calling emergency services immediately, according to the US CDC, the UK's NHS and the World Health Organization.
As temperatures rise, health authorities repeat the same warning every summer: know the difference between heat exhaustion and heat stroke, because the second one can kill within minutes if untreated. The US Centers for Disease Control and Prevention (CDC), the UK's National Health Service (NHS) and the World Health Organization (WHO) publish near-identical guidance on recognizing and preventing both conditions.
Why this matters
The WHO estimates that roughly 489,000 heat-related deaths occurred each year between 2000 and 2019, with 45% of them in Asia and 36% in Europe. The organization also says heat-related mortality among people over 65 rose by about 85% between the periods 2000–2004 and 2017–2021. A single European summer, 2022, was linked to an estimated 61,672 excess deaths, according to WHO.
Heat exhaustion vs. heat stroke
Both conditions start when the body cannot cool itself fast enough, but they differ in severity and in what to do about them, according to the CDC's National Institute for Occupational Safety and Health (NIOSH) and the NHS.
Comparing heat exhaustion and heat stroke
Feature
Heat exhaustion
Heat stroke
Skin
Heavy sweating, pale and clammy skin (NHS)
Hot, dry skin, or profuse sweating; skin may look red (NHS, CDC/NIOSH)
Fast heartbeat, fast breathing, seizures, loss of consciousness (NHS, CDC/NIOSH)
Recommended response
Cool the person down; should recover within about 30 minutes (NHS)
Emergency medical care is required immediately (NHS: call 999; CDC: call 911)
What to do for heat exhaustion
The NHS describes four steps to take if someone shows signs of heat exhaustion, which should help them feel better within roughly 30 minutes.
Move the person to a cool place.
Remove unnecessary clothing such as a jacket or socks.
Give them plenty of water to drink; isotonic sports drinks are also suitable, per the NHS.
Cool their skin by spraying or sponging them with cool water, and fan them.
If the person does not improve within 30 minutes of these steps, the NHS says to call 999 (or the local emergency number), because this may indicate heat stroke.
What to do for heat stroke
Heat stroke is described by both the NHS and CDC/NIOSH as a medical emergency. The NHS instructs people to call 999 immediately if someone has a very high temperature, hot skin that is not sweating, confusion, a seizure, or loses consciousness. While waiting for help, the NHS recommends wrapping the person in a cool, wet sheet or fanning and sponging them with cold water, and placing them in the recovery position if they become unconscious, checking breathing and pulse, and starting CPR if there are no signs of life. CDC/NIOSH similarly advises moving the person to shade, removing outer clothing, and cooling them rapidly with ice baths, wet cloths or cold compresses applied to the head, neck, armpits and groin while waiting for emergency responders.
Heatstroke is a medical emergency.
— NHS, heat exhaustion and heatstroke guidance
Who is most at risk
The NHS lists children, older people, and people with long-term health conditions such as diabetes or heart problems as being at higher risk. The WHO adds outdoor and manual workers, people living in inadequate housing, and infants to this list. The CDC's broader climate-health guidance also names people aged 65 and older and those with chronic medical conditions among those most vulnerable to extreme heat.
Prevention
Stay in an air-conditioned space as much as possible during hot weather (CDC).
Drink fluids regularly rather than waiting to feel thirsty — the WHO suggests about one cup of water per hour and at least 2–3 litres a day in heat.
Avoid being outdoors in strong sun between 11 a.m. and 3 p.m., per NHS advice.
Limit alcohol consumption and strenuous exercise during very hot periods (NHS).
The WHO advises that, where air conditioning is used, setting the thermostat to 27°C and also running an electric fan can make a room feel about 4°C cooler; it adds that fans should only be used when temperatures are below 40°C, since above that they can heat the body instead.
Never leave children or pets inside parked cars, the CDC warns.
None of this guidance replaces direct medical advice. Anyone showing signs of heat stroke should receive emergency care immediately, and anyone with heat exhaustion that does not improve within 30 minutes of cooling and fluids should also seek emergency help, according to the NHS.
Questions
What is the main difference between heat exhaustion and heat stroke?
Heat exhaustion causes heavy sweating, nausea and dizziness but the skin stays pale and clammy; it usually improves within 30 minutes of cooling and fluids. Heat stroke involves a very high body temperature, hot skin that may stop sweating, confusion and possible loss of consciousness, and is a medical emergency, according to the NHS and CDC/NIOSH.
What should I do if someone has heat exhaustion?
The NHS recommends moving the person to a cool place, removing unnecessary clothing, giving them water or an isotonic sports drink, and cooling their skin with cool water and fanning. If they do not improve within 30 minutes, call emergency services.
When should I call emergency services for heat-related illness?
Call emergency services (999 in the UK, 911 in the US) immediately if someone has a very high temperature, hot skin without sweating, confusion, a seizure or loses consciousness — signs of heat stroke — or if heat exhaustion symptoms do not improve after 30 minutes of cooling, per the NHS.
Who is most at risk of heat-related illness?
The NHS and WHO identify children, older adults, people with chronic conditions such as diabetes or heart disease, outdoor and manual workers, and people without access to cooling or adequate housing as most at risk.
How can heat-related illness be prevented?
The CDC, NHS and WHO recommend staying in cool or air-conditioned spaces during peak heat, drinking fluids regularly, wearing light loose clothing, avoiding strenuous activity in the hottest part of the day, and never leaving children or pets in parked cars.
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