Heatstroke occurs when the body is no longer able to cool itself and starts to overheat.

The human body is able to maintain a stable body temperature in a wide range of different environments. This is known as thermoregulation.

How the body cools itself

There are four ways that the body can cool itself. They are:

• heat radiating out of the body in the same way as it radiates out of a fire (radiation) 
• cold air or water crossing the skin and cooling the body (convection)
• a cooler object coming into direct contact with the skin, drawing out or conducting heat from the body (conduction)
• the body producing sweat that cools on the skin and lowers the body’s temperature (evaporation)

Normal skin temperature is 32-34°C (89.6-93.2°F). If the outside temperature is higher than your skin temperature, your body will produce sweat to help it cool down.

Anything that reduces the amount of sweat your body produces can cause it to overheat. Examples include:

• dehydration – where the body’s normal water content is reduced
• wearing tight-fitting clothing
• very hot and humid weather that doesn’t allow the sweat to cool

Types of heatstroke

There are two types of heatstroke. They are:

• classic heatstroke
• exertional heatstroke

Classic heatstroke usually affects the elderly, babies and people with long-term health conditions. It develops during unusually hot weather, such as during a heatwave.

Exertional heatstroke usually affects young, active people while they are doing strenuous, physical activity for a long time in hot conditions. It can affect:

• athletes
• military personnel
• fire-fighters

Vulnerable groups

Sweat is controlled by a part of the brain called the hypothalamus. The hypothalamus acts like a thermostat that tells the body to produce more sweat when it detects that it is getting hotter.

However, in some elderly people and people with long-term health problems, such as diabetes, the hypothalamus does not work as well as it should, putting these groups at increased risk of getting heat exhaustion and heatstroke.

Older women are thought to have a slightly higher risk of getting heatstroke than older men. This is because women have fewer sweat glands than men, and older women are more likely to live alone without immediate help available.

Babies and young children are also at risk because they sweat less and are more vulnerable to the effects of dehydration.

People who are unable to keep cool during a heatwave are also more at risk of getting heatstroke. This group may include:

• people with Alzheimer’s disease 
• people with a disability
• people who are bed-bound
• those who have had too much alcohol
• babies and young children

Read more about how to keep your baby safe during hot weather.

Drugs and medications

There are a number of drugs and medications that can interfere with the body’s ability to regulate temperature. It puts those who use them at increased risk of developing heat exhaustion and heatstroke. These include:

• amphetamines – stimulants such as speed and crystal meth
• cocaine
• antipsychotics – used to help treat mental health conditions such as schizophrenia
• benzodiazepines – a type of tranquiliser
• beta-blockers and calcium-channel blockers – used to treat high blood pressure (hypertension) and some types of heart disease
• diuretics – used to reduce the amount of fluid in your body

Heat exhaustion and heatstroke are two related health conditions that can be very serious if not treated quickly.

Heat exhaustion

Heat exhaustion is where a person experiences fatigue (extreme tiredness) as a result of a decrease in blood pressure and blood volume. It’s caused by a loss of body fluids and salts after being exposed to heat for a prolonged period of time. 

Someone with heat exhaustion may feel sick, faint and sweat heavily. Read more about the symptoms of heat exhaustion.

If a person with heat exhaustion is quickly taken to a cool place and is given water to drink, and if excess clothing is removed, they should start to feel better within half an hour and have no long-term complications. However, without treatment, they could develop heatstroke (see below).

Certain groups are more at risk of developing heatstroke or suffering complications from dehydration, and should be taken to hospital. These include: 

• children under two years of age 
• very elderly people
• people with kidney, heart or circulation problems
• people with diabetes who use insulin

Read more about treating heat exhaustion.

Heatstroke

Heatstroke is a more serious condition than heat exhaustion. It occurs when the body’s temperature becomes dangerously high due to excessive heat exposure. The body is no longer able to cool itself and starts to overheat.

Signs of heatstroke include dry skin, vertigo, confusion, headache, thirst, nausea, rapid shallow breathing (hyperventilation) and muscle cramps. Read more about the symptoms of heatstroke.

Suspected heatstroke should always be regarded as a medical emergency, and you should dial 999 to request an ambulance.

While waiting for the ambulance to arrive you should:

• immediately move the person to a cool area
• increase ventilation by opening windows or using a fan
• give water to drink (if the person is conscious), but don’t give them medication, such as aspirin or paracetamol
• shower their skin with cool, but not cold, water (15-18°C); alternatively, cover their body with cool, damp towels or sheets, or immerse them in cool water (not cold)

Read more about how to treat heatstroke.

Left untreated, heatstroke can lead to complications, such as brain damage and organ failure. It’s also possible to die from heatstroke.

Avoiding heat exhaustion and heatstroke

There are steps you can take to reduce the risk of getting heat exhaustion and heatstroke during a heatwave. These include:

• staying out of the sun during the hottest parts of the day, particularly between 11am and 3pm
• if you have to go out in the heat, walk in the shade, apply sunscreen and wear a hat
• do not leave anyone in a parked car
• avoid extreme physical exertion
• have plenty of cold drinks, but avoid drinks that contain caffeine and alcohol
• eat cold foods, particularly salads and fruit with a high water content
• take a cool shower, bath or body wash
• sprinkle water over your skin or clothing, or keep a damp cloth on the back of your neck
• keep your environment cool

Read more about preventing heatstroke and how to prepare for a heatwave.

In the UK, heat exhaustion and heatstroke usually only occur during periods of unusually hot weather, such as a heatwave.

The risk may be greater when travelling abroad in hot countries.

At home

Each year, the government operates Heat-Health Watch, a system of four different warning levels based on the expected temperatures. This can be useful when planning activities or preparing for a heatwave.

Public Health England (PHE) has also published a Heatwave plan for England (PDF, 1.15 MB), which provides advice about how to cope during a heatwave and recommendations for preventing heat-related illnesses. The main recommendations are outlined below.

Stay out of the sun 

Stay out of the sun during the hottest parts of the day, particularly 11am-3pm. However, it is important to remember that it can also be very hot before and after these times.

If you have to go out when it is hot, make sure you apply sunscreen, wear a sun hat and stay in the shade where possible.

Keep hydrated

Make sure you drink plenty of cold drinks when it is hot, but avoid drinks containing caffeine and alcohol. Eat cold foods, such as salad and fruit, which have a high water content.

Spraying water on to your skin or placing a damp cloth on the back of your neck will help keep you cool. You could also take a cool bath or shower.

Cool environment

Place a thermometer in your main living room and bedroom to keep a check on the temperature. If possible, move into a cooler room, particularly for sleeping.

Keep windows that are exposed to the sun closed during the day. Open windows at night when the temperature has dropped. Consider replacing metal blinds or dark curtains because they absorb heat.

Have your loft and cavity walls insulated. This will keep the heat in when it is cold and keep it out when it is hot. Use light coloured, reflective external paint.

Turn off non-essential lights and electrical equipment because they generate heat.

Placing bowls of water around your house will also help cool the air through evaporation.

Look out for others

During hot weather, keep an eye on people who are isolated, elderly, ill or very young. Make sure they are able to keep cool. Ensure they are not left alone in stationary cars during a heatwave. This advice also applies to animals.

In hot weather, check on elderly or sick neighbours and elderly family members or friends every day, if possible. Be alert and call a doctor or social services if someone is unwell or if further help is needed.

Travelling abroad

The above advice also applies if you are travelling abroad to a hot country.

It is important to remember that it takes the body 7-10 days to acclimatise to a hotter environment. Until that time, you will sweat less than you should. Drink water regularly, avoid staying in the heat too long and wear loose, light clothing. 

Athletes should acclimatise for three to four days before undertaking anything but light physical activity. Build up activity levels gradually over a two to three week period.

Alcohol is also an important factor to consider when travelling abroad. Drinking too much alcohol can affect your ability to judge temperature and move to somewhere cooler. It can also lead to dehydration, increasing the risk of heat exhaustion and heatstroke.

Heat exhaustion and heatstroke can be very serious if they are not treated quickly.

Heat exhaustion 

The symptoms of heat exhaustion can develop rapidly. They include:

• very hot skin that feels ‘flushed’
• heavy sweating 
• dizziness 
• extreme tiredness (fatigue)
• nausea (feeling sick) 
• vomiting
• a rapid heartbeat (tachycardia)
• confusion
• urinating less often and much darker urine than usual 

A person with heat exhaustion should be moved quickly to somewhere cool and given fluids, preferably water, to drink. They should start to feel better within half an hour.

However, certain groups of people are more at risk of getting heatstroke, or developing complications from dehydration, and should be taken to hospital. These include:

• children under two years of age 
• elderly people
• people with kidney, heart or circulation problems
• people with diabetes who use insulin

Read more about treating heat exhaustion.

Heatstroke

The symptoms of heatstroke can develop over several days in vulnerable people, such as the elderly and those with long-term health problems. These groups are particularly at risk during spells of hot weather.

Symptoms develop more quickly when associated with physical activity. This type of heatstroke, known as exertional heatstroke, usually affects young, active people.  

Symptoms of heatstroke include:

• high temperature – a temperature of 40°C (104°F) or above is one of the main signs of heatstroke (although it can be diagnosed at lower temperatures and some people can reach these temperatures during physical activity without developing heat exhaustion or heatstroke)
• heavy sweating that suddenly stops – if the body can’t produce any more sweat, the skin will become dry which is a major warning sign that the body has become over-heated and dehydrated
• a rapid heartbeat
• rapid breathing (hyperventilation)
• muscle cramps 

The extreme heat that causes heatstroke also affects the nervous system, which can cause other symptoms such as:

• confusion
• lack of co-ordination
• fits (seizures)
• headache
  
• vertigo (the sensation that you’re moving or spinning when standing still)
• restlessness or anxiety
• problems understanding or speaking to others
• seeing or hearing things that aren’t real (hallucinations) 
• loss of consciousness

Heatstroke is a medical emergency. Dial 999 immediately to request an ambulance if you think that you or someone you know has heatstroke.

While waiting for the ambulance to arrive, move the person somewhere cool and give them fluids to drink (preferably water). You could also cool their skin with water by placing a damp flannel or sheet on them or spraying them gently.

It’s better to wait for medical supervision before immersing someone fully in water because it could increase their blood pressure significantly (hypertensive response), which could be dangerous for those with cardiovascular disease or those at risk of stroke, such as the elderly.

Read more about treating heatstroke.

Heat exhaustion and heatstroke need to be treated quickly.

Heat exhaustion

If you suspect someone has heat exhaustion, follow the advice outlined below.

• Get them to rest in a cool place – ideally a room with air conditioning or, if this is not possible, somewhere in the shade.
• Get them to drink fluids – this should be water or a rehydration drink, such as a sports drink; they should stop taking fluid on board once their symptoms have significantly decreased (usually within 2-3 hours).
• Avoid alcohol or caffeine because they can increase levels of dehydration.
• Use cool water (not cold) on their skin – if available, use a cool shower or bath to cool them down, otherwise apply a cool, wet flannel or facecloth to their skin.
• Loosen clothing and ensure the person gets plenty of ventilation.

Dial 999 to request an ambulance if the person doesn’t respond to the above treatment within 30 minutes.

If the person is at increased risk of developing heatstroke or complications from dehydration, they should be taken to hospital. This group includes:

• children under two years of age
• elderly people
• people with kidney disease, heart failure or circulation problems
• people with diabetes who use insulin

Heatstroke

Always call an ambulance in cases of suspected heatstroke. While you are waiting for the ambulance to arrive, you should:

• move the person to a cool area as quickly as possible
• increase ventilation by opening windows or using a fan
• give them water to drink (if they are conscious), but do not give them medication, such as aspirin or paracetamol
• shower their skin with cool, but not cold, water (15-18°C); alternatively, cover their body with cool, damp towels or sheets or immerse them in cool water (not cold)
• wait for medical supervision to arrive before fully immersing the person in water because the body’s response could cause them harm
• gently massage their skin to encourage circulation
• if they have a seizure (fit), move nearby objects out of the way to prevent injury (do not use force or put anything in their mouth)
• if they are unconscious and vomiting, move them into the recovery position by turning them on their side and ensuring their airways are clear

Hospital treatment

After a person with heatstroke has been admitted to hospital, the most important treatment aim is to lower their temperature as quickly as possible. This can be done in two ways:

• immersing their body in an ice bath
• spraying their body with a mist of cool water while warm air is fanned over the body; the combination of cool water and warm air encourages rapid heat loss through evaporation