Heat exhaustion and heatstroke

Page last reviewed: 13/07/2011

Heat exhaustion and heatstroke are two heat-related health conditions. Both can be very serious.

Heat exhaustion

Heat exhaustion is when the temperature inside the body, known as the core temperature, rises to 37-40°C (98.6-104°F).

At that temperature, the levels of water and salt in the body begin to drop. This causes symptoms such as nausea, feeling faint and heavy sweating.

If left untreated, heat exhaustion can sometimes lead to heatstroke.


Heatstroke happens when a person's core temperature rises above 40°C (104°F). Cells inside the body begin to break down and important parts of the body stop working.

Symptoms of heatstroke can include mental confusion, hyperventilation (rapid shallow breathing) and loss of consciousness.

Heatstroke is a medical emergency. If left untreated, it can cause multiple organ failure, brain damage and death.

Types of heatstroke

There are two main types of heatstroke:

  • classic heatstroke, and
  • exertional heatstroke.

Classic heatstroke

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

Exertional heatstroke

Exertional heatstroke usually affects young, active people doing strenuous physical activity for a long time in the heat. For example, cases of exertional heatstroke have occurred in:

  • athletes,
  • people serving in the military, and
  • fire-fighters.

How common are heat exhaustion and heatstroke?

It is hard to tell exactly how common heat exhaustion is, as many people who get it do not go to their GP for treatment.

Heatstroke is rare in Ireland, but during a heatwave the death rate among the elderly is much higher than usual.


If a person with heat exhaustion is taken quickly to a cool place and given plenty of water to drink, they should begin to feel better within half an hour and experience no long-term complications. Without treatment, they could develop heatstroke.

Heatstroke is very serious and should be treated immediately. Treatment involves quickly cooling down the body to lower the core temperature.

If treated swiftly, 90% of people with heatstroke survive. If not, the survival rate is as low as 20% among vulnerable people such as the elderly.

Dehydration is when too many fluids and minerals are lost from the body.

Page last reviewed: 13/07/2011

Heat exhaustion  

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

  • your skin feeling very hot and flushed,
  • heavy sweating,
  • dizziness,
  • fatigue,
  • nausea,
  • vomiting,
  • tachycardia (a rapid heartbeat), 
  • mental confusion, and
  • urinating less often and the colour of your urine being much darker than usual.


The symptoms of classic heatstroke can develop over several days if you are spending a long time somewhere hot. The symptoms of exertional heatstroke can appear more quickly, usually after physical activity.

Symptoms of heatstroke include:

  • high body temperature: having a temperature of 40°C (104°F) or above is a major sign of heatstroke,
  • heavy sweating that suddenly stops: if the body is unable to produce any more sweat then this is a big warning sign that the body has become over-heated and dehydrated,
  • tachycardia (a rapid heartbeat),
  • hyperventilation (rapid breathing), and
  • muscle cramps.

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

  • mental confusion,
  • lack of co-ordination,
  • seizures (fits),
  • restlessness or anxiety,
  • problems understanding or speaking to others,
  • hallucinations (seeing or hearing things that are not real),
  • loss of consciousness.

Heatstroke is a medical emergency. If you are worried that you or someone you know may have heatstroke symptoms, dial 999 immediately and ask for an ambulance.


High temperature
A high temperature is when your body temperature goes above the normal 37°C (98.6°F). Also known as a fever.
Loss of appetite
Loss of appetite is when you do not feel hungry or want to eat.
Nausea is when you feel like you are going to be sick.
Your stomach is an organ in your digestive system that helps digest food by mixing it with acids to break it down into smaller pieces.
Vomiting is when you bring up the contents of your stomach through your mouth.
A hallucination is when you are seeing, hearing or feeling something or someone that isn't really there.

Page last reviewed: 13/07/2011

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

There are four ways that the body can cool itself:

  • Radiation: heat comes directly out of the body in the same way as heat comes out of a fire.
  • Convection: cold air or water crossing the skin can cool the body.
  • Conduction: a cooler object that is in direct contact with the skin can draw out, or conduct, heat from the body.
  • Evaporation: the body produces sweat that then cools on the skin, lowering the temperature of the body.

Your skin temperature ranges from around 32 to 34°C (89.6 to 93.2°F). Once the outside temperature rises higher than your skin temperature, the only way your body can cool itself is through sweating.

So anything that reduces sweat can cause the body to overheat. Examples include:

  • dehydration,
  • wearing tight-fighting clothing, or
  • when the weather is very hot and humid, which does not allow the sweat to cool.

Vulnerable groups

Your sweat is controlled by part of the brain called the hypothalamus. The hypothalamus acts like a thermostat, producing more sweat when it detects that the body is getting hotter.

In some elderly people and people with chronic health problems such as diabetes, the hypothalamus does not work as well as it should. These people are more at risk of heat exhaustion and heatstroke.

Babies and young children are also more at risk because they sweat less.

Drugs and medications

There are a number of different drugs and medications that can interfere with the body's ability to regulate its temperature and so make people more vulnerable to heat exhaustion and heatstroke. These include:

  • amphetamines,
  • cocaine,
  • alcohol,
  • antipsychotics (medication 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 and some types of heart disease), and
  • diuretics (used to reduce the amount of fluid in your body).


Diuretic medicine increases the production and flow of urine out of the body. It is used to remove excess fluid from the body.
Chronic usually means a condition that continues for a long time or keeps coming back.
Thyroid gland
The thyroid gland in the throat makes hormones to help control growth and metabolism (the process that turns the food we eat into energy).
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
The heart is a muscular organ that pumps blood around the body.
Dehydration is when too many fluids and minerals are lost from the body.

Page last reviewed: 13/07/2011

Diagnosing both heat exhaustion and heatstroke can be done by reviewing the symptoms you have and taking your temperature.

In cases of heatstroke, you may need further testing to see if your body has experienced any more damage. These tests can include:

  • Blood tests, to see if damage to your nervous system has lead to gases and acids being released into your blood. Blood tests can also be used to check the state of your liver.
  • A urine test, to see if your kidneys have been damaged.
  • Imaging tests, such as MRI or CT scans, to see if any other organs have been damaged.


Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
A hallucination is when you are seeing, hearing or feeling something or someone that isn't really there.

Page last reviewed: 13/07/2011

At home

Heat exhaustion and heatstroke usually only happen in Ireland during unusually hot weather such as a heatwave. You should take notice of the following advice on how to prevent heat-related illnesses during a heatwave.

Stay out of the heat

  • Keep out of the sun between 11am and 3pm.
  • If you have to go out in the heat, walk in the shade, apply sunscreen and wear a hat and light scarf.
  • Avoid extreme physical exertion.
  • Wear light, loose-fitting cotton clothes.

Cool yourself down

  • Have plenty of cold drinks, but no caffeine and alcohol.
  • Eat cold foods, particularly salads and fruits with a high water content.
  • Take a cool shower, bath or body wash.
  • Sprinkle water over the skin or clothing, or keep a damp cloth on the back of your neck.

Keep your environment cool

  • Place a thermometer in your main living room and bedroom to keep a check on the temperature.
  • Keep windows that are exposed to the sun closed during the day, and open windows at night when the temperature has dropped.
  • Care should be taken with metal blinds and dark curtains, as these can absorb heat. Consider replacing or putting reflective material in between them and the window space.
  • Consider putting up external shading outside windows.
  • Have your loft and cavity walls insulated. This keeps the heat in when it is cold and out when it is hot.
  • Use pale, reflective external paints.
  • Turn off non-essential lights and electrical equipment, as they generate more heat.
  • Grow trees and leafy plants near windows to act as natural air conditioners.
  • Keep indoor plants and bowls of water in the house, as evaporation helps cool the air.
  • If possible, move into a cooler room, especially for sleeping.

Look out for others

  • Keep an eye on isolated, elderly, ill or very young people and make sure they are able to keep cool.
  • Ensure that babies, children or elderly people are not left alone in parked cars.
  • Check on elderly or sick neighbours, family or friends every day during a heatwave.
  • Be alert and call a doctor if someone is unwell or further help is needed.

Travelling abroad

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

Another important thing to remember is that it takes the body between a week and 10 days to adjust to a hotter environment. Before that time, you will sweat less than you should do. So until then, you should avoid doing any strenuous physical activity, even if you are very fit and healthy.


Dehydration is when too many fluids and minerals from the body are lost.

Page last reviewed: 13/07/2011

Heat exhaustion

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

  • Get them to rest in a cool place, ideally in a room with air conditioning, or at least somewhere that is in the shade.
  • Give them plenty of fluids to drink. This should either be water or a rehydration drink such as a sports drink. Avoid alcohol or caffeine as this can increase dehydration.
  • Cool their skin with cold water. If available, use a shower or cold bath to cool them down. If not, then apply wet flannels to their skin.
  • Loosen any unnecessary clothing, and make sure that the person gets plenty of air.

If the person does not respond to this treatment within half an hour, call 999 and ask for an ambulance.


First aid

You should 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 air supply by opening windows or using a fan.
  • Give them water to drink if possible, but do not give medication such as aspirin or paracetamol.
  • Shower the skin with cool, but not cold, water (15°C-18°C). If there is no shower nearby, cover the body with cool, damp towels or sheets, or immerse in cool water.
  • Gently massage the skin to encourage circulation.
  • If seizures (fits) start, move nearby objects out of the way to prevent injury (do not use force or put anything in the mouth).
  • If the person is unconscious and vomiting, move them into the recovery position by turning them on their side and making sure their airways are clear and they can breathe as easily as possible.

Hospital treatment

Once a person has been admitted to hospital the most important thing is to lower their temperature as quickly as possible.

This can be done in two ways:

  • immersing their body in an ice bath, or
  • 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 will encourage rapid heat loss through evaporation.          


Oxygen is gas with no colour or smell. It makes up about 20% of the air we breathe.
Vomiting is when you bring up the contents of your stomach through your mouth.
Intravenous (IV) is the injection of blood, drugs or fluids into the bloodstream through a vein.
A drip is used to pass fluid or blood into your bloodstream, through a plastic tube and needle that goes into one of your arteries or veins.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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