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

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.