Heat Exhaustion and Heatstroke

Authored by Dr Sarah Jarvis MBE, Reviewed by Dr Hayley Willacy on | Certified by The Information Standard

The risks of excess sun exposure to the skin are well known. Too much sun increases the risk of skin cancer, and burning carries a particularly high risk for malignant melanoma.

But in the short term, excess heat also carries a risk of heat exhaustion which, if untreated, can lead to life-threatening heatstroke.

If you are exposed to too much heat, your body works hard to keep your temperature constant. This can lead to you feeling generally unwell, lacking in energy and feeling dizzy or sick. Heatstroke happens when your body's normal mechanisms for regulating your temperature break down, and your temperature rises to more than 40°C. Heat exhaustion should be treated quickly to prevent it turning into heatstroke, which is a medical emergency.

Anyone can experience the physical effects of excess heat, but people at highest risk include:

  • Babies and toddlers.
  • Elderly people.
  • People who are physically active.
  • People with chronic health conditions like diabetes or heart, lung or kidney disease.
  • People taking 'water tablets' (diuretics - usually used for high blood pressure or heart failure), antipsychotic medication or recreational drugs like ecstasy
  • People with tummy troubles like gastroenteritis or inflammatory bowel conditions like Crohn's disease or ulcerative colitis that can cause diarrhoea.

Your body's internal processes work best when levels of water, body salts and temperature are just right. Your body has many mechanisms to keep your internal environment constant, regardless of what's going on outside. These processes are known as homeostasis.

So when you become hot, your body's protection mechanisms kick in. You sweat to lose heat; your urine becomes more concentrated to preserve water; and your body tells you that you need to drink more and get out of the sun. If you stay in the heat, your body starts to struggle.

Key signs and symptoms of heat exhaustion include:

  • Passing less urine (which is very dark).
  • Dizziness and feeling faint.
  • Tiredness.
  • Headache.
  • Feeling sick, lack of appetite, and cramping stomach pain.
  • Muscle cramps in legs and arms.
  • Rapid pulse.
  • Clammy, pale skin and profuse sweating.

Fluid and a cool environment are key to treating heat exhaustion. If you think you're showing signs, immediately:

  • Move to a cool room.
  • Take off as much clothing as possible, particularly tight clothes.
  • Take a cool bath or shower.
  • If there are no bathing facilities, sponge down with cool water.
  • Put on a fan if possible (if your skin is damp, the evaporation will cool you down quicker).
  • Lie down if possible.
  • Drink as much non-alcoholic fluid as you can.
  • Seek medical help if you're not feeling better within about half an hour.

Sunstroke is a form of heatstroke. The only different is that it's caused specifically by being exposed to too much direct sunlight. Symptoms and treatment are exactly the same as for heatstroke - they're both medical emergencies.

The only added issue is that there's a risk of severe sunburn on top of other complications. Blistering of the skin is a sign of severe sunburn - sponge the skin gently with cool clean water while waiting for medical help.

In heatstroke, your body can no longer cope. The normal mechanisms to keep your body cool give up, and your temperature soars to over 40°C. Other symptoms include:

  • Severe, throbbing headache.
  • Feeling very sick and being sick (vomiting).
  • Fast pulse (which can be weak or very strong and 'bounding' along).
  • Shallow, rapid breathing.
  • Very hot, flushed skin.
  • Weak or cramping muscles.
  • No sweating despite being very hot.
  • Dizziness, feeling faint or fainting.
  • Confusion and agitation.
  • Seizures.
  • Sometimes collapse and loss of consciousness.

Left untreated, heatstroke can lead to swelling of your vital organs, including your brain.

Heatstroke is a medical emergency and can cause serious damage to kidneys and other vital organs if not treated promptly. Someone experiencing heatstroke is unlikely to be able to treat themself. If you think someone has heatstroke, immediately:

  • Dial 999/112/911 for an ambulance. While waiting:
    • Move them to a cool place and lay them down.
    • Stay with them at all times.
    • Remove as much of their clothing as possible.
    • Wet their skin with cool water; then fan them.
    • If possible, put them in a cool bath or shower.
    • For a young, healthy person who has developed heatstroke from vigorous exercise in hot weather, consider applying ice packs to their armpits, neck and back.
    • Put them in the recovery position if they lose consciousness.

Heat exhaustion often occurs as a result of direct sun exposure. To find out more about avoiding harm from excess sun exposure, see the separate leaflet called Sun and Health.

However, it is important to remember that it is possible to develop heat exhaustion even when you are not in direct sunlight. To avoid damage from heat as well as sun:

  • Keep your fluid intake up. Caffeinated drinks in moderation can contribute to your fluid intake - about 400 mg a day in total (eight cups of tea or four cups of coffee in total). However, you should avoid drinking more caffeinated drinks than this, and top up with water.
  • Do not drink alcohol, which dehydrates you and reduces your awareness of the warning symptoms. It also impairs your ability to take early steps to move yourself to somewhere cooler and take steps to avoid heat exhaustion turning into heatstroke.
  • Avoid strenuous exercise in hot weather (especially between 11 am and 3 pm, when temperatures are usually highest).
  • Wear loose clothes made from natural fibres such as cotton or linen, which allow your body to evaporate heat readily by sweating.
  • Keep your home cool by pulling curtains in daytime. If the weather outside is hotter than inside, keep windows closed and use a fan. When the weather outside cools down, particularly at night, keep windows open.
  • Take regular cool showers or baths.
  • If you are feeling hot, put a damp flannel on the back of your neck and keep changing it. As the water evaporates from your skin, it will cool you down.
  • Remember that south-facing rooms with windows in direct sunlight are hotter than north-facing ones.
  • Keep indoor plants which help cool the air as they evaporate water.

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