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Heat Stroke

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:
people serving in the military, and

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

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.

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