Heat exhaustion and heatstroke - Causes of heat exhaustion and heatstroke

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