Hypothermia - Causes of hypothermia

Hypothermia is caused by getting too cold as the body loses more heat than it can generate and the body temperature drops below 35°C (95°F).

There are different types of hypothermia depending on how quickly the body loses heat. The three types are described by doctors as:

acute or immersion hypothermia, which happens when a person loses heat very rapidly – for example, after falling into cold water
exhaustion hypothermia – this happens when a person’s body is so tired it can no longer generate heat
chronic hypothermia – heat is lost slowly over time; this is common in elderly people who live in poorly heated accommodation or in people sleeping rough

Hypothermia is most common in cold environments. You're more at risk if you don't wear enough layers to keep warm or you don't cover your head (a large amount of body heat is lost through your head).

It's also possible to get hypothermia in mild weather. For example, if you're soaked in the rain and don't dry off properly soon afterwards (particularly if there is a cool wind), the water evaporates from your skin and lowers your body temperature.

Who's at risk?

Some people are at an increased risk of getting hypothermia because they're vulnerable to cold environments or they're unable to keep warm. These include:

Babies can lose heat quickly if they're left in a cold room because they can't regulate their body temperature as well as older children and adults. Newborn babies in particular are at risk for the first 12 hours of their life.
Older people, particularly if they're not very active, do not eat enough, have other illnesses or take medication that can interfere with the body's ability to regulate temperature.
Homeless people who are unable to find shelter.
Heavy drug and/or alcohol users – these substances affect the body's ability to retain heat. The blood vessels stay widened (dilated), allowing heat to escape. Someone who is under the influence of drugs or alcohol may also not realise they have hypothermia.
People with a condition that affects their memory, such as Alzheimer's disease, may not be able to recognise the symptoms of hypothermia or recognise when they're cold.
People with certain health conditions, such as heart problems, severe arthritis or someone who has had a stroke. These conditions can change the body's ability to respond to temperature changes – for example, by affecting the fingers and toes (where you may first feel cold).
People taking sedatives, which can interfere with the body's ability to regulate temperature.
Someone who has fallen into cold water, which can cause the body's core temperature to fall very quickly.
People who spend long periods in cold weather conditions, such as climbers, walkers and skiers.
Someone who has had a severe injury, particularly a head injury.

Perioperative hypothermia

It is also sometimes possible for hypothermia to occur during a stay in hospital - particularly before, during and after an operation. This is known as perioperative hypothermia.

Hospital staff will try to ensure you stay warm during your stay in hospital. They will monitor your temperature and may use a special blanket into which warm air is blown to help stop you getting too cold. This is called ‘forced air warming’.

You should tell staff if you feel cold at any time during your stay in hospital.

You can read the NICE guidelines for more information about Keeping patients warm before,during and after an operation (PDF, 207kb).

Therapeutic hypothermia

In some cases, medical professionals may deliberately make someone develop hypothermia as a treatment. This is known as therapeutic hypothermia.

There's evidence to suggest that, in some circumstances, inducing a state of hypothermia in the body can reduce the risk of death and increase the chances of a good recovery.

This group of people includes those who have suffered a cardiac arrest due to a heart attack outside of hospital, but who have been successfully resuscitated and are in an intensive care unit.