Symptoms and causes

Shivering is likely the first thing you'll notice as the temperature starts to drop because it's your body's automatic defense against cold temperature — an attempt to warm itself.

Signs and symptoms of hypothermia include:

  • Shivering
  • Slurred speech or mumbling
  • Slow, shallow breathing
  • Weak pulse
  • Clumsiness or lack of coordination
  • Drowsiness or very low energy
  • Confusion or memory loss
  • Loss of consciousness
  • Bright red, cold skin (in infants)

Someone with hypothermia usually isn't aware of his or her condition because the symptoms often begin gradually. Also, the confused thinking associated with hypothermia prevents self-awareness. The confused thinking can also lead to risk-taking behavior.

When to see a doctor

Call 911 or your local emergency number if you suspect someone has hypothermia.

While you wait for emergency help to arrive, gently move the person inside if possible. Jarring movements can trigger dangerous irregular heartbeats. Carefully remove his or her wet clothing, replacing it with warm, dry coats or blankets.

Hypothermia occurs when your body loses heat faster than it produces it. The most common causes of hypothermia are exposure to cold-weather conditions or cold water. But prolonged exposure to any environment colder than your body can lead to hypothermia if you aren't dressed appropriately or can't control the conditions.

Specific conditions leading to hypothermia include:

  • Wearing clothes that aren't warm enough for weather conditions
  • Staying out in the cold too long
  • Being unable to get out of wet clothes or move to a warm, dry location
  • Falling into the water, as in a boating accident
  • Living in a house that's too cold, either from poor heating or too much air conditioning

How your body loses heat

The mechanisms of heat loss from your body include the following:

  • Radiated heat. Most heat loss is due to heat radiated from unprotected surfaces of your body.
  • Direct contact. If you're in direct contact with something very cold, such as cold water or the cold ground, heat is conducted away from your body. Because water is very good at transferring heat from your body, body heat is lost much faster in cold water than in cold air. Similarly, heat loss from your body is much faster if your clothes are wet, as when you're caught out in the rain.
  • Wind. Wind removes body heat by carrying away the thin layer of warm air at the surface of your skin. A wind chill factor is important in causing heat loss.

Risk factors for hypothermia include:

  • Exhaustion. Your tolerance for cold diminishes when you are fatigued.
  • Older age. The body's ability to regulate temperature and to sense cold may lessen with age. And some older adults may not be able to communicate when they are cold or to move to a warm location if they do feel cold.
  • Very young age. Children lose heat faster than adults do. Children may also ignore the cold because they're having too much fun to think about it. And they may not have the judgment to dress properly in cold weather or to get out of the cold when they should.
  • Mental problems. People with a mental illness, dementia or other conditions that interfere with judgment may not dress appropriately for the weather or understand the risk of cold weather. People with dementia may wander from home or get lost easily, making them more likely to be stranded outside in cold or wet weather.
  • Alcohol and drug use. Alcohol may make your body feel warm inside, but it causes your blood vessels to expand, resulting in more rapid heat loss from the surface of your skin. The body's natural shivering response is diminished in people who've been drinking alcohol. In addition, the use of alcohol or recreational drugs can affect your judgment about the need to get inside or wear warm clothes in cold weather conditions. If a person is intoxicated and passes out in cold weather, he or she is likely to develop hypothermia.
  • Certain medical conditions. Some health disorders affect your body's ability to regulate body temperature. Examples include an underactive thyroid (hypothyroidism), poor nutrition or anorexia nervosa, diabetes, stroke, severe arthritis, Parkinson's disease, trauma, and spinal cord injuries.
  • Medications. Some drugs can change the body's ability to regulate its temperature. Examples include certain antidepressants, antipsychotics, narcotic pain medications and sedatives.

People who develop hypothermia because of exposure to cold weather or cold water are also vulnerable to other cold-related injuries, including:

  • Freezing of body tissues (frostbite)
  • Decay and death of tissue resulting from an interruption in blood flow (gangrene)