Information and Resources

What Is Hypothermia?

Hypothermia is a potentially dangerous drop in body temperature, usually caused by prolonged exposure to cold temperatures. The risk of cold exposure increases as the winter months arrive. But if you're exposed to cold temperatures on a spring hike or capsized on a summer sail, you can also be at risk of hypothermia.

Normal body temperature averages 98.6 degrees. With hypothermia, core temperature drops below 95 degrees.In severe hypothermia, core body temperature drops to 86 degrees or lower.

What Causes Hypothermia?

The causes of hypothermia include:

Cold exposure. When the balance between the body's heat production and heat loss tips toward heat loss for a prolonged period, hypothermia can occur. Accidental hypothermia usually happens after cold temperature exposure without enough warm, dry clothing for protection. Mountain climbers on Mount Everest avoid hypothermia by wearing specialized, high-tech gear designed for that windy, icy environment.

However, much milder environments can also lead to hypothermia, depending on a person's age, body mass, body fat, overall health, and length of time exposed to cold temperatures. A frail, older adult in a 60-degree house after a power outage can develop mild hypothermia overnight. Infants and babies sleeping in cold bedrooms are also at risk.

Other causes. Certain medical conditions such as diabetes and thyroid conditions, some medications, severe trauma, or using drugs or alcohol all increase the risk of hypothermia.

How Does Cold Exposure Cause Hypothermia?

During exposure to cold temperatures, most heat loss -- up to 90% -- escapes through your skin; the rest, you exhale from your lungs. Heat loss through the skin happens primarily through radiation and speeds up when skin is exposed to wind or moisture. If cold exposure is due to being immersed in cold water, the movement of waves and water can increase heat loss up to 50%.

The hypothalamus, the brain's temperature-control center, works to raise body temperature by triggering processes that heat and cool the body. During cold temperature exposure, shivering is a protective response to produce heat through muscle activity. In another heat-preserving response -- called vasoconstriction -- blood vessels temporarily narrow.

Normally, the activity of the heart and liver produce most of your body heat. But as core body temperature cools, these organs produce less heat, in essence causing a protective "shut down" to preserve heat and protect the brain. Low body temperature can slow brain activity, breathing, and heart rate.

Confusion and fatigue can set in, hampering a person's ability to understand what's happening and make intelligent choices to get to safety.