The skin and the tissues under it are kept at a constant temperature (about 98.6° F, or 37°C) by the circulating blood and other mechanisms. The blood gets its heat mainly from the energy given off by cells when they burn (metabolize) food—a process that requires a steady supply of food and oxygen. A normal body temperature is necessary for proper functioning of all the cells and tissues in the body. In a person with low body temperature, most organs, especially the heart and brain, become sluggish and eventually stop working.

Body temperature falls when the skin is exposed to colder surroundings. In response to this fall in temperature, the body uses several protective mechanisms to generate additional heat. For example, the muscles produce additional heat through shivering. Also, the small blood vessels in the skin narrow (constrict), so that more blood is diverted to vital organs, such as the heart and brain. However, as less warm blood reaches the skin, body parts such as the fingers, toes, ears, and nose cool more rapidly. If body temperature falls much below about 88° F (about 31° C), these protective mechanisms stop working, and the body cannot warm itself. If body temperature falls below 83° F (about 28° C), death is likely.

Cold injuries are less likely to occur, even in extremely cold weather, if the skin, fingers, toes, ears, and nose are well protected or are exposed only briefly. The risk of cold injuries increases in the following circumstances:

• When the flow of blood is too slow
• When food intake is inadequate
• When dehydration or exhaustion occur
• When the environment is wet or when a body part contacts something wet
• When the person comes into contact with a metal surface
• When insufficient oxygen is available, as occurs at high altitude

Keeping warm in a cold environment requires several layers of clothing—preferably wool or synthetics such as polypropylene, because these materials insulate even when wet. Because the body loses a large amount of heat from the head, a warm hat is essential. Eating enough food and drinking enough fluids (particularly warm fluids) also help. Food provides fuel to be burned, and warm fluids directly provide heat and prevent dehydration. Alcoholic beverages should be avoided, because alcohol widens (dilates) blood vessels in the skin, which makes the body temporarily feel warm but actually causes greater heat loss.

Cold injuries include hypothermia, frostnip, chilblains, immersion foot, and frostbite. Other problems related to the cold include Raynaud syndrome (see Peripheral Arterial Disease: Raynaud's Syndrome) and allergic reactions to the cold (see Allergic Reactions and Other Hypersensitivity Disorders: Physical Allergy).

Did You Know... Drinking alcoholic beverages actually makes the body colder because the widening of blood vessels that makes a person feel warm allows more heat to escape from the body.

Last full review/revision September 2012 by Daniel F. Danzl, MD