Frostbite is the freezing of body tissues (skin, muscle, bone) in extreme cold. At or below 59 degrees Fahrenheit, blood vessels close to the skin start to narrow (constrict). This helps to preserve your core body temperature. In extreme cold or when the body is exposed to cold for long periods, this protective strategy can reduce blood flow in some areas of your body to dangerously low levels. The combination of cold temperature and poor blood flow can cause tissue injury. Frostbite is most likely to happen in body parts farthest from the heart, and those with a lot of surface area exposed to cold. These areas include the toes, fingers, ears and nose.
Body tissue will not freeze until the outside temperature is at or below 28 degrees Fahrenheit. If areas of tissue exposed to extreme cold begin to freeze, ice crystals form in some cells and fluid flows into these cells. This can cause the cells to burst. Additional damage can occur when the tissue is warmed again, because damaged blood vessels can leak fluid and proteins into tissue, causing swelling and blistering.
Frostbite ranges from the superficial freezing of the topmost layers of skin, which is called frostnip, to severe frostbite that affects deeper tissues, such as muscles and bones. The amount of damage depends on several factors besides the cold temperature, including altitude, wind chill, blood circulation and body composition. Factors that increase your risk of frostbite include:
Impaired thinking (from psychiatric problems, medical illness or substance abuse)
Older age, especially if you already have circulation problems
Contact with metal
Large areas of exposed skin
Previous frostbite or other injury caused by cold
A low percentage of body fat
Wearing tight clothes, which impairs circulation
Drinking alcohol, which increases loss of body heat
Drinking caffeine, which increases dehydration
Using nicotine, which decreases blood flow to your limbs
Poorly controlled diabetes