When temperatures drop below freezing, frostbite can occur.

Frostbite produces tissue damage in two ways. The most obvious involves actual freezing of the tissues. Ice crystals form between the tissue cells and get larger by extracting water from them. This cell dehydration and resultant chemical imbalances cause damage, but not as much as the second way. If ice crystals formed within the cells, much more damage would occur, but this usually does not happen with frostbite.The second way frostbite injures is much more significant. This involves obstruction of the blood supply to the tissues. The blood cells "sludge" within the capillaries and small veins. These sludged blood clots obstruct the blood flow to the tissues. This obstruction by sludging and clotting produces more irreversible tissue damage than freezing does.

Frostbite is almost always limited to the feet, hands, ears or nose. These areas do not contain large heat-producing muscle and are some distance from the heat generation sources. Moreover, when the body conserves heat, a reduction of blood supply happens in these areas first.

The toes represent the most common site of frostbite. Factors contributing to frostbite of the hands and feet by reducing the blood supply are hypothermia (which causes blood vessel constriction) and constricting clothing (for instance, boots that are too tight).

Contact with cold metal (i.e., door handles) can produce rapid and severe frostbite. Even more severe injuries can occur through contact with gasoline or solvents left outdoors in sub-freezing temperatures.

ategories of frostbite

After thawing, frostbite can be divided into four categories or degrees, with the first considered superficial and the other three designated as deep frostbite.

1. First-degree frostbite: warm, swollen and tender with no or few blisters.

2. Second-degree frostbite: blisters form within minutes to hours after thawing and enlarge over several days. Blisters are filled with pink or reddish fluid.

3. Third-degree frostbite: small, reddish-blue or purplish fluid in the blisters. Surrounding skin may be red or blue and may not blanch when pressure is applied.

4. Fourth-degree frostbite: no blisters or swelling. The part remains numb, cold and bloodless. Gangrene develops quickly. The color varies from white to dark purple.