Frostbite is damage occurring in skin and adjacent tissues caused by freezing. (The numbness caused by cold allows considerable damage without pain.)
Frostbite is an effect of the body's defensive response to intense cold, which is to shut down blood vessels at the extremities in order to preserve warmth at the core of the body. Consequently, it mostly occurs in the face, ears, hands, and feet. In superficial frostbite the affected part turns white and cold; it can be treated by gentle thawing. If freezing continues, ice crystals form in the tissues, the flesh hardens and there is no sensation. Deep frostbite, which causes tissue death, require urgent medical treatment.
Judicious rewarming, pain relief, and measures to maximize skin blood flow may reduce tissue loss. No attempt should be made at rewarming if there is a risk of refreezing as this increases the likelihood of gangrene.