In severe cases of frostbite, the skin and deeper tissues may become gangrenous and turn dark green or black as they die. Feeling, movement and blood flow are all lost. If the tissue is not surgically removed, the gangrene can spread and develop into a severe infection.
Frostbite can happen in just a few minutes in the worst conditions, such as the subzero temperatures of Mount Everest. In such a remote location there is almost no possibility of thawing the frostbitten tissue before permanent damage is done.
Since frostbite is worse in areas where blood flow is restricted -- like the fingertips -- there isn't a lot of warm blood to stop the frostbitten tissue from cooling the tissue next to it. That's how frostbite spreads. Getting the tissues out of the cold environment is the only way to stop the spreading frostbite.
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Any opinions expressed here are for educational purposes only and are not intended for diagnosis.