David Craig, age 13, of Keen, N.H., for his question:

WHAT HAPPENS WHEN YOU GET FROSTBITE?

Frostbite is an injury to the skin and sometimes deeper tissues of the body. It is due to the freezing or formation of ice crystals in the tissue cells.

Frostbite develops in three states: a reddening of the skin, formation of blisters and finally death of some of the skin cells and the underlying tissues. Clots often form in the blood vessels. Mild cases of frostbite often result in chilblain while more severe cases may result in gangrene.

Frostbite usually develops when the air temperature is below 10 degrees Fahrenheit, but may occur at a temperature nearer the freezing point, 32 degrees Fahrenheit, when other elements such as high winds, dampness or general chilling of the body are present.

The parts of the body most often affected by frostbite are the hands, feet (especially the heels and toes), ears, cheeks, chin and nose.

The recommended first aid treatment for frostbite is the immediate application of warmth to the injured parts. If possible, the affected areas should be soaked in warm water. Vigorous massage should be avoided as it would cause further harm to the damaged tissues.