Frostbite is a medical condition involving damage to the skin and underlying tissues caused by freezing temperatures. It is a localized injury that most commonly affects exposed areas such as the fingers, toes, nose, and ears. Understanding the conditions under which tissue freezing occurs is necessary for prevention and immediate care.
The Critical Temperature Threshold
Skin tissue begins to freeze when its temperature drops to or below 32°F (0°C). Prolonged exposure to slightly lower temperatures, such as 28°F (-2.2°C), is necessary for ice crystals to form. However, ambient air temperature is not the sole factor determining frostbite risk. The effective temperature felt on exposed skin is dramatically reduced by wind, a measurement known as the wind chill index. Wind rapidly removes the insulating layer of warm air surrounding the body, accelerating heat loss.
The National Weather Service considers wind chill values near -25°F to be dangerous for exposed skin. At this calculated temperature, frostbite can occur in as little as 15 minutes. Frostbite is possible even when the thermometer reads above zero, provided the wind speed is high.
How Tissue Damage Occurs
Frostbite damage results from two interconnected physiological processes: direct cellular injury and vascular damage. During freezing, ice crystals form in the space between cells, physically damaging cell membranes. This crystallization draws water out of the cells, leading to cellular dehydration and increased electrolyte concentration. Simultaneously, the cold causes blood vessels to constrict, leading to blood flow cessation (stasis) and the formation of micro-clots (thrombosis).
This loss of blood flow results in oxygen and nutrient deprivation, known as ischemia, which contributes to tissue death. Additional damage occurs during rewarming, triggering an inflammatory response called reperfusion injury. This secondary injury involves the generation of free radicals and further microvascular damage, potentially leading to progressive tissue necrosis.
Recognizing the Stages of Injury
The progression of frostbite is classified into stages that reflect the depth of tissue damage, beginning with the mildest form known as frostnip. Frostnip involves numbness, tingling, and a cold feeling, but the skin remains soft and does not sustain permanent injury.
Progression to superficial frostbite means the skin appears white or waxy. While the surface is firm, the tissue underneath is still pliable to the touch. Upon rewarming, this stage is characterized by swelling, a stinging or burning sensation, and the development of clear, fluid-filled blisters within 12 to 36 hours.
Deep frostbite is the most severe stage, affecting all layers of skin, as well as the underlying muscles and bone. The affected area becomes hard, cold, and completely lacks sensation, which makes the injury difficult to recognize. Rewarming deep frostbite results in large, dark, blood-filled blisters and the eventual hardening and blackening of the tissue. This dead tissue is called gangrene, and it may indicate the need for surgical removal.
Immediate Steps for Treatment
Immediate action involves moving the affected person out of the cold and removing any wet or constricting clothing and jewelry. The primary goal of first aid is rapid and gentle rewarming, ideally using a water bath heated between 104°F and 108°F for about 30 minutes. If a warm bath is unavailable, body heat, such as tucking frostbitten fingers into an armpit, can be used. Nonprescription pain relievers should be administered, as the rewarming process can be intensely painful.
Avoid thawing the injured area if there is any chance it might refreeze before reaching medical care, as a thaw-refreeze cycle causes greater tissue destruction. Direct heat sources like fires or heating pads should never be used, and the frostbitten area must not be rubbed, as both actions cause further damage. If the feet are injured, the person should avoid walking. Seeking professional medical attention is advised for any frostbite extending beyond frostnip.