Cold Injuries: Frostbite, Hypothermia, and Immersion Foot
Cold injuries are not accidents — they are failures. FM 31-70 is unambiguous on this point: the vast majority of cold weather casualties result from preventable failures in training, planning, and supervision. Understanding cold injuries is only useful if paired with the discipline to prevent them, because by the time a cold injury is obvious, significant damage has already occurred.
How Cold Destroys Tissue
The body prioritizes core temperature above all else. When you are cold, blood flow to the extremities is reduced — this is vasoconstriction, and it is your body sacrificing your fingers and toes to maintain heart and brain temperature. It is an effective survival mechanism in short exposures, but in sustained cold, the tissues being starved of blood begin to freeze.
Ice crystals forming in tissue cause two types of damage. The crystals themselves mechanically rupture cell walls. More destructively, as tissue freezes, water is drawn out of cells into the spaces between them, concentrating intracellular fluids to toxic levels. The cells die from dehydration and electrolyte imbalance even where ice crystals haven't directly ruptured them.
Rewarm tissue too quickly and you add a third insult: ice crystals melt rapidly, releasing inflammatory mediators that cause swelling, blistering, and tissue death in tissue that might otherwise have been salvageable. This is why the Army prohibits rewarming frostbitten tissue in the field unless evacuation is impossible — frozen tissue can walk on, but thawed and refrozen tissue cannot.
Frostbite: Staging and Field Recognition
The manual describes frostbite in stages based on depth of tissue freezing:
Frostnip (superficial, first degree): The skin appears white or grayish-yellow, feels firm on the surface but soft and normal underneath when pressed. The affected person typically feels numbness or stinging. This stage is fully reversible with immediate rewarming — face the affected area against a partner's warm skin (neck or armpit), or cup it with bare hands. Do not rub frostnipped tissue; friction does not warm frozen cells and the ice crystals cause damage when the tissue is moved against itself.
The Buddy Check System
FM 31-70 mandates regular buddy checks for cold injuries during operations. Partners examine each other's face, ears, and neck for white or grayish patches every 20-30 minutes in extreme cold. The affected person almost never feels frostbite because numbness is a symptom — you cannot reliably self-diagnose.
Superficial frostbite (second degree): The entire skin surface feels hard and wooden, but deeper tissue remains pliable. After rewarming, blisters filled with clear or milky fluid develop within hours. These blisters indicate injury to the full skin thickness but not to deeper tissue. Do not break blisters — they protect underlying tissue from infection and desiccation.
Deep frostbite (third and fourth degree): The entire area — skin, subcutaneous tissue, and potentially muscle and bone — is frozen solid. The area is hard, cold, and completely insensate. After rewarming, blisters filled with bloody fluid develop, indicating injury to blood vessels beneath the skin. Deep frostbite almost always results in partial tissue loss. The final extent of damage cannot be determined for days or weeks after rewarming — tissue that appears dead often recovers, and tissue that appears viable sometimes does not.
Field treatment rule: If evacuation to definitive medical care is possible within 24 hours, do not rewarm deep frostbite in the field. Keep the area frozen and insulated. A soldier can march out of the wilderness on frozen feet — he cannot walk on thawed, blistered feet that have refrozen.
If evacuation is not possible, rewarm in a water bath at 100-108°F (measured with a thermometer — guessing is not acceptable). The process is extremely painful as circulation returns to damaged tissue. Oral pain medication should be administered if available. Tobacco and alcohol are prohibited — both cause vasoconstriction that worsens tissue damage.
Never Rub Frostbitten Tissue
Rubbing frostbitten areas was once a widely taught treatment. FM 31-70 explicitly prohibits it. Ice crystals in frozen tissue are sharp; movement causes them to lacerate surrounding cells. The only exception is frostnip, where the skin surface is affected but no actual freezing has occurred in deeper tissue.
Hypothermia: Recognition and Field Management
Hypothermia is the general cooling of core body temperature below 95°F (35°C). Unlike frostbite, which affects extremities first, hypothermia attacks the brain and core organs. The progression from shivering to unconsciousness can take hours in moderate cold — or under 30 minutes in cold water.
Mild hypothermia (90-95°F core temp): Intense shivering, poor coordination, slurred speech, mild confusion. The person will deny being cold. They may be irritable. This stage is fully reversible with rewarming and caloric intake.
Moderate hypothermia (82-90°F): Shivering stops — not because the person is warming up, but because the body has exhausted its shivering mechanism. Muscle rigidity increases. Judgment becomes severely impaired. The casualty may appear deceptively calm and "fine" to untrained observers.
Severe hypothermia (below 82°F): Unconsciousness, cardiac arrhythmia, and potential cardiac arrest. The classic hypothermia maxim applies: a person is not dead until they are warm and dead. Successful resuscitation has occurred after core temperatures as low as 57°F, particularly in cold water drowning. Never abandon resuscitation of a hypothermic patient based solely on their apparent lifelessness.
Handle Hypothermics Gently
A severely hypothermic heart is electrically unstable. Rough handling — including unnecessary movement, jostling, or attempts at CPR on a still-beating heart — can trigger ventricular fibrillation. Move hypothermic casualties horizontally and gently. Do not stand them upright; this causes cold blood from extremities to rush to the core, dropping core temperature further (afterdrop).
Field rewarming: Remove the casualty from wind and wet. Remove wet clothing if dry replacement is available. Apply heat to the high heat-transfer zones: neck, armpits, and groin. Use chemical heat packs, body heat from a partner inside a sleeping bag, or warm (not hot) water bottles wrapped in cloth. Provide warm, sweet liquids if the person is conscious and able to swallow without risk of aspiration. Never give alcohol — it causes peripheral vasodilation that increases heat loss.
Immersion Foot (Trenchfoot)
Immersion foot develops when feet are wet and cold — but not frozen — for extended periods, typically above 32°F. The mechanism is different from frostbite: sustained vasoconstriction in wet conditions leads to tissue hypoxia and eventually to cellular damage without freezing occurring.
The condition develops insidiously over 12-72 hours. Early symptoms include numbness and discomfort. Late-stage immersion foot produces severe pain, blistering, and in extreme cases, tissue necrosis. Unlike frostbite, the injury is largely invisible in its early stages — the skin may appear normal or mildly mottled even as significant damage is occurring in deeper tissue.
Prevention is absolute: dry socks at regular intervals, at minimum twice daily. The Army issues this as a command responsibility — leaders must ensure their soldiers change socks. For preppers, this translates to carrying spare socks as a non-negotiable item in any winter kit. Wet socks dry faster against the skin while marching than inside a pack.
Wind Chill Is Not Temperature
The wind chill factor indicates how quickly exposed skin loses heat, not the actual air temperature. Skin freezes at roughly -5°F in calm conditions. The same skin freezes at the same rate in 20°F temperatures with a 30 mph wind. Always dress for wind chill, not thermometer readings. FM 31-70 includes wind chill charts that all cold weather preppers should study.
Dehydration: The Hidden Cold Weather Killer
Cold air is extremely dry, and heavy breathing in cold weather removes significant water from the body. Physical exertion in cold weather clothing produces heavy sweating, which is lost without the obvious visual cues of sweating in warm weather. The sensation of thirst is suppressed in cold conditions — cold-induced diuresis causes increased urination, further accelerating fluid losses.
The manual requires a minimum of 3-4 quarts of fluid daily in cold weather operations, more during heavy exertion. Water collection and treatment become logistical priorities: snow must be melted, not eaten — consuming snow directly lowers core temperature and costs more energy than it provides in water. Streams and lakes may be contaminated despite appearing pristine. A canteen should never be filled more than two-thirds full in freezing temperatures to allow expansion if the water freezes.
High-calorie food is equally critical. The body burns significantly more calories maintaining temperature in cold weather than in temperate conditions. The Army's cold weather ration requirement is 4,500-5,000 calories per day during active operations — roughly double the temperate zone requirement. Fats are particularly valuable because they provide more than twice the calories per gram of carbohydrates or proteins.