Hypothermia is the unintentional cooling of the body core to below 35 °C. It develops when the body loses more heat than it produces. Hypothermia can be fatal. For isolated personnel, declining muscle and nerve function also makes equipment harder to operate, walking less reliable and personal decisions less sound, well before recovery may become possible. The German terms are Hypothermie and Unterkühlung.
How cold, wetness and exhaustion combine
The body produces heat through metabolism and muscle activity. In cold conditions, it constricts skin blood vessels to retain heat internally; shivering generates additional heat through muscle activity. Both responses have limits. Exhaustion and lack of food energy reduce the ability to sustain work and shivering. Injury can also impair temperature regulation. Injured personnel can therefore become hypothermic even in a warm climate.
Wind removes the warmed layer of air at the body surface. Contact with cold ground conducts heat away. Wet clothing insulates less effectively, and evaporating water removes further heat. After a water crossing, these losses continue even after the person has left the water. Freezing conditions are not required. In cold water, heat loss is accompanied by the immediate risk of drowning through cold shock and declining swimming capability, before substantial hypothermia has developed.
Moving quickly can initially produce heat while also soaking clothing with sweat. When the person subsequently hides or waits, heat production falls while damp clothing continues to cool them. While still functioning normally, personnel need to adjust their available layers to the effort and restore the necessary wind protection and insulation before a halt. Moisture left in the clothing reduces insulation for the following period of rest.
Recognising changes in personal function
Shivering shows that the body needs to generate heat. By itself, it does not establish hypothermia of the body core. Cold fingers can also become clumsy while core temperature is still normal: fasteners, knots and small controls become harder to handle. This loss of function still matters to self-care. Wind protection, additional insulation and required equipment must be accessible before the hands can no longer do the work.
After cold exposure, increasing stumbling, unusually slow reactions, slurred speech, confusion and marked drowsiness are warning signs. Personnel who can no longer perform simple actions reliably or keep track of their route and basic needs must immediately limit further cooling. Self-assessment is also affected: the impression of coping becomes an unreliable guide to the actual condition. A single sign cannot reliably identify a particular core temperature.
If shivering decreases while the person remains cold and becomes weaker or less alert, this indicates deterioration. It then means less internal heat production. A person who is still shivering can also be significantly hypothermic. Waiting for shivering to stop wastes the period in which protection, food and equipment can still be organised without assistance.
Limiting further heat loss with available means
The first requirement is protection from wind and precipitation that can actually be reached. A person who is still thinking clearly and moving safely can use a nearby sheltered position; a long journey over unknown ground is not a rewarming method. At the resting place, an insulating layer goes between the body and cold ground. An available mat, additional clothing or other suitable dry material can serve this purpose. Available layers protect the torso, head and neck, while the face remains clear for breathing.
Under shelter, wet clothing is exchanged for dry clothing or another adequate insulating layer. Replacement material and covering need to be ready before undressing. If dry items and a sheltered place to change are absent, completely undressing in the wind adds to heat loss. An available windproof, impermeable covering over wet clothing can then reduce further evaporation and wind exposure. Available insulating layers supplement this protection; the covering alone does not replace them.
A thin emergency foil or poncho generates no heat and provides little of the cushioning of an insulating ground layer. The barrier slows particular heat losses, insulation retains a protective layer of air, and the body or a suitable external heat source supplies the energy. When equipment is absent, a better protected position and the material actually available remain the person's options. Collecting further material must still be safely achievable; with increasing clumsiness, extensive construction can consume more heat and strength than the resulting shelter can save in the time available.
Adding heat and energy safely
Passive rewarming uses internal heat production: insulation and wind protection retain heat produced by metabolism and shivering. This can be effective in a person who remains alert and shivering. As shivering declines and energy reserves are exhausted, this option becomes less effective. Wrapping someone up therefore does not necessarily restore warmth.
If an appropriate large-area body warmer is available, it can add heat to the upper torso in accordance with its instructions. Active heat sources must not be placed directly on the skin; the required separating layer and skin checks limit burns. Pressure, such as lying on a warmer, can particularly damage the skin. Small chemical hand warmers cannot provide adequate rewarming of the body core. Hot baths, vigorous rubbing of the limbs and uncontrolled heating of cold hands or feet are not substitute methods.
A person who is fully awake and can swallow safely can use available high-carbohydrate food and a warm, non-alcoholic drink. The energy supports heat production; the drink must not be hot enough to burn. Drowsiness, unconsciousness or unsafe swallowing rules out eating and drinking because food and fluid can enter the airway. Alcohol dilates skin blood vessels, increasing heat loss, and further impairs decisions.
Movement and progressing hypothermia
After substantial cooling, core temperature can initially continue to fall even though cold exposure has ended. This continued fall is called afterdrop. Heat continues to pass from the warmer body core into cold outer tissues; increased blood flow can also return cold blood from the arms and legs to the torso. Especially after cold-water exposure, immediate exertion can worsen this problem. When hypothermia is suspected, protection and rewarming therefore take priority over trying to warm up by running.
Immediate danger can force a short change of position. The ability to do so without assistance depends on clear orientation and movement that is actually safe. Increasing confusion, difficulty standing or marked drowsiness argues against continuing an escape journey. Available covering and insulation then need to be maintained. If a usable connection to friendly forces or a reachable emergency contact exists, the report should include position, cold exposure and declining mobility while the equipment can still be operated. Recovery requires forces able to reach and care for the person.
Unconsciousness ends all ability to provide self-care. Advanced hypothermia can markedly slow breathing and circulation and cause dangerous cardiac arrhythmias; it requires care from helpers who can actually reach the person and medical rewarming. If others reach the casualty, handling should be as gentle and horizontal as possible. Unnecessary lifting into an upright position, vigorous movement or asking the casualty to walk can further stress an unstable circulation. Protection from wind and cold surfaces remains necessary during possible pickup and transport.
Whole-body hypothermia and local cold injuries
British military terminology separates hypothermia, or whole-body cooling, from Freezing Cold Injury and Non-Freezing Cold Injury. This has a practical consequence: numb hands or injured feet need protection but do not, by themselves, establish hypothermia of the body core; local injury and hypothermia can also occur together. US military care additionally gives particular attention to hypothermia associated with serious injury. Blood loss, burns or injury to the nervous system can worsen heat balance without a winter environment. Thermal protection therefore belongs in the early care of an injured person.