cold exposure

The body's adjustment

Like other mammals, man normally keeps his body core temperature remarkably constant, so that the temperature of the heart, brain, and other central organs seldom rises more than 2°C above 37°C, or falls more than 2°C below this. The main way that this is achieved in a cold environment is through conscious protective measures, such as putting on warm clothing or entering warm accommodation, when the body's sensory system records exposure to cold. The sensory system itself is a two stage one. Receptors in the skin respond immediately when the body is exposed to cold. If the response to these is insufficient to maintain body core temperature, receptors in the brain record a fall in temperature which induces further sensation of cold.

If the cold exposure continues despite conscious protective measures, the cold receptors trigger internal mechanisms to restore heat balance. The first of these is that sympathetic nerves release noradrenaline, to shut down blood flow to the skin. The effect is that blood no longer carries heat from the body core to the surface of the body, and the skin and tissues under it are allowed to cool down. These tissues then provide a layer of insulation between the body core and the surface. An important point is that the thickness of this insulating layer, which is determined in turn by the thickness of the layer of fat under the skin, largely determines the amount of cold stress that the person can undergo without developing hypothermia. Hypothermia is defined simply as a fall in body core temperature below 35°C.

The thickness of fat under the skin, described as subcutaneous fat, is particularly important to survival during immersion in cold water, because most of the external clothing is then lost, and internal insulation of the body largely determines the rate of heat loss. When you consider that fat also provides buoyancy to help keep people afloat, and that it provides energy reserves that are needed for many hours of physical exertion, it is obvious why relatively fat people have dramatically better ability than their thinner colleagues to survive in water after shipwreck, and to swim long distances in cold water as in Channel crossings and other long-distance swims.

The exertion of swimming increases heat production, but it also increases heat loss in various ways, and on balance people at risk of hypothermia after shipwreck maintain their body core temperature better by floating still in life jackets than by active swimming. In cold air, physical exertion has a positive effect, helping to maintain heat balance, but it can only be continued for a limited time. It is therefore better for people lost in cold conditions to shelter from the wind than to continue exercise until they are exhausted. Shelter from wind can greatly reduce heat loss, and snow shelters are an important means of achieving this in exposed snow-covered country. If body core temperature drops despite measures of this kind, the cold receptors of the brain trigger shivering, which increases heat production in muscles but without increasing heat loss as much as active movement does. This will normally stabilize heat balance and body temperature, but represents the last defence against cold. If it is insufficient, or if it fails as a result of exhaustion, body temperature will fall progressively and death will eventually occur from hypothermia. The newborn baby has an even more effective method of heat production in the cold by metabolizing brown fat, but this is almost entirely replaced by shivering within the first year of life.

Illness in winter

Illness and death from heart attacks, strokes, and respiratory disease increase in winter, even in countries with mild winter climates such as Britain and Greece. Despite some popular perceptions, hypothermia with serious falls in body core temperature from any cause is rare, and is not an important cause of death in urban populations except at times of war or natural disasters when housing is damaged, people are living rough, and food supplies are disrupted.

It has recently become clear that the shutting down of blood flow to the skin, which is the normal first defence against cold, can explain some of the large numbers of deaths that are associated with cold weather. The blood volume has to be reduced to prevent the blood that was displaced from the skin from overloading the circulation; fluid is accordingly lost from the blood to the tissues, which removes the excess volume but leaves the blood more concentrated and therefore more liable to clot. This does no harm to young people with healthy arteries, but increases the chance of a blood clot forming in the arteries of middle-aged or elderly people, resulting in increased heart attacks and strokes.

Respiratory disease is also commoner in winter. The reasons for this are not fully understood, but cross-infection due to crowding in poorly ventilated rooms, and stress hormone responses to cold that reduce immune responses to infection, probably both play a part. The respiratory infections in turn cause changes in blood composition that further increase the risk of blood clots.

Prevention and treatment

Prevention of hypothermia during hill walking, mountaineering, and water sports or shipwreck is a matter for special training and equipment among a special group of people. Although very important in this group, it only relates to around a dozen deaths per year in Britain. With regard to urban illness and death in winter, prevention is directed at around 50 000 excess deaths that still occur annually in Britain in cold weather. Both indoor and outdoor cold exposures seem to play important parts in this mortality; the importance of warm housing has been appreciated for many years, but the realization that outdoor cold exposure is an important factor in causing illness and death in winter has been more recent. Outdoor activity in which people keep warm by exercise seems to be beneficial, but long waits by elderly people in cold weather with inadequate clothing (for example at bus stops) are not. Official action to improve outdoor facilities such as bus shelters can be important, but much of that kind of cold exposure can best be prevented by action by individuals. Waterproof and windproof clothing when appropriate, as well as sufficient over-clothing, particularly wearing hats, and keeping moving while out, can do much to prevent cold stress outdoors. Keeping enough durable food in the house to prevent the need for shopping excursions in very cold weather, or help by neighbours with shopping, can make outdoor excursions unnecessary for elderly people in the worst weather. Such measures may seem obvious once the awareness of cold hazards is realized, but surveys show that many people in countries with mild winters, such as Britain, do not automatically take the advance measures needed to have suitable clothing and supplies available before a cold spell arrives, or do not use them when it does.

Treatment of people exposed to cold

People who are found confused or unconscious in cold surroundings, particularly in wind and either wet or very cold conditions, may be hypothermic. Many, however, are found to be suffering instead from other illness, or from exhaustion or malnutrition. A mouth temperature above 35°C will exclude hypothermia. A mouth temperature below 35°C does not prove hypothermia as the mouth can be cold when body core temperature is normal, and a rectal temperature below 35°C is then needed to establish whether hypothermia is in fact present.

The first aid treatment for hypothermia is to cover the patient with blankets or other insulation, keep him lying flat, and move him to warm surroundings and to hospital with as little disturbance as possible. This gives minimum risk of precipitating sudden death from ventricular fibrillation, an unco-ordinated contraction of the ventricles of the heart which can result from almost any form of exertion or active interference during hypothermia. The only major situation in which another course is needed is when the victim has cooled so far that the heart has stopped, or has ceased to pump blood because of ventricular fibrillation. In this case, and if a trained person is at hand and establishes that there is no pulse in the carotid artery, external cardiac massage may be useful while the victim is rushed to hospital for specialist treatment. Cardiac massage should never be given if the heart is beating, even slowly, in hypothermia. There is still debate over whether a warm bath (no hotter than is comfortable to your own elbow) is useful for victims who have been rescued from cold water and have suffered rapid body cooling. A warm bath can probably help, but only if given within about 20 minutes of rescue and without great disturbance to the patient, and this is in any case rarely feasible.

Frostbite and immersion injury

Frostbite is the freezing of body tissue. It is obvious in the appearance of hard white areas of skin, developing in air colder than 0°C. Small areas of frostbite, usually on fingers, toes, ears, or face, are best treated by immediate thawing, preferably in lukewarm water. Bending or other injury to the frozen tissue must be avoided. Large areas of frozen tissue, such as whole limbs, must not be thawed rapidly, as potassium ions and other agents can than be released from the thawing tissue into the blood in such quantity as to cause death. Such cases, which are seldom seen except in very cold regions, must be treated by wrapping the frozen area in clothing and transporting the patient as quickly as possible to hospital where limbs can be thawed under full biochemical control.

W. R. Keatinge

Bibliography

Burton, A. C. and and Edholm, O. G. (1955). Man in a cold environment. Edward Arnold, London.
W. R. Keatinge (1978). Survival in cold water. (Reprinted.) Blackwell Scientific Publications.
Neild, P. J.,, Syndercombe-Court, D.,, Keatinge, W. R.,, Donaldson, G. C.,, Mattock, M.,, and and Caunce, M. (1994). Cold-induced increases in erythrocyte count, plasma cholesterol and plasma fibrinogen of elderly people without comparable rise in protein C or factor X. Clinical Science, 86, 43–8.

See also diving; hypothermia; survival at sea; temperature regulation.