he human body needs to maintain a constant internal temperature of 37o centigrade to operate effectively. When travelling off-piste, body heat is generated by metabolising food, but lost through heat transfer to the colder outside environment.
In cold conditions, body heat is lost through four basic mechanisms:
- Conduction: heat transfer between two surfaces in direct contact. When standing, walking or skiing conduction plays only a small role in heat transfer since only the feet are in contact with the snow. When a casualty is lying down a third of the body will be in contact with the ground and heat loss can be dramatically increased.
- Convection: heat transfer from a surface to a gas or liquid. The body warms air (or water) it is contact with. The warm air rises or is blown away taking body heat with it. Wind or cold water immersion dramatically increases the rate of heat loss through convection.
- Evaporation: heat is needed to convert a liquid into a vapour. Working at a high intensity will cause evaporative heat loss through sweating.
- Radiation: is direct energy transmission through electromagnetic radiation and is the main source of heat loss. Conversely, radiation is how the sun heats the earth.
The body’s metabolic functions are extremely sensitive to changes in its internal temperature. These processes take place in the head and in the torso – the so-called body core – and this is the area which is particularly sensitive to changes in temperature. Hypothermia occurs when the body cannot generate enough heat to prevent the core from cooling down. A cooling of just 2 degrees from a core temperature of 37oC to 35oC represents the onset of mild hypothermia when the victim can becomes confused, lose muscle co-ordination and begin to slow down. Further cooling below this level leads to profound hypothermia (at 32 oC) and ultimately to death. Hypothermia is the most serious risk in cold conditions.
The second type of cold injury is frostbite (or frostnip in mild cases). Frostbite is the freezing of deep tissue (frostnip is superficial freezing of tissue). Frostbite is most commonly seen in the extremities: ears, nose, fingers and toes. In extreme cases the damage this causes can lead to amputation of fingers and toes or even hands and feet (read Maurice Herzog's "Annupurna" for a particularly graphic account). In very cold conditions, frostbite is a real risk, particularly where constrictions (such as tight fitting boots) reduce the amount of blood circulating to the fingers and toes.
The body has a number of physiological response to cold temperatures (shivering, hair standing on end, reduction in peripheral circulation) but by far the most important methods of heat control are shelter and clothing. This section shows you how to use these to stay warm and dry off-piste.
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