‘Cold’, as far as the human body is concerned has a number of possible interpretations. In this context, it simply denotes an immediate environment that is below body temperature and is likely to cause the body temperature to fall.
As stated earlier, heat always flows from a hot source to one that is lower in temperature, and the human body can lose heat through the avenues of conduction, convection, and radiation.
The requirement for a stable central or core temperature within ± 0-5°C of 36*9°C has already been stressed. If this temperature falls by a generalized chilling of the body to 35-0°C (95 0°F), the resulting condition is called hypothermia (deficient heat). A special low-reading clinical thermometer is necessary to record such low body temperatures. Hypothermia leads to deterioration of the function of organs and cells, and if not controlled and corrected, ultimately to death. The brain tolerates hypothermia badly, and the manifestations of its malfunction are virtually identical to those which occur under conditions of excessive heat—a point that has received little or no attention—possibly because few investigators have had the opportunity to study both.
Central or core temperature is maintained in two ways: first, and of greatest importance when the body is at rest, heat is generated by the use of the body’s fuel reserves of fat and carbohydrates; second, any muscular activity generates further heat. In a fit person, the muscular activity may increase heat production by a factor of 10 or 15. Heat is conveyed from the core of the body to the skin and extremities by the circulating blood. Heat is then lost to the environment, and the cooled blood returns, tending to lower the core temperature. In cold conditions, therefore, the body’s heat can be lost through the inadequate insulation of the skin and extremities in generalized chilling. If there is severely localized chilling, cold injury can occur even if the core temperature is not substantially altered—and such injury can be sufficiently serious to require hospital treatment.
Different forms of cold injury include the non-freezing type called ‘immersion foot’ (‘trench foot’ of World War I) from long exposure to cold, damp conditions, and frostbite, which follows freezing of the tissues of the extremities, especially the nose, cheeks, chin, ears, fingers, and feet in cold dry environments. Less serious injuries—though often annoying ones—include the chapped skin of the lips, nose, and hands in cold windy weather and sunburn and windburn which, with snow- blindness, typically occur in cold dry climates.
The human body has no mechanism for acclimatizing to cold which corresponds to the way it adapts to hot climates. Instead, it must rely on insulation, provided not only by our layers of body fat but also by modern clothing and shelter technology. Travellers generally have to rely on trapped still dry air in clothing and housing to keep them warm, and this remains the best insulant known.
Prevention of the effects of cold thus depends upon the maintenance of body heat by an adequate supply of energy (food and drink) for internal energy production; on blocking avenues of heat loss, and finally upon the preservation of an insulating shell of still dry air.
It is pertinent at this point to discuss a source of possible confusion in terminology. The World Health Organization has decreed that hypothermia in this context should be known as ‘accidental hypothermia’. Terms such as exposure, mountain hypothermia, and immersion hypothermia are no longer recognized.
Wind: an additional risk
Wind speed has an important effect on human heat balance in both cold/wet and cold/dry conditions. Much of the important research on this subject was carried out by the late Paul Siple and his colleague C.F. Passel, who in 1945 published a method of assessing the effects of wind, which has stood the test of time and is known as the Wind Chill Factor. A slight modification of the original is reproduced here as the Wind Chill Index. By measuring or assessing the present and possible trend of temperature and wind speed, the traveler can allocate the prevailing conditions to one of the zones A-G on the graph, and then decide whether it is safe to stray away from a shelter.
When using this method, I recommend deeming the air temperature to be 6°C (11°F) lower than actual temperature if the clothing is soaked through with water. This adjustment is necessary because of the extra heat loss by conduction through waterlogged apparel.
Use of the method obviously requires some means of measuring air temperature and wind speed. Air temperature is simple to
A guide to estimating wind speed (Beaufort scale of wind force) Wind speed
|(miles per hour)||BMufort |
|1||0||calm||smoke rises vertically|
|1-3||1||light air||wind direction shown by smoke; wind vane does not move|
|4-7||2||slight breeze||wind felt on face; leaves rustle; wind vane moves|
|8-12||3||gentle breeze||leaves and small twigs in constant motion; light flag lifts|
|13-18||4||moderate breeze||dust and loose paper raised; small branches move; snow begins to drift|
|19-24||5||fresh breeze||small trees begin to sway; wavelets created on inland waterways|
|25-31||6||strong breeze||large branches in motion; umbrellas difficult to use; high snow-drifts|
|32-38||7||high wind||whole trees in motion; visibility obscured by drifting snow|
|39-46||8||gale||twigs break off trees; walking increasingly difficult|
|47-54||9||strong gale||slight structural damage to buildings|
|55-63||10||whole gale||inland trees uprooted|
|64-72||11||storm||rare; widespread damage|
|73-82||12||hurricane||very rare; widespread severe damage|
measure using inexpensive equipment such as the sling or whirling psychrometer. As the name implies, this apparatus is rotated rapidly like a (British!) football rattle for a minimum of thirty seconds and the reading taken. Of great use in cold and hot climates, the instrument can be small enough to slip into the pocket.
If very cold conditions are expected, a thermometer containing colored alcohol that will not freeze is required. Measurement of wind speed requires an anemometer, which is generally a rather delicate and expensive instrument. The Beaufort table of wind- speed, shown in Table above, supplies an alternative to the use of an instrument but does require some practice in its use.
Cold effects and injuries
The most important harmful effect of cold is that already referred to as accidental hypothermia, defined as a result of the general sized chilling of a person such that he or she has a core temperature of 35°C (95 0°F) or less.
In the field, the measurement of temperature in the mouth, armpit or ear is useless. Placing the thermometer in the stream of urine as it is passed is reliable, but requires a full bladder, or a minimum of 200 ml of urine. Usually, the only reliable method with simple apparatus is to place the thermometer in the rectum.
In practice, the onset of the condition may be fast or slow, and this is a convenient approach to its description as acute or chronic.