9

Heat, cold and the design of the physical environment

The incidence of heat-related illness is as high as 1 in every 100 men working for 1 year 50 US football players are said to have died of heat illness in the 10 years to 1975 Heat strain occurs whenever the body generates more heat than it can lose even in cold conditions A British soldier marching in outdoor temperatures close to 12 degrees Celsius died of heat-related illness.

(G. W. Crockford, 1999)

Because the rates of most biochemical reactions are temperature dependent, it is advantageous to control the rates of these reactions by means of a thermoregulatory system.


Fundamentals of human thermoregulation

Humans have a remarkably well-adapted ability to tolerate heat compared with other primates. This applies equally to 'Eskimos' as to tropical rainforest dwellers, despite small differences. This is because humans are hairless and have a large proportion of high-capacity sweat glands - known as eccrine glands - in their skin.


Thermal balance

Thermoregulation is achieved by balancing the two main factors that determine body temperature - the metabolic heat produced and the rate of heat loss. The thermoregulatory goal is to maintain the core temperature at approximately 36-37°C. Core temperatures over 39.C are disabling and over 42°C they are usually fatal. The lower acceptable limit is 35.C and 33°C marks the onset of cardiac disturbances. Further drops in core temperature are extremely dangerous and temperatures as low as 25°C are fatal. The temperature of the peripheral body tissues, particularly the skin, can safely vary over a much wider range. From a thermal point of view, the body can be considered to have a warm core where much of its heat is produced. This is surrounded by a shell of cooler, insulating tissues, particularly subcutaneous fat.

The principal sources of heat are the liver, the brain, the heart and the working muscles. Muscular work is a source of heat because the mechanical efficiency of muscles is only about 20%. Little heat transfer from the interior to the surface of the body takes place via conduction: the body tissues are poor heat conductors. Heat is transferred to the skin from the deep body tissues by convection. Blood is an ideal

-233-

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