Cold and Exercise

Cold Air

The severity of cold air exposure, expressed as the corrected effective temperature, depends on the wind velocity (or the effective wind velocity, adjusted for the individual's speed of movement), the temperature recorded by a wet bulb thermometer, and the temperature recorded by a globe thermometer (which allows for radiant heating from sunlight). Values can be calculated fairly simply, using nomograms published by the American Society of Heating, Refrigerating and Air Conditioning Engineers, and limits can be specified below which it is wise to cancel events such as a cross-country ski run, where competitors will face the adverse environment for a long period.

A simple example illustrates the likelihood that a person will have difficulty in regulating body temperature. Given a typical skin temperature of 35±C, an air temperature of 0±C, and a wind speed of only 5 m/s, the convective heat loss in normal clothing is about 75 kJ/min, more than twice the likely heat production when engaged in walking in the hills at an oxygen consumption of 2 L/min. Thus, unless additional clothing is provided, a progressive chilling of the body will occur.

The effectiveness of thermal protection depends greatly on the film of still air in immediate contact with the skin and additional air trapped within the clothing. These barriers are readily displaced by the wind, unless the outer garment is windproof. The still air is equally displaced by body movement, particularly in downhill skiing or cycling. Insulation is greatly degraded if the clothing becomes saturated with water, whether externally (from rain or snow) or internally (from sweating).

Cold Water

Cooling occurs much more quickly in water than in air because the rate of heat convection is greater in water. Thus, a temperature of 18±C may be thermally neutral for a person who is exercising in still air, but the thermally neutral temperature in water is around 29±C for a distance swimmer and [Page 269]may be as high as 33±C to 35±C for a sailor whose craft has capsized. Heat loss is particularly rapid in divers who are using heliox gas mixtures, as helium has a high thermal conductivity.

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