Sweat, Page 2

Getting Rid of Useless Metabolic Heat: Radiation and Convection.

Sweat evaporation, although the most powerful means of heat dissipation in hot weather, is not the only one. Humans also lose heat by radiation (the emission of infra-red waves) and convection (the transfer of heat from one place to another by the circulation of heated particles of a gas or liquid, such as wind blowing across the skin).

In 1996, Bodil Nielsen, a Danish exercise physiologist, published an intriguing paper. Entitled "Olympics in Atlanta: a fight against physics", this study examines the theoretical limits of heat dissipation imposed by weather conditions. Nielsen begins by pointing out that heat liberation during running amounts to approximately 4 kJ per kilogram of body weight for each kilometer run. The body can store a small amount of this heat, but the remainder must be dissipated or core temperature will rise. Nielsen presents us with this analysis of the theoretical limits of heat loss by radiation and convection:

                                        Radiation and Convection losses at various temps.gif (3309 bytes)

This graph shows that heat loss by radiation and convection, while very effective at low temperatures, decreases with rising temperatures. At an air temperature of 95F, heat loss by radiation and convection ceases. Exposure to temperatures above 95F leads to flow of heat into the athlete. The environment has become an oven, and the athlete is being cooked.

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