Human bodies dissipate heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and as the last extremity is reached--by panting, when blood is heated above 98.6 degrees. The heart begins to pump more blood, blood vessels dilate to accomodate the increased flow, and the bundles of tiny capillaries threading through the upper layers of the skin are put into operation. The body's blood is circulated closer to the skin's surface, and excess heat drains off into the cooler atmosphere. At the same time, water diffuses through the skin as perspiration. The skin handles about 90 percent of the body's heat dissipating function.

Sweating, by itself, does nothing to cool the body, unless the water is removed by evaporation--and high relative humidity retards evaporation. The evaporation process itself works this way: the heat energy required to evaporate the sweat is extracted from the body, thereby cooling it. Under conditions of high temperature (above 90 degrees) and high relative humidity, the body is doing everything it can to maintain 98.6 degrees inside. The heart is pumping a torrent of blood through dilated circulatory vessels; the sweat glands are pouring liquid--including essential dissolved chemicals, like sodium and chloride--onto the surface of the skin.