from emedicine.com

Heat transfer to and from the body occurs via the following 4 mechanisms:


* Conduction is the transfer of heat via direct physical contact; it accounts for 2% of the body's heat loss.
* Convection is the transfer of heat from the body to the air and water vapor surrounding the body; it accounts for 10% of the body's heat loss. When air temperature exceeds body temperature, the body gains heat energy.
* Radiation is the transfer of heat via electromagnetic waves; it accounts for most heat dissipation. As long as air temperature is less than body temperature, 65% of the body's heat is lost by radiation.
* Evaporation is the transfer of heat by transformation of a liquid into a vapor; it accounts for 30% of the body's heat loss.

The body's dominant forms of heat loss in a hot environment are radiation and evaporation. However, when air temperature exceeds 95°F, radiation of heat from the body ceases and evaporation becomes the only means of heat loss. Evaporation of 1 mL of sweat results in the loss of 0.58 kcal of heat; thus, 1 L of sweat evaporated from the body accounts for the loss of 580 kcal of heat. An individual exercising in the heat easily can sweat 1-2 L/h. If humidity reaches 100%, evaporation of sweat is no longer possible and the body loses its ability to dissipate heat.