The average skin temperature for a typical person is around 34 deg C as opposed to the normal core body temperature of 37 deg C. A function of human physiology is to regulate body temperature by means of perspiration in conditions where the skin temperature approaches or exceeds 37 deg. As skin temperature increases above 37 deg C, the amount of perspiration will increase (Freudenrich).
To convert sweat from a liquid to a vapor, the system requires a particular amount of heat, the heat of vaporization. This heat is the energy used to speed up the movement of the water molecules so that they can escape into the air, or evaporate. As evaporation occurs, the remaining liquid cools because heat has been removed from it to supply the heat of vaporization that is necessary to make the phase change to the gaseous state (Nave).
At waters' boiling point, the heat of vaporization is 2256 J/g, but at normal skin temperature the heat of vaporization is higher, 2427 J/g, because the cohesion of water molecules is greater at that lower temperature, so it takes more energy to break off into the gaseous state (Kotz, Treichel, and Weaver 247).
The bodily response of perspiration takes advantage of this intrinsic property of water. In other words, because water requires a lot of heat to convert from liquid to gas, a lot of heat can be exported out of the body by the evaporation of sweat, therefore cooling the body. This efficiently uses the body's heat as an effective means for cooling it off, even in conditions where the ambient temperature is greater than normal body temperature (Nave).
Freudenrich, Craig. "How Sweat Works." How Stuff Works. 17 Jul 2006 .
Kotz, John, Paul Treichel, and Gabriela Weaver. Chemistry & Chemical Reactivity. 6th. Belmont: Thompson Brooks/Cole, 2006.
Nave, R. "Cooling of the Body." Evaporation of Perspiration. 17 Jul 2006 .