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. When heat gain exceeds the level the body can remove, or when the body cannot compensate for fluids and salt lost through perspiration, the temperature of the body's inner core begins to rise, and heat-related illness may develop. Sunburn, with its ultraviolet radiation burns, can significantly retard the skin's ability to shed excess heat. Studies indicate that, other things being equal, the severity of heat disorders tend to increase with age heat cramps in a 17-year old may be heat exhaustion in someone 40, and heat stroke in a person over 60.
Acclimatization has to do with adjusting sweat-salt concentrations, among other things.
The idea is to lose enough water to regulate body temperature, with the least possible chemical disturbance. Reprinted with permission of NOAA - National Weather Service.