Human core temperature is closely regulated and kept within a narrow range: 36.5°-37.5°C at rest and up to 38.5°C during physical effort in a hot environment. In order to maintain a constant body temperature, the heat generated in the body (metabolic heat which depends on muscular work) and the heat absorbed from the surroundings (external heat load) must be dissipated. In warm environments two mechanisms participate in heat dissipation: the cardiovascular system conducts heat to the surface of the body (vasodilation) from which it is then lost to the environment by conduction and convection and the evaporation of sweat. Sweating is the main mechanism by which heat is dissipated from the skin to the environment. When heat accumulation exceeds heat dissipation, body temperature rises.