First introduced by Belding and Hatch (1955) based on the fact that required evaporative heat loss and maximum evaporation together incorporate all components of thermal stress. The Index of Heat Stress is calculated by the following equation:
If the index of heat stress is less than 100 %, sweat evaporation can match , and body temperature can be con-
trolled. If the index of heat stress is greater than 100 %, sweat evaporation, and cooling, is limited to (Brotherhood 2008). Hence, evaporative heat cannot be lost, and heat is stored in the body, core temperature rises, and heat stress is likely. The index of heat stress differs from the heat stress index (HSI).