The temperature of the environment is one of the factors affecting human performance. At body temperature substantially higher than the optimal levels (36.5–37.5°C), both physical and mental performance may deteriorate due to the complicated interplay of physiological and pathophysiological processes. The main source of heat gain is the body's own internal heat. When the air temperature or humidity rises above the optimal ranges for comfort, problems can arise. From these problems: heat stroke, heat exhaustion, heat syncope, heat cramps, skin rash and fainting. Transient heat fatigue refers to the temporary state of discomfort and mental or psychological strain arising from prolonged heat exposure. The risk of heat stress arises from a combination of human physical effort, which increases body temperature, and the ability of the body to shed the excess heat in order to maintain a viable core body temperature. Heat loss from the body depends on environmental factors such as radiant heat, air temperature, relative humidity and air movement. Heat control measures include: (1) Engineering controls to provide a cooler workplace. (2) Personal Protection to reduce worker exposure. (3) Training employees to recognize and prevent heat illnesses (Health education).