The human body maintains a fairly constant core body temperature, even though it may be exposed to varying environmental temperatures. In this case , evaporation is the only way the body can cool itself. Physical work contributes to the total heat stress of the job by producing metabolic heat in proportion to the intensity of the work. The environmental factors of heat stress include air temperature, radiant temperature, humidity and air velocity.
Clothing also affects the heat stress.
When the body's cooling system has to work too hard to reduce heat stress, it can strain itself. The severity of strain will depend not only on the magnitude of the prevailing stress, but also on the age, physical fitness, degree of acclimatization, and dehydration of the worker. Excessive exposure to a hot work environment can lead to a variety of heat-induced disorders and safety problems. The risk of heat-related illness varies from person to person. Older and obese people are generally at high risk.