Working creates increased metabolic heat, and the blood flow to the skin increases in an attempt to carry the heat to the skin surface where evaporation and the subsequent cooling effect takes place .   The body dissipates heat through the skin via the cooling mechanism provided by evaporation of sweat with air flow.  Increase the work rate or the external temperature and the heart will pump faster in order to carry the blood faster to the surface. Factors that limit the body evaporative process from taking place are:

Air temperature is too high.

Humidity is too high.

A high radiant heat load is present.

Too much insulating clothing constricting the worker.

If the above factors are present then the body begins to experience physiological heat stress with differing symptoms depending on the degree of stress.