Common logic says that workers who must wear protective apparel and equipment face a greater risk of heat-related illness as the temperature and humidity rise. Recognizing and dealing with the heat stress risk for protected workers can be a daunting task for even the best safety experts. An effective preventive program requires an understanding of how heat affects the human body, the chemistry of moisture transmission, and the physical properties of fabrics used in protective garments. Fortunately, leading garment manufacturers have studied the effects of heat stress on protected workers, and guidelines are available that can assist users in understanding and dealing with this widespread safety risk.
A simple explanation is this: When faced with an unacceptably hot environment, the body reacts with two defense mechanisms: First, the capillaries near the skin's surface dilate and act as a "radiator" to exhaust excess heat away from the body (vaso dilation). If this does not provide enough cooling, then the individual starts to sweat, which further cools the body through evaporation.