Responses to the thermal environment.

Human responses to the thermal environment and to internal heat production serve to maintain a narrow range of internal body temperatures of 36-38 C. There are two categories of responses: voluntary or behavioral responses, and involuntary or physiological autonomic responses. Voluntary or behavioral responses consist generally of avoidance or reduction of thermal stress by modification of the body's immediate environment by modification of clothing insulation or by comfort conditioning of his microenvironment. Physiological responses consist of peripheral vasoconstriction to reduce the body's thermal conductance and increased heat production by involuntary shivering in the cold, and peripheral vasodilation to increase thermal conductance and secretion of sweat for evaporative cooling in hot environments. Autonomic responses are proportional to changes in internal and mean skin temperatures. Repeated exposures to heat, humidity, and exercise will modify the physiological response mechanisms by acclimation to produce more effective responses. Physiological responses also depend on the point in a diurnal cycle, on physical fitness, and on the sex of the individual. Behavioral responses rely on thermal sensations and thermal discomfort. Thermal discomfort appears to be closely related to the level of autonomic responses so that warm discomfort is closely correlated with skin wettedness, and cold discomfort similarly relates to cold extremities and shivering activity.