Perceived comfort temperature results from the energy balance of the body, which includes heat loss by convection and conduction to the surrounding air, by evaporation and by radiation to and from neighbouring surfaces. When too much heat is lost, the body perceives a sensation of cold through temperature sensors in the skin. When not enough heat is lost, the temperature of the skin rises, resulting in a sensation of warmth. A healthy body always maintains equilibrium between heat gains (metabolic heat and heat gains from external environment by convection, conduction and radiation) and heat losses to the environment by convection, conduction, radiation and evaporation (or transpiration). This equilibrium is necessary to maintain the internal body temperature at a constant of nearly 37°C. It is reached by automatic changes of the blood circulation and skin temperature, on the one hand, and by conscious changes of clothing and activity on the other. Another way of ensuring thermal equilibrium is to change the environment by using available controls such as fans, heaters or window blinds. When the temperature is comfortable, heat loss is almost equally shared between radiation to surrounding surfaces, conduction/convection to the air and transpiration (evaporation). When temperature increases, evaporation becomes the predominant mode of heat loss, and at temperatures in excess of 37°C, becomes the only way to lose heat. At the same time, the production of metabolic heat may fall. When temperature decreases, evaporation is reduced but it is compensated for by an increase of radiation and convection losses. At low temperatures, the metabolism may increase (shivering) to compensate for heat losses by additional heat production. This clearly shows that thermal comfort does not result from air temperature alone, but also from the temperature of the surrounding surfaces, humidity and air movement.
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