The human organism is so designed that it strives to maintain a constant internal temperature even during high levels of physical workload. For low physical work­loads this temperature is 37°C, with certain upward variations in daytime and down­ward variations at night. If the environmental conditions are such that the body’s ability to maintain its temperature at the required level is exceeded, a lack of comfort is experienced, and if the stress is sufficiently great there may be detrimental effects on health. But problems or lack of comfort can also occur without any change in the body’s core temperature. Local cooling or warming can be experienced as uncom­fortable and help to reduce the level of a person’s mental performance. Local cooling may arise, for example, from too great a temperature difference between feet and head or between the two sides of the body.

Thermal comfort is a condition where people experience no discomfort from the thermal climate, that is, they do not know whether they want it warmer or cooler. People are different in their individual preference for climate, and it is therefore impossible to create a climate that satisfies everyone whenever there is more than one person in a room.