A unit of thermal insulation, usually applied to clothing or bedcovers. It is defined as the amount of insulation necessary to maintain comfort and a mean skin temperature of 33°C (92°F) for a person who is producing heat at the standard metabolic rate (50 Kcal m−2 of body surface per hour; one met) in an indoor environment characterized by a temperature of 21°C (70°F), relative humidity of less than 50%, and air motion of 6. 1 m min−1. If if is assumed that 76% of the metabolic heat is lost through the clothing, the unit can be defined in physical terms as the insulation that will restrict heat loss to 1 Kcal m−2h−1 with a temperature gradient of 0. 18°C across the fabric. In the first approximation, an insulation of one clo is provided by clothing material with a total thickness of 0. 64 cm and air layers (between skin and clothing and between inner and outer garments) of about 0. 51 cm.