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Human thermal comfort rests upon the interaction between six key factors, four from the environment and two regulated by the individual and his job. The ambient environmental factors include air temperature, air motion, vapor pressure, and the mean radiant temperature of the surroundings, while the heat production and clothing of the human are the individual factors. A body of studies, largely encouraged by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers over the past decade, has led to the recognition of such trade-offs as that changing air motion by 0.18 m/s (35 ft/m) roughly corresponds to a change of 1°C (1.8°F) in comfort temperature; a change of 36 percent relative humidity similarly corresponds to a 1°C (1.8°F) change, as does a change of 1°C (1.8°F) in mean radiant temperature. These trade-offs only operate within finite limits. Greater potentials for enhancement of thermal comfort and energy conservation are associated with regulation of the two key factors controlled by the occupant of the space. Increasingly since the 1940s, we have been substituting machines for manual labor and are less active, yet a heat production increase of 50 W by the individual allows comfort temperature to drop by 3°C (5.4°F). The clothing worn by the individual has also been decreasing from the 1940s, when the average office business clothing provided 1.0 clo unit of thermal insulation; today, the average office wear provides less than 0.6 clo of insulation. The 0.4 clo difference in clothing insulation represents a change in the comfort temperature of about 2.2°C (4°F); that is from the 20°C (68°F) average acceptable low thermostat setting of our childhood to the 22.2°C (72°F) low accepted today (pre-OPEC). Looking to the future, economics alone forecasts a return to the heavier clothing of our grandparents (with 1.0 kg of clothing providing about 2.0 clo) as the most feasible offset to diminishing energy sources and their increasing costs.
thermal comfort, discomfort (hot and cold), environment (thermal), clothing, heat exchange (human), sweat discomfort
Director of Military Ergonomics Division, U.S. Army Research Institute of Environmental Medicine, Natick, Mass.