As my college professor once said, “Human bodies are like a bunch of little furnaces running around.” Truer words were never spoken, since each human body generates heat. As warm-blooded mammals, humans produce energy by metabolizing food, with most of this energy taking the form of heat. Metabolic heat is produced by the body all the time, mainly as a result of muscular activity, although almost all bodily functions produce some heat. It is no secret that the more active we are, the more heat we produce.1
Heat is transported around the body by the blood. To balance the metabolic input, heat is lost continually to the environment through the skin and through the surfaces of the lungs. Subsequently, human thermal comfort is also determined by the body’s ability to dissipate the heat and moisture that are produced continuously by metabolic action.
Heat is measured in British thermal units (a Btu is the quantity of heat required to raise one pound of water one degree Fahrenheit). For men of average size, seated and doing light work, the metabolic rate is about 450 Btu/h. Women under similar circumstances generate about 385 Btu/h. For a 155-lb man, seated and doing moderate to heavy work, the rate ranges from 650 to 800 Btu/h; standing and walking about while doing moderately heavy work will raise the rate to 1000 Btu/h, whereas the hardest sustained work will result in a metabolic rate of 2000 to 2400 Btu/h.
Thermal comfort is said to be attained when the environment surrounding the individual is in a state of equilibrium; i. e., the heat and moisture produced by the body are removed at the rate at which they are being produced. One method in which the human body maintains thermal equilibrium with its environment is by means of physiologic thermoregulation. For example, in situations of prolonged sweating, skin wetness slowly increases with time because of accumulating salt on the skin. The increasing salt occurs because the water in perspiration evaporates, while the dissolved materials, principally sodium chloride, remain on the surface. It is also thought that part of the relief that bathing brings after a warm day or strenuous activity is that by cleaning the skin, the salt is removed and the perspiration can evaporate more efficiently with reduced skin wetness. Another method of physiologic thermoregulation is shivering. The muscle tensions that cause shivering create a 300 percent increase in heat production, while the body also tries to cut heat loss by limiting blood flow to the skin and extremities.1
While some heat is removed through breathing, heat loss through the skin is by far the major path. Cold receptors in the skin signal the brain if the temperature on the skin drops at a rate faster than 0.5°F (0.25°C) per minute. This process allows people to adapt to indoor and outdoor climates by means of behavioral adjustments such as light or heavy clothing, low – or high-speed fans, open or closed windows, etc. An event as mundane as a cool draft of air can change local air temperatures by 2°F (1°C) in a matter of seconds.