Thermal Comfort

a functional condition of the human body characterized by a certain content and distribution of heat in the superficial and deep tissues, with minimum exertion of the heat-regulation mechanism. Such a condition is subjectively regarded as the most preferable.

Objectively, thermal comfort is characterized by a constant body temperature, minimum activity of the sweat glands (imperceptible perspiration at a rate of 40–60 g/hr), and small periodic fluctuations of temperature in the extremities, especially the hands and feet (in the 30°–31°C range). The skin temperature in the trunk region remains almost unchanged at about 33°C, the overall skin temperature is comparatively constant at 32°–33°C, and the cardiovascular, respiratory, digestive, excretory, and other physiological systems function at optimum levels. In addition, thermal comfort is characterized by the highest level of mental readiness. Man experiences thermal comfort in a state of muscular rest with heat production of about 80 kilocalories (kcal) per hr (1 kcal = 4.19 kilojoules) or doing light work, with heat production of no more than 150 kcal/hr (such as the duties of a clerk, engineer, operator, or researcher) and a certain combination of microclimatic parameters—temperature, relative humidity, and rate of air movement and heat emission.

Standards for the microclimate in residential and public buildings to ensure thermal comfort are established in differing ways to take into account various climatic zones, seasons, and age groups. In most healthy adults living permanently in a temperate zone and wearing ordinary indoor clothing, thermal comfort is achieved at an ambient temperature of 18°–22°C in winter and 23°–25°C in summer, with a difference of not more than 3°C between the ambient temperature and the temperature of enclosed areas, with relative humidity of 30–60 percent, and with a rate of air movement of 0.05–0.15 m/sec in winter and 0.2–0.4 m/sec in summer. For an unclothed person the comfort zone is at an ambient temperature of 28°–30°C. The zone of thermal comfort changes somewhat under the influence of such factors as physical exertion, adaptation to heat or cold, and some pathological conditions. It can be broadened by conditioning and hardening the body by taking progressively colder air and water baths, as well as through dynamic microclimatic actions that extend the lower comfort limit. Such steps increase the body’s resistance to colds. At night, lowering of the air temperature by 1°–2°C, together with good heat insulation of the body, is recommended as being conducive to deep sleep. In children during the first years of life (especially in neonates) and in the elderly, the microclimatic comfort zone is narrow because of the functional insufficiency of the heat-regulation mechanisms.

Individual differences in the boundaries of the thermal comfort zone depend on basal metabolism, adaptation, development of subcutaneous fat, and the habit of wearing clothing that provides more or less heat insulation.