AIR : HEATING AND VENTILATION I. Air is man's immediate environment and in relation to health serves two important functions. The oxygen supply which it furnishes may justify us in regarding air as a food, while it also serves an important function in the regulation of the body's temperature. It varies in the following directions: (a) Chemical is very constant out of doors all over the world. Variations in outdoor air are slight and are only found over small areas. On the other hand, the composition of indoor air is very variable, due to the constant admixture with expired air. It is a simple physical mixture, the approximate proportions being shown in the following table, ignoring rare and unusual constituents.
(b) Temperature and pressure show seasonal and attitudinal variations.
(c) The possible moisture content varies directly with the temperature and pressure.
(d) Odors are variable, being more numerous and concentrated in indoor air.
(e) Air is subject to rapid movement, and to rapid changes in its temperature, pressure and moisture content.
Air must be regarded as a most necessary and essential food. In this respect the nitrogen serves as a harmless diluent for the active oxygen, the essential constituent. The oxygen effects the combustion of the food eaten and stimulates digestion and metabolism.
2. By good air is meant air that is free from dust, smoke and odors, of moderate coolness and humidity, free from accumula tions of respiratory products and body excretions. It is usually outside air, the air of rural, residential or unsettled areas.
3. By bad or injurious air is meant air that produces discom fort, which is chiefly caused by heat and humidity, and odors in the air of inclosed spaces. Odors are the chief cause of dis comfort and their intensity is increased by heat and humidity. Ordinarily these conditions are due to the self pollution of air about a sedentary person, the aerial blanket about the individual being saturated with his excrement. Odors are of two types, extraneous and intrinsic. Extraneous odors include those due to gas works, bone boiling plants, rendering plants, slaughter houses, stables and meat markets. Intrinsic odors arise from the sudoriferous glands, from bad teeth, bad breath, gases from the stomach and rectum, or urine decomposing on the clothing.
These are not detectable on chemical analysis. The necessary temperature and humidity to accentuate odors may be produced by the body, the heat being furnished by direct radiation and the humidity from the expired air and perspiration.
4. In addition air may contain certain various undesirable or injurious substances not due to respiration. Thus there may be present dust from various sources (metals, stone, wood, soil, manure, cotton, wool, etc.) soot and smoke. The latter are products of combustion and add sulphurous acid and carbon dioxide. The air of industrial establishments may be contami nated by dust and fumes of varying kinds. These have already been considered.
5. Effect of Temperature and Humidity.—The human body is readily adaptable to temperatures from o degrees F. to 80. degrees F. Temperatures above or below these limits produce discomfort. Discomfort from both of these factors arises from the disturbances they produce in the heat regulating devices of the body. The bodily activities produce an excess of heat which must be lost or heatstroke will result. Heat is lost by (t) heat transfer which is affected by radiation, conduction, and convection, and by (2) evaporation (perspiration) whose amount diminishes as the surrounding humidity and tem perature rise. Without free perspiration the temperature of the body rises if the external temperature goes above 7o degrees F. As long as free evaporation persists the heat production and heat loss are balanced. When humidity is high evapora tion is lessened and the balance is maintained by an increased flow of blood to the skin and consequently an increase in the loss of heat by transfer. Humidity affects the heat output by (r) increasing the conductivity of the atmosphere for neat thus cold moist air is chilling) and (2) interferes with the evaporation of perspiration (thus warm moist air is ennerva ting). Around a temperature of 68 degrees F. humidity exer cises a minimum influence. Humidity should not exceed 7o degrees F. as measured on a wet bulb thermometer. Tempera ture and humidity are perhaps the most important factors in ventilation.