Page:Popular Science Monthly Volume 70.djvu/32

28 effects. Special provision must be made for drying the air to keep down the relative humidity in the rooms so cooled. In moist and warm weather it would otherwise reach the saturation point. Such a condition is not only uncomfortable, but can become very unhealthy. The science of artificial cooling is as yet very little understood by the average layman and any devices which do not give perfect control over humidity must be cautioned against.

Ventilating apparatus itself may become a source of contamination if improperly designed, operated or maintained. Air filters have been found, for instance, which were intended to arrest the dust, but actually also arrest nearly all the fresh air. Some of these filters can not be cleaned or renewed without spilling the very impurities collected into the air ducts and thence into the rooms. Mechanical ventilating devices too often defeat their usefulness by lack of control over air currents and temperature, which either puts them out of service, or the persons for whose benefit they were intended.

Vitiation through Animal Life.—The last, but not the least, among the sources of vitiation is the presence of animal life or of man. Theoretically, perhaps, this may be called the only unavoidable factor, or the one which must be met by ventilation. The exhalation of carbonic acid in place of the oxygen inhaled reduces the life-giving quality of the air, or its power of regenerating the blood. Exhaled air, moreover, is charged with vapor and organic matter. The substance, called effluvia, which emanates from the surface of the human body is also of organic nature. It is harmless enough when permitted to dry and disperse, but in the moist and warm air of over-crowded rooms it quickly putrefies and becomes obnoxious. It can be recognized by that pungent odor characteristic of a sweltering mass of people. Whatever ill effects may be due to effluvia come through the action of odor on the nerves, rather than through inhaling this comparatively innocuous matter. The excess of heat and moisture produced by an audience as previously mentioned is now regarded as more than a temporary discomfort, quite aside from the danger in subsequent exposure to cold. Exhaled air, effluvia and heat thus combine, in varying proportions, to make room air unfit for breathing. In crowded meeting places they are the principal sources of vitiation, which may practically determine the artificial supply of air, while, for instance, in dwellings, offices and shops with liberal space allowance and plenty of exposure they are often a negligible quantity compared with the sources of contamination.

The remedy for the unhealthy conditions described naturally lies in systematic sanitation of the air; indoors as well as out-of-doors. The methods of carrying on such work are indicated by the causes