The commonest causes of impurities of the air in houses are the expired air and the transudation of the skin; the production of the combustion of lights or unconsumed gas may come from the burner when lit, if the pressure is very strong, or the rubber fittings may retain the gas; tobacco smoke; the effluvia of simple uncleanliness of rooms and persons; and the products of the fluid or solid excreta retained in the room. In addition, there may be special conditions which allow the impure air to flow into the room, as from the basement or cellar of a house, from imperfectly trapped soil and waste-pipes, or from other impurities outside of the house.
In respiration the air is vitiated by a decrease in the amount of oxygen and an increase in the amount of carbonic acid; the expired air contains about 4 or 5 per cent. less oxygen and about that amount more of carbonic acid than the inspired air. It has been estimated that an individual takes into her lungs about 500 cubic inches of air per minute and exhales the same amount of vitiated air. The expired air is of a higher temperature, and is loaded with aqueous vapor. The organic substances present in expired air are in part the causes of the odor of the breath; it is probable that many of them are of a poisonous nature. The air is still further vitiated by the products of decomposition of persons having decayed teeth, nasal catarrh, and disorders of the digestive systems, as well as by personal emanations.
When the sensibilities of the sense of smell become dulled, they give no warning of the sense of danger, and the individual may not feel conscious of the harm, although the nervous centers may be greatly depressed, and, because discomfort has not been experienced in a vitiated atmosphere, it does not follow that harm has not been done. The effects are slowly and imperceptibly cumulative, but are on this account none the less injurious, and are now recognized as being among the most potent and wide-spread of all the predisposing causes of disease.
The physiologic effects of breathing vitiated air are that, owing to the impurities of the air, the respirations become quicker and shallower, the heart’s action more rapid and feeble; there is a more or less irritation of the mucous membranes lining the nose, throat, and larynx. In extreme cases, where many people are crowded together and the ventilation is totally inadequate, the air often becomes so impure as to cause headache, lassitude, nausea, and fainting.
The long-continued action of such impurities on the olfactory nerves may ultimately induce, through the central nervous system, alterations in the respiration, circulation, and nutrition. When moderately vitiated air is breathed more or less continuously, the individual becomes pale and loses her appetite; after a time there is a decline in the muscular strength and animal spirits. The aëration and nutrition of the blood is interfered with, and the general tone of the system falls below par.
It has further been maintained that metabolism is hindered by much-breathed atmosphere. In addition to the ordinary symptoms of discomfort, the long occupancy of so-called stuffy rooms so lowers the resistance as to be conducive to the contraction of colds and even to more serious infections.
People in this lowered condition of health, which is very common among those who spend the greater part of the day indoors, in offices, houses, schools, factories, and workrooms, offer much less resistance to attacks of acute diseases than do people who lead an outdoor life.
In considering the ventilation of a house, the purity of the air, the temperature, and the dryness of the air must all be considered.
The test now generally accepted as the standard of purity of the air is not the chemical one of the estimation of the amount of carbonic acid contained in the air of a closed space, but that, on entering a room or closed space from the outside fresh air, no sense of impurity or closeness should be noticeable.
The so-called natural ventilation of houses, which takes place through the porosity of the walls, the cracks around the doors and windows, is generally too inconsiderable to be taken into account.