The air of the house must be fresh, pure, and cool, to allow proper ventilation of the lungs and skin. Colds are prevalent in winter, because that is the season when people are housed up and breathe impure air. 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.