The effects usually attributed to the continued breathing of diluted sewer-air are general _malaise_, loss of appetite, anæmia, impaired nutrition, and therefore diminished power of resistance to attacks of disease which are not directly attributable to sewer-air poisoning. It is doubtful whether these effects are due to the constant introduction of sewer-air in minute quantities into the blood, or to the inhalation of particles of organic matter floating in such a contaminated atmosphere. =The greatest danger, however, in the breathing of sewer-air is that of inhaling with it the living particles= (bacilli, etc.) =contained or developed in the excreta of diseased persons=. The diseases believed to be propagated in this way are =cholera=, =typhoid fever=, and =dysentery=. The discharges both from the mouth (stomach) and bowels are known to be poisonous. It believed by many that the poisons of typhoid fever and diphtheria may be developed _de novo_ by the decomposition of the mixtures found in cesspools and sewers. There also seems to be a connection, imperfectly understood, between bad drainage and malarial fevers, and perhaps cerebro-spinal meningitis. The origin of yellow fever is not yet ascertained. Surgical erysipelas, puerperal fever, and hospital gangrene, are only developed on and about wounded surfaces, and seem to be due to the organisms developed in the secretions of such surfaces, where ventilation and drainage are bad. =Air may be contaminated by the products of organic decomposition rising from the ground and drawn into the house through furnace-flues=, etc. =Ground-air= contains from 1.49 to 80 parts per 1,000 of carbonic acid, and frequently contains products of organic decomposition. A damp soil is also very unhealthy, as shown by Bowditch and others. “A persistently low ground-water, say fifteen feet down or more, is healthy; a persistently high ground-water, less than five feet from the surface, is unhealthy; and a fluctuating level, especially if the changes are sudden and violent, is very unhealthy” (De Chaumont). Such soils are especially productive of consumption. VENTILATION. The contamination of the atmosphere by the respiration and bodily emanations of human beings and other animals is unavoidable, but the noxious matters thus added to the air are being constantly changed in the following ways: =1. Oxidation.= The organic matters, which have been mentioned as especially injurious, are gradually decomposed by the oxygen of the air, and changed into harmless substances, which either remain as constituents of the atmosphere, or are washed into the earth by rains.