Persons with decayed teeth, catarrh, indigestion, diseased lungs, or other unsoundness give off still more of this material. When many people are assembled in a badly ventilated room, the amount of injurious animal matter in the air is much increased, and is called “_crowd poison_.” Its odor is strong and repulsive to one who just enters the room, but the sense of smell becomes dull to it in a few minutes. It would seem that nature gives a fair warning against harm; but if we disregard the warning it soon ceases. =People who are really Unclean.=--Nature’s plan seems to be for us to live out of doors. Air once breathed is impure. It is just as unfit to enter our bodies as muddy water or decayed food. Yet many who call themselves cleanly and refined, and _will not allow a speck of dirt to remain on their clothes, nor use a spoon just used by another, do not object to breathing into their lungs, over and over again, the cast-off air from the lungs of others_. If a window is opened for ventilation, they are horror-stricken for fear of drafts. Drafts are injurious only to persons perspiring, or to those who have coddled the skin by continually overheating it. There are thousands of schools, churches, and theaters all over the land which reek daily with the malodorous particles from the lungs of their occupants. Although the air in them is odorless to those who occupy them, it is disgusting to any one who enters from the fresh air. Figure 80 shows the correct ventilation of a stove-heated schoolroom. =Dust= causes catarrh of the bronchial tubes and chronic inflammation of the lungs; it prepares for consumption, by gradually weakening the lungs of those who breathe it. Intelligence and common sense are necessary to prevent it from accumulating in the house. The chief purpose of the house cleaning should be not only to remove bits of paper from the floor, which do no harm even to the shoes, but _to remove impurities from the air_. _It does no good to stir up the dust and allow it to settle down again_ (Exp. 12). In many houses dust is thus allowed to accumulate for months. Experiments show that dust and germs floating in the air are not diminished to a great extent by a gentle draft through the room. The windows must be open and sweeping done in the direction of the air currents; the windows should be _left open for a long while after the sweeping_. A windy day is best for sweeping.