Others again, where the kitchen is not properly ventilated, get the smoke of frying and the smell of cooking all through them.

But such sources of impurity, while injurious and always to be strictly avoided, are neither half so dangerous when they occur, nor one-tenth so common as the great chief cause of impure air--our breaths and the other gases from our bodies, with the germs they contain.
Drafts not Dangerous.

Now comes the practical question, How are we to get rid of these breath-poisons?
From the carelessness of builders, and the porous materials of which buildings are made, most houses are very far from air-tight, and a considerable amount of pure air will leak in around window-casings, door-frames, knot-holes, and other cracks, and a corresponding amount of foul air leak out.

But this is not more than one-fifth enough to keep the air fresh when the rooms are even partially occupied, still less when they are crowded full of people.

As each individual, breathing quietly, requires about four bushels of air (one and a half cubic yards) a minute, it is easy to see that, when there are ten or more people in a room, there ought to be a steady current of air pouring into that room; and when there are twenty or even forty people, as in an average schoolroom, the current of air (provided there _is_ one) must move so fast to keep up the supply that the people in the room begin to notice it and call it "a draft." It would be difficult to ventilate a room for even four or five persons without producing, in parts of it, a noticeable draft of air.