This is done sometimes by forcing it through fine mesh cloth bags, or the most modern scheme is by passing it through a thin screen of water and so washing the dirt and smoke out of it. The second practical difficulty with flooding rooms with outside air is the expense in cold weather of warming the large volumes that would be required. For this reason it has been found feasible in churches and public halls that are occupied only occasionally to use the same air over and over by keeping down the temperature and moisture. Of course, this cuts down very greatly the expense of heating. There is one other source of harmful effect from bad air besides the high humidity and undue warmth; this is the presence in it of ammonia and other poisonous compounds that are given off from the bodies of people. It used to be believed that organic poisons were exhaled from the lungs with every breath, but we now know that the amount of these, if any are present, is too small to be important in comparison with the very much larger amounts that come off from the evaporating sweat, from decaying teeth, and from the digestive tract; there is no doubt that in any assemblage of people the air will be vitiated by organic poisons from these sources. The more cleanly the individuals are, the less will be the contamination. It is generally believed, although perhaps not absolutely proven, that the bad health found in sweatshops and crowded slums generally is due largely to chronic poisoning from the constant breathing of effluvia from the unwashed bodies and clothing of the inhabitants. The obvious remedy is insistence upon personal cleanliness, although this does not lessen the desirability of breathing as pure air as can be gotten. The point to be emphasized is that where personal cleanliness prevails, the closeness of rooms is chiefly due to excessive moisture ordinarily accompanied by too high a temperature. Ventilation measures should be carried out with this in mind. CHAPTER XVII THE SERVICE OF REMOVAL OF WASTE