KEEPING WELL 409
The air in any limited space becomes exhausted as it is used in respiration. At the same time it becomes impure and unsuitable for respiration by the addition of carbonic acid gas, which is exhaled with every expiration of the lungs. It is further contaminated by exhalations from the body, by the vapors from cooked food, by coal and sewer gas, by the combustion of lamps and illuminating gas, by dust and dirt which find an entrance into every house, and by the germs of disease. The problem is, therefore, to get rid of the impure air and to have a constant supply of fresh and pure air. Few, if any, houses are so carefully made that air cannot enter by cracks in doors and windows, and by the loose joints which have resulted from bad workmanship or shrinking materials. But this will seldom result in effective ventilation, even when the winds are high and resist all attempts to shut them out, except, of course, in the most primitive or the most dilapidated houses. In warm weather, ventilation is easily effected by keeping doors and windows open most of the time. In cold weather (except, of course, in the extreme of cold weather) let the house be freely opened morning and evening for half an hour or so, and flushed, as it were, with fresh air. In the meantime, try and arrange a draught by the stairways, or by the air-shafts and windows and doors, if one's home is on a single floor, which shall provide a continuous inflow of pure air, and an outflow of impure air. Ventilation may be provided in windows or in walls, one for the heavy gases at the bottom, and another for the lighter ones at the top of the room. Many systems of ventilation involving more or less expense have been advocated, but for private dwellings, especially when the question of expense is to be considered, an efficient system can always be devised with the exercise of ordinary common sense, by utilizing doors, windows and stairways. For public institutions, or very large buildings, some definite system of ventilation is imperative, just as one would follow a definite system of lighting and heating. In all cases, the simplest method should be followed when possible.