Close air, just as much as stagnant water, promotes the growth of dangerous microbes, and the chances of infection are greatly enhanced where a number of people are gathered together in such places. Many of them may be suffering from infectious diseases of the respiratory organs; they exhale, and also eject by coughing or sneezing, an enormous number of microbes, which mingle with the air and multiply at their leisure in such close atmospheres; and this is especially so when they are assisted in their growth by the great heat prevalent in such places, particularly in winter time. Bacteriological examinations made of the air of such localities have shown an enormous number of dangerous microbes. We need, therefore, not be surprised when persons, and especially children whose resistance is diminished, often contract tonsillitis, diphtheria, bronchitis, or pneumonia, etc., after having passed an evening in such a place, the air being hot, and particularly if, at the same time, the temperature outside was very low. Living in a close room will soon tell on the general health, and this is easily visible in the appearance of such persons. Their pale faces form a striking contrast to the fresh rosy cheeks of those who habitually live in the fresh air. Those who have to pass their lives in offices are to be pitied, although, to a large extent, it is their own fault; they deprive themselves of the benefit of fresh air, which, after office hours, they would have ample opportunity to obtain if they would not persist in spending their leisure time in a club or other close place, instead of taking a brisk walk and exercise in the fresh air. No wonder that such people easily acquire dyspepsia and stomach troubles! Exercise in the open air is most valuable for promoting an appetite, and persons sitting constantly in close places often lack this; thus their gastric juice, which is indispensable to a thorough digestion (see chapter on hygiene of eating—how to obtain an appetite) fails, and eating without this juice their food will not be well digested and will create stomach troubles, which are extremely frequent in such office workers, but rarer in the case of peasants, coachmen, and other fortunate individuals whose occupation keeps them in the fresh open air. There is an increased amount of carbonic acid in the air of all localities where many persons are present. According to Pettenkofer, even in our living-rooms the carbonic acid content of the air is increased above the normal; and still more is this so in lecture-halls (3.2 per cent., against a normal content of 0.5 per cent.), in public houses (4.9 per cent.), and most of all in school rooms (7.2 per cent.).