THE RESPIRATION OF INJURIOUS ATMOSPHERES Carbon Dioxide.—Until within recent years it was supposed that carbon dioxide was harmful when inhaled even in small quantities. In any but the most recent textbooks the estimates of the quantity of air necessary for the efficient ventilation of a room are based on the assumption that the carbon dioxide present must not rise above a certain level. The figure usually given is o.1%. This rule is probably not a bad one, but it is now known that carbon dioxide in such small quantities is quite innocuous and even in much greater quantities would have to be breathed before an injurious level was reached. Men can inhale 5% for some hours without suffering from much more than discomfort, and untutored persons would not be conscious of the presence of 2% of carbon dioxide in the air if it were otherwise pure.
The rule that the air of dwelling rooms should not contain more than o.i% carbon dioxide is therefore useful, because air laden beyond that limit with carbon dioxide is also probably laden with other things to an injurious degree.
Considering the above points, the benefits of ultra-violet rays are treated elsewhere (LIGHT and RADIATIONS in relation to health). Here it is only necessary to say that ordinary window glass is relatively opaque to ultra-violet light and even specially manufactured glasses are often much less penetrable than is the open window. The beneficial effects of changing the air in a room on the disposition of germs has been demonstrated beyond dispute by experiments carried out by Leonard Hill. He dissipated a certain number of germs into a room with the windows shut; 20 min. later he exposed a plate of gelatine and found on it 39 germs. On a second occasion he dissipated the same number of germs into the room, opened the windows, and on exposure of the plate after 20 min. only one germ settled on it.
The advantage of keeping the air in motion may be illustrated by the following experience. On two successive days (on each of which the outside air was hot and still) a small over-crowded room was occupied for 8 hours by II typewriting clerks at work, a doctor and two experimenters. On the first day all avenues of ventilation were as far as possible closed, the chimney stuffed up, curtains put over the doors, etc., but the air in the room was actively circulated by electric fans. The percentage of carbon dioxide rose to about 2%. On the second day the fans were not in motion, but panes were abstracted from the windows. Chemical analysis showed that the air, though still, was pure. The steno graphers who were unaware of the point of the experiment agreed that the conditions were more tolerable the first day than the second, and examination of their work bore out that statement. The benefits of movement are probably due to two causes: (I) the actual stimulating effect of moving air passing over the skin, and (2) the fact that moving air evaporates moisture from the skin much more readily than still air. The relative importance of these factors probably differs much in different persons. The moral of the above experiment is not to disparage purity. It is to emphasize the necessity of combining purity with movement.
In the above table time is measured in hours and the concentra tion in parts of carbon monoxide per million of air. The figures assume that the subject is at rest and inhaling about seven litres of air per minute. If he is active and therefore inspiring greater quantities of air, the time necessary to produce death or uncon sciousness is cut down directly in proportion to the magnification of the quantity of air breathed per minute.