There are, of course, two things that must be true of any air that is to be breathed; these are that it must contain enough oxygen and must not contain too much carbon dioxide. So far as the oxygen supply is concerned we may state that only with the greatest difficulty are conditions reached in which there is not enough oxygen in the air. As we all know, the air becomes thinner the higher we go above the surface of the earth; both mountain climbers and aviators have attained heights at which the amount of oxygen in the air is only about one-third that of ordinary air and have been able to obtain enough oxygen for their bodily needs even under those extreme conditions. It is quite evident that a room could scarcely be so poorly ventilated as to bring the oxygen supply down below this figure, so that no attention need be paid to the oxygen supply in working out practical methods of ventilation. Air which contains carbon dioxide to the extent of four per cent could not be breathed because the carbon dioxide being produced in the body would not diffuse out fast enough into an atmosphere containing that amount of carbon dioxide to keep the body alive. This again is a percentage of carbon dioxide that is practically never reached. Probably the most famous case in history of death from poor ventilation is the “Black Hole of Calcutta,” a dungeon room about twenty feet square with only two small windows, in which one hundred and fifty British soldiers were imprisoned over one night; all but twenty-three of these died, but it is doubtful whether their death was actually due either to deficiency of oxygen or to excess of carbon dioxide. This is because there were enough other factors which would make the air unbreathable to bring on death before either of these could come into play. The modern science of ventilation concerns itself with these other factors; chief among them is the factor of moisture. As we shall see in the next chapter our bodies are constantly giving off from the lungs and by evaporation from the sweat glands water vapor into the air. This causes the humidity to go up rapidly in rooms where people are congregated, and particularly so where there are many people present. Also everyone gives off a great deal of heat. We now know that the feeling of closeness which we ascribe to a poorly ventilated room is due to the combination of warmth and moisture. We also know that the discomfort which comes from being in such rooms is due to the same causes. Actual vitiation of the air is much less disagreeable than is the accumulation of heat and moisture. In theory, of course, the best ventilation is secured by keeping rooms flooded with outdoor air. In practice, however, this does not always work out; for example, in many cities the air is so laden with dust and smoke as to be bad for everybody and even dangerous for sick people. Before such air is breathed the smoke and dirt should be gotten out of it.