We have noted the fact that of the myriads upon myriads of swarming cells which the blood contains, a large proportion are the oxygen-conveyers. When you take air into your lungs, these cells absorb the precious element, and rush with it to all parts of the body. After distributing the oxygen wherever it is needed, they pick up for the return journey to the lungs all manner of débris and gases—the poisons which are produced by the organs of the body as they carry on their work. As Metchnikoff has shown us, it is the accumulation of poisons produced by the activity of our various organs which, unless properly disposed of, or kept below excessive quantities, bring about premature old age, the majority of all diseases, and early death. The amount of poisons which the average person throws off from the body with a single breath, as has been shown by delicate laboratory experiments, is enough to contaminate and render unfit for breathing three cubic feet or three-quarters of a barrel of air. Assuming an average of twenty breaths per minute, which is the normal rate for breathing for adults, the amount of air each person contaminates per minute will be sixty cubic feet, or one cubic foot a second. If you hold your breath for a minute, you will be conscious of an extremely unpleasant feeling, which is the way in which the body manifests its urgent need for oxygen. The need of ventilation is not merely the need of oxygen, however. There may be plenty of oxygen in the air of a room which has been closed for some time, and which has been breathed in and out of the lungs of the people in the room; the trouble is that this oxygen is unfit for breathing, being full of impurities thrown off by the bodies of these people. HOW TO CALCULATE ROOM VENTILATION Dr. Kellogg has supplied some exceedingly useful calculations of the degree of ventilation needed in rooms of various sizes. “Every one,” he says, “should become intelligent in relation to the matter of ventilation, and should appreciate its importance. Vast and sometimes irreparable injury frequently results from the confinement of several scores or hundreds of people in a school room, church or lecture room, without adequate means of removing the impurities thrown off from their lungs and bodies. The same air being breathed over and over becomes intensely charged with poisons which render the blood impure, lessen resistance and induce susceptibility to taking cold and to infection with germs of pneumonia, consumption and other infectious diseases which are always present in a very crowded audience room.