How are we to determine the quantity of air which an individual requires? We can but make the general statement that it must be sufficient to dilute the carbonic acid exhaled to an extent which precludes poisoning. It is impossible to fix a limit. Breathing becomes embarrassed, and frontal headache and other symptoms make themselves felt when 10 per cent. of pure carbonic acid is mixed with air. Even in so large a proportion as this, carbonic acid is not fatal to life. Yet an atmosphere in which there is present a hundredth part of this amount of carbonic acid, produced by respiration, is extremely injurious to health under the ordinary conditions in which people live. It may be asserted, therefore, that under ordinary conditions 0·1 per cent. is the extreme limit for wholesome living. But again we are obliged to add that air contaminated to this extent is not under all circumstances injurious to health. The explorers on the recent Antarctic Expedition were obliged at times to sleep three men in one sleeping-bag, with the aperture of the bag tightly closed. The atmosphere must have been heavily laden with carbonic acid. Dr. Wilson assures us that it was impossible to keep a pipe alight inside the bag. Not that any man so placed would desire, one would imagine, to add the combustion-products of tobacco to those given off from the lungs! The survival of the explorers proves that it is impossible to fix a limit of safety even for the carbonic acid in air vitiated by respiration. It is, however, a matter of common observation that air which is moist and warm, owing to respiration, and tainted with the odours of humanity, is extremely prejudicial to those who live in it. Such an atmosphere is a favourable medium for the conveyance of germs, whether of the common cold or of a more virulent type. At one time it was supposed that the volatile emanations which can be condensed, along with water, by hanging a vessel of ice to the ceiling of a crowded room, were actively poisonous; but this statement has not been confirmed by recent research. It is unnecessary to call any such evidence in support of the thesis that human beings thrive better in fresh air than in foul. The admirable results achieved by the “fresh air cure” show that there is no degree of vitiation which can be pronounced innocuous. Nevertheless, public opinion demands that sanitarians should give some figure as a guide. Commonly they fix the maximum of carbonic acid compatible with health at 0·06 per cent., the quantity of carbonic acid being taken as the measure of all impurities present. An adult exhales about 0·6 cubic foot of CO₂ per hour. Fresh air already contains about 0·04 per cent. If, therefore, the percentage is not to rise higher than 0·06 per cent., each adult must be supplied with 3,000 cubic feet of air per hour. With good ventilation air may be changed four times an hour, and therefore 800 cubic feet is regarded as sufficient space for each occupant of a room.