In each minute a man breathes out 450 cubic inches of air containing rather more than 4 per cent, of carbon dioxide. This mixed with three times its bulk of pure air would give a little over one cubic foot containing one per cent, of carbon dioxide. Such air is no longer respirable with safety. The result of breathing it for an hour or two is headache and drowsiness; of breathing it for weeks or months several hours daily, a:

What percentage of carbon dioxide produced by breathing shows that the air is unfit for use? Is the proportion of carbon dioxide found ordinarily in a room poisonous? Why is the percentage of carbon dioxide in air usually employed in deciding whether the air is fit to breathe? What must be borne in mind in deciding from the percentage of carbon dioxide in it that air is no longer wholesome? Is air containing much carbon dioxide fit to breathe? Give an example.

What bulk of air does a man contaminate with carbon dioxide to the extent of one per cent, in each minute? is air so contaminated fit to breathe? What arc the consequences of breathing it for a few hours? What of breathing it for months?

In order to have air to breathe in a fairly pure state every man should have for his own allowance at least about 800 cubic feet of space to begin with, and the arrangements for ventilation should at the very least renew this at the rate of one cubic foot per minute. The nose is, however, the best guide, and it is found that at least five times this supply of fresh air is necessary to keep free from any odor the room inhabited by one adult. If an inhabited room smells "close" to one coming into it from "out of doors," the air in it is unwholesome to breathe for any length of time.