(1) 1000 cubic feet of air for every 20 minutes Explanation: Breathing sixteen times every minute an average volume of air amounting to 30 cubic inches, we find that the expired air amounts to no less than 17 cubic feet per hour. This contaminated air contains only 16 per cent of oxygen, and 4.5 per cent of carbonic acid gas, and is sufficient to vitiate no less than 3,000 cubic feet of fresh air. Pure fresh air contains from 0.03 to 0.04 per cent of carbonic acid gas, or at the most, .