247. Air is rendered unfit for breathing by a great variety of causes, that of respiration being the most conspicuous.

Each adult person breathes about 20 times per minute and inhales about 30 cubic inches of air at each breath. The air on entering the lungs contains about 79 per cent., by volume, of nitrogen, and 20.8 per cent. of oxygen, with a very small fraction of carbonic acid; but when it is expired it contains only about 15.4 per cent. of oxygen, while the carbonic acid is increased to about 4.3 per cent. of the total volume. The quantity of available oxygen is thus reduced from 20.8 parts to 15.4 parts, or very nearly 26 per cent.

Oxygen is the only part of the air (excepting the moisture) which serves to sustain life. The nitrogen is wholly useless, being perfectly inert, and is expelled from the lungs without undergoing any change whatever. It is inhaled simply because it is so thoroughly mixed with the oxygen that its inhalation cannot be avoided. Only about 21 per cent. of the air is of any use for sustaining life; the remainder merely acts in a mechanical way to dilute the oxygen and increase the volume of the mixture. Air which is breathed once thereby loses 26 per cent. of its oxygen, that is, 26 per cent. of its total life-sustaining power.

248. The amount of oxygen consumed per hour varies with the age and state of health of the person, and also with the degree of activity-whether asleep or awake, or engaged in muscular labor. Animals require more oxygen than men.

The amounts of oxygen consumed in equal times, per pound of actual weight (not per head), are in the following proportions:

Man, 100. Sheep, 117. Dog, 283.

Horse, 135. Ox, 132. Chicken, 312.

The average adult man consumes oxygen at the rate of 20 cubic feet per day, or about 1 cubic foot per hour while engaged in active labor, and less when quiet or asleep.

249. The formation of carbonic acid in the lungs is practically continuous, only a portion being expelled at each breath. The oxygen which is absorbed into the lung cells comes into contact with carbonaceous matter derived from the food, and combination takes place, resulting in the formation of carbon dioxide, commonly called carbonic acid, (symbol CO2). This process, in fact, is one of combustion, and is in every respect the same as that which takes place in a furnace, except that it is less intense. The carbon is supplied at a rate which maintains the temperature at about 98° F. The amount of C02 which is thus produced in 24 hours averages about 16 1/4 cubic feet for each adult person. Children produce a little less, and sick people often considerably more. The rate of production varies from hour to hour, being commensurate with the muscular activity of the individual.

If this carbonic-acid gas is not contaminated with anything else, air containing 1 1/2 per cent. of it may be breathed for an hour or more without harm, but in most cases it is accompanied by other poisonous compounds, which make one-tenth of that proportion hardly endurable.