255. There is only one way by which the fitness of air for respiration can be determined with any certainty, and that is by chemical examination. The sense of smell cannot be depended on for this purpose, because it is so easily blunted. After remaining in a room full of bad air for 10 or 15 minutes a person will usually be unable to perceive any unpleasant odor about it. It is only upon passing from the fresh outdoor air into a tainted atmosphere that the sense of smell can be relied upon to discover the bad quality of the air, but even then no accurate estimate can be made of the real degree of pollution.

When the carbonic acid gas due to respiration and exhalation does not exceed 2 parts in 10,000, the air is considered fresh and wholesome. When a definite animal or musty odor begins to be perceptible, the air is said to be rather close, and the exhaled carbonic acid gas is found to amount to 4 or 5 parts. When the proportion increases to 7 or 8 parts, the air is called close or very close, and when it reaches 12 parts in 10,000 the air is pronounced very bad. Above this point the sense of smell fails to perceive any marked difference.

256. Proper ventilation requires that the exhaled carbonic-acid gas should not, at any time, exceed 2 parts in 10,000. The amount of air that will be required, per minute, to maintain this degree of freshness may be readily computed.

Each adult person in good health breathes out about. 7 of a cubic foot of carbonic acid per hour, and in the same time exhales from the lungs and skin about .091 pound of water, which at 70° becomes about 80 cubic feet of steam or vapor. He also imparts about 400 heat units per hour to the air of the room by conduction and radiation from his body. The air supplied for ventilation must, therefore, serve three purposes: to dilute the carbonic acid to a proper degree; to absorb the vapors exhaled, without permitting any noticeable increase in the humidity; and to absorb the heat as rapidly as emitted, without perceptible rise of temperature.

In order to dilute .7 of a cubic foot of carbonic acid to the proportion of 2 parts in 10,000, it is necessary to mix it with 10,000 X .7 / 2 = 3,500 cubic feet of air. Therefore, the air supply should be 3,500 cubic feet per hour for each adult person, or nearly 1 cubic foot per second. Taking into consideration the smaller amount of carbonic acid produced by children, the supply for schoolrooms and similar places may be reckoned at 3,000 cubic feet per person, per hour.

257. The volume of the air supply required varies with the season and the condition of the outer atmosphere. In clear, cold weather, 3,000 cubic feet per hour per head is sufficient for good ventilation; but on a mild spring day, with a damp, muggy atmosphere, it is difficult to get enough air without getting too much heat at the same time. The air is not dried in the least by passing through heaters, and is so moist and warm that it fails to remove the animal heat as fast as necessary. Where people are assembled in considerable numbers, as in schools, etc., it is likely to produce feelings of great lassitude, and even fainting spells. On such occasions as these, it is highly desirable to have some means for reducing the humidity of the air.

258. Cubic space is not an important factor in ventilation, but there is a certain minimum space required for each person which must never be disregarded. The carbonic acid and other exhalations from the body diffuse themselves through the air with comparative slowness, and in order to secure their dispersion into the atmosphere with proper rapidity, it is necessary that every person should have a certain amount of "breathing room."

The minimum space that may be permitted, in cubic feet per person, is as follows:

Floor space must be considered as much as mere cubic space. Thus, in a schoolroom, there must be an aggregate of 25 square feet of floor surface for each pupil; and in hospitals each bed must have 100 square feet of space.

In stables, each horse or cow should have 100 square feet of floor space. A horse should have 1,600 cubic feet of air space, and a cow not less than 1,200. As cows are usually kept to furnish milk for food, it is important that they should be kept in a healthy condition, and that the air around them should be clean. The practice of furnishing their quarters with plenty of good air has been found highly beneficial.