The purity of air in any habitable enclosure is determined by the amount of CO2 (Carbonic acid gas) included in its composition. The process of ventilation is that of adding fresh air to the impure atmosphere of houses, until a desirable quality is attained. In the opinion of hygienists, when air does not exceed 6 to 8 parts of CO2, by volume in 10,000, the ventilation is desirable. Ordinary outdoor air contains about 4 parts of C02 to 10,000, while very bad air may contain as high as 80 parts to the same quantity. The quantity of air required for the ventilation of a building is determined by the number of people to be provided. The amount of air required per individual per hour necessary to produce a desired condition of ventilation is determined by adopting a standard of purity to suit the prevailing circumstances.

In hospitals where pure air is considered of greatest importance 4000 and 5000 cubic feet per inmate per hour is not uncommon. The practice of supplying 30 cubic feet of air per person per minute (1800 cubic feet per hour) seems to fulfill the average requirements. It is the amount commonly specified for school-rooms.

The quantity of fresh air required per person to insure good ventilation will depend on the type of building to be supplied and varies somewhat with different authorities. The De Chau-mont standard is that of 1 cubic foot of air per second or 3600 cubic feet per hour, for each person to be accommodated. De Chaumont assumed a condition of purity which will permit less than 2 parts in 10,000 of C02 over that carried by country air. In considering the same problem from the basis of permissible C02, if 6 parts of C02 in 10,000 represents purity of the required air, then 3000 cubic feet per person per hour is necessary. Likewise, the varying amounts for different degrees of purity are given by Kent in the following table. The upper line gives the permissible number of parts of CO2 per 10,000. while below each factor appears the number of cubic feet of air required per hour for each person supplied.

It is generally recognized, that it is possible to live under conditions where no attempt is made to change the air in a building. It is also an established fact that the only preventive and cure for tuberculosis is that of living constantly in an atmosphere of the purest air. The greatest attainable degree of health is enjoyed by those who live in the open air, because oxidation is one of the most efficient forms of prevention and elimin ation of disease, and an abundance of pure air is the only assured means of sufficient oxidation.

The De Chaumont standard is intended to represent the limit beyond which the sense of smell fails to detect body odors or "closeness" in an occupied room. The amount of CO2 that air contains is not an absolute index of its purity, but it gives a standard under ordinary conditions, makes possible the requirement of a definite quantity of air. If it were possible to express the amount of oxygen contained in the atmosphere, the same relative condition might be attained.

The ordinary man exhales 0.6 cubic foot of CO2 per hour. Some forms of lighting apparatus produces this gas in greater amounts. The ordinary kerosene lamp gives out 1 cubic foot of CO2 per hour. A gas light using 5 cubic feet of gas per hour produces 3.75 cubic feet of CO2 in the same time. Any form of combustion permitting the products to escape into the air of the room tends to lower the quality of the atmosphere by adding to its content of CO2.

The prevailing impression that impure air is heavy and settles to the floor is erroneous. Impurities in the form of gases and vapors (principally carbonic acid gas and odors) diffuse throughout the entire space, and the entering fresh air tends to dilute the entire volume.

As a quantative problem, ventilation consists in admitting pure air into an impure atmosphere in amount to give a definite degree of purity. This is accomplished by admitting sufficient air to completely change the atmosphere at stated intervals, or to provide a definite quantity for each inhabitant.

The methods by which ventilation may be accomplished will depend on the type of building to be ventilated and the apparatus it is possible to use. When the use of mechanical ventilation appliances are permissible, any desired degree of atmospheric purity may be maintained at all times, under any condition of climate or change of weather.

In buildings where mechanical ventilation cannot be considered as that of the average dwelling, the problem is one of producing an average condition of reasonably pure air by natural convection. In the average dwelling, ventilation is accomplished by the natural draft produced in chimneys or air flues, by partially opened windows and by the force produced by the movement of the outside air. In some buildings a better condition of ventilation is attained by ordinary means than at first sight seems possible.

The fact that it is difficult to keep a house at the desired temperature during cold weather indicates that a considerable quantity of outside air is constantly entering and heated air is leaving the building. It may be, however, that the ventilation under such condition is unsatisfactory, even though the amount of air which enters the building is sufficient in quantity to produce a desirable atmosphere. If the places of entrance and exit are so located that the entering air has no opportunity to mix with the air of the building, the advantage of its presence is lost.

In the burning of fuel in stoves and furnaces, the amount of oxygen necessary for combustion is supplied by the air which is first taken into the house and thus forms its atmosphere before it can enter the heater. Theoretically, about 12 pounds of air are required for the combustion of a pound of coal, but in practice a much larger amount actually passes through the heater. As given by Suplee, from 18 to 24 pounds of air are actually used in burning 1 pound of coal. If 20 pounds of air per pound of fuel is taken as an average, there will be required 198 cubic feet of air per pound of coal consumed. In a building that requires 10 tons of coal to be used during the winter months, this would necessitate the average use of 1977 cubic feet of air per hour, which must be drawn into the house before it can enter the stoves. This air acts as a means of ventilation and if it is used to advantage would furnish a supply sufficient in amount to produce excellent ventilation, considerably more than enough for two people. The amount of air drawn into the house in this way is further increased by that which passes into the chimney flue through the check-draft dampers, when the fires are burning low.