In practice the room will show a higher amount than 50 per cent. humidity with this addition of the amount of water, because of the water vapor that is exhaled from the lungs of the pupils. That a considerable amount of water vapor is added to the atmosphere by breath exhalation is made evident from the moisture condensed by breathing on a cold pane of glass. In any unventilated room occupied by a considerable number of people the humidity is thus increased a very noticeable amount. The change in humidity of the air in a closed room filled with people is very pronounced. The constant exhalation of moisture from the lungs is sufficient to saturate the air in a short time. The heavy atmosphere of overcrowded, unventilated rooms is due to moisture exhalation, body odors and increased carbonic acid gas. As the humidity of the atmosphere is increased a sensation of uncomfortable warmth is the result of the lesser evaporation. CHAPTER XI VENTILATION 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 CO₂ 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.