PRINCIPLES OF VENTILATION 313 



parts in 10,000, of carbon dioxide. It was assumed many years 

 ago that air had become too vitiated for use when the propor- 

 tion of carbon dioxide had been increased more than about 

 0.03 per cent., or 3 parts in 10,000, due to breathing, i.e., when the 

 proportion of carbon dioxide had been increased from 0.03 per 

 cent, to more than 0.06 per cent. While it has been conclusively 

 proved that the breathing of air containing as much as 5 per 

 cent., or 500 parts in 10,000, of carbon dioxide, has not the 

 slightest depressing effect, still the old rule laid down years ago 

 is the rule which controls in nearly all ventilating systems 

 today. Even those who admit that a large amount of carbon 

 dioxide in the air is harmless and that the theory of crowd 

 poison can not be proved, still maintain that the percentage 

 of carbon dioxide in the air is a good indication of the whole- 

 someness or degree of vitiation of the air. 



343. Calculation of the Amount of Fresh Air Needed per 

 Minute by Each Person. If we admit that the amount of 

 carbon dioxide in the air produced by breathing must not be 

 permitted to rise above 0.03 per cent., or 3 parts in 10,000, we 

 need only to know how much carbon dioxide is exhaled per 

 hour by each person in order to determine the amount of fresh 

 air which must be supplied him. This is easily calculated as 

 follows: Physiologists tell us that the average amount of the 

 TIDAL AIR, i.e., of the air inhaled and exhaled at each breath, is 

 about 28 or 30 cu. in. and that a person ordinarily breathes 

 about 17 times per^ minute. Many analyses of exhaled breath 

 have been made and they show that exhaled breath usually con- 

 tains about 4 per cent, by volume of carbon dioxide. From 

 these facts, we see that each person exhales about %o cu. ft. 

 of carbon dioxide per hour. (Show that this calculation is 

 correct.) Now, if the air of the room is to be so diluted by the 

 admission of fresh air that the proportion of carbon dioxide 

 derived from the breath is not greater than 0.03 percent, or 0.0003 

 of the whole, we see that there should be provided 0.7 -r- 0.0003 

 or 2333 cu. ft. of fresh air per hour for each person in the 

 room. 



