184 THE HYGIENE OF RESPIRATION 



respiratory * carbon dioxide in 10,000 parts of air indi- 

 cates that the air is as bad as can safely be tolerated 

 by human beings. As expired breath contains 4 per 

 cent of carbon dioxide, or 400 parts in 10,000, each 

 breath must be diluted with at least 200 parts of fresh 

 air to make it safe for breathing. Since the amount of 

 air expired by one person in an hour 2 is 15.6 cubic 

 feet, it is evident that each person in a room must be 

 supplied every hour with about 3,000 cubic feet of 

 fresh air, if he is to avoid the debilitating effects of 

 bad air. 



Ventilation. To supply this amount of fresh air 

 economically is the problem of ventilation. In summer, 

 ventilation is not difficult because the windows can be 

 kept open so that the air can circulate throughout the 

 rooms of a house. Under these circumstances, all the 

 air of a room can be changed as frequently as once a 

 minute. In cold weather, when the windows cannot 

 be freely used in this way, special provision has to be 

 made to get rid of the bad air and to introduce in its 

 place air that is fresh and pure. In addition, the air 

 which is thus introduced must be warmed, so that it 

 is customary to consider heating and ventilation to- 

 gether and to provide for them at one and the same 

 time. 



1 By respiratory carbon dioxide is meant that which is added to 

 atmospheric air by the breath. Atmospheric air itself contains 

 from 31/2 to 4 parts of carbon dioxide in 10,000 parts. For ex- 

 ample, when by test 10 parts of carbon dioxide in 10,000 are found 

 in a school-room, 4 parts are the atmospheric carbon dioxide, and 

 the remaining 6 parts are respiratory carbon dioxide. In this 

 case, it is evident that there is being supplied to the room only 

 | the amount of air which is necessary. 



2 In a year, each adult breathes about 10,000 Ibs. of air. From 

 it, he takes 657 Ibs. of oxygen, and to it he gives 730 Ibs. of carbon 

 dioxide. 



