MECHANISM AND CHEMISTRY OF RESPIRATION 207 



open fireplaces were the rule, so many chances were left for 

 air to enter houses that special ventilating apparatus wag 

 almost unnecessary. With more skilled workmanship, with 

 machine-made building materials and especially with improved 

 methods of heating, a necessity has arisen for fresh air radia- 

 tors, fans, transoms and the various ventilating devices. 



The need for ventilating a room depends on the rate at 

 which the air is breathed by its occupants. The normal per- 

 son breathes from fifteen to twenty times a minute. When 

 quiet the muscles oxidize materials slowly and the breathing 

 rate is lowered, but any hurry or excitement causes an im- 

 mediate increase in the breathing rate. Since we know that 

 with each breath about thirty cubic inches of air is taken into 

 the lungs, it would be easy to calculate the amount of air 

 breathed in any given time, but this would not really tell us 

 how much air is needed. To know this it would be necessary 

 to find out to what degree air can be impure before it is not 

 properly respirable. Fresh air keeps one active and alert but 

 the air of a close room makes one feel stupid and sleepy, and 

 even produces headache. This is not, as sometimes supposed, 

 because there is not enough oxygen in the air, for all rooms 

 contain sufficisnt oxygen to furnish the hemoglobin with all 

 it can hold; nor is the reason to be found in the presence of 

 an unusually large amount of carbon dioxid. There may be 

 about three per cent of pure carbon dioxid in the air with- 

 out interfering in any degree with its wholesomeness. So long 

 as there is not enough to interfere with the elimination of this 

 gas from the lungs, it will do no injury, and the air of no or- 

 dinary room, however poorly ventilated, contains enough to 

 do this. The ill effects of breathing air already breathed by 

 other persons are partly due to the large amount of water it 

 contains and partly to its high temperature. Perhaps other 

 factors are concerned, but the trouble is generally neither lack 

 of oxygen nor the presence of too much carbon dioxid. 



It is therefore evident that it is no simple matter to tell just 



