IMPURITIES IN AIR. 137 



breathe two different breaths ; you take in one, you give out 

 another. The composition of these two breaths is different. 

 Their effects are different. The breath which has been 

 breathed out must not be breathed in again." KINGSLEY. 



The air in the vesicles receives from the blood carbon 

 dioxid, water vapor, and other impurities above mentioned. 

 It has been believed for a number of years that the organic 

 impurities constitute the most dangerous element in expired 

 air. Carbon dioxid, though to some extent a poison, is not 

 very injurious in sucli quantities as ordinarily exist in the 

 air, even in poorly ventilated rooms ; while the headache and 

 drowsiness that one experiences in a close room where there 

 are a number of people is due to the reabsorption of these 

 organic matters. It is not due to lack of oxygen, for the oxy- 

 gen may be reduced to 13 per cent without causing discom- 

 fort. A person may breathe air containing one per cent of 

 carbon dioxid, with a corresponding reduction of oxygen, when 

 the carbon dioxid is generated by ordinary chemical processes 

 (as in a small room with a large kerosene lamp, or a gasoline 

 stove) ; but air having one per cent of carbon dioxid produced 

 by breathing is highly injurious, because it contains the or- 

 ganic impurities above noted, and the term "crowd-poison" 

 has been employed for this material. Later investigators, 

 however, maintain that there is nothing poisonous in the 

 freshly expired breath. 



HEATING AND VENTILATION. 



We often hear the expression " Free as Air." While air 

 is free, and there is an abundance of it, still the expression is 

 practically untrue. In the temperate and colder parts of the 

 world, and for many indoor workers, especially those follow- 



