CHAPTER VI. 



THE SAFEGUARDS OF THE BODY AGAINST IN- 

 HALED DUST. 



HAVING now gathered together a con- 

 siderable number of facts about the 

 distribution in the air of dust particles and 

 among them of living germs, we are ready to 

 consider their significance if they have any 

 to human beings, who must live in and breathe 

 this more or less dust-laden air. 



The average amount of air which a healthy 

 grown person takes in at each breath has been 

 estimated to be about one half a litre (about 

 30 cubic inches). We have seen from our 

 various analyses of the air of different places in 

 and about New York, under ordinarily favor- 

 able conditions, that the number of living 

 germs in 10 litres of air varies from 1 1 to 376. 

 So that basing our estimate upon these studies 

 36 



