Seasonal Bacteria in Air of Dwellings 55 



of large numbers to the atmosphere, as well as the sur- 

 vival of the bacteria already there. On the contrary, 

 the air of city streets and of human dweUings is par- 

 ticularly rich in microorganisms owing to conditions 

 favoring both the addition of large numbers to the 

 atmosphere and their survival there. 



These facts are of very considerable moment from 

 the standpoint of hygiene and sanitation. We know 

 that, notwithstanding the large number of bacteria 

 in the atmosphere, the air expired from the human lungs 

 is practically germ-free. This means that the microor- 

 ganisms are retained in the nose, mouth, and throat, 

 and that many of them are carried with the dust parti- 

 cles into the lungs. Enormous numbers of bacteria are 

 thus retained, and it is obvious that, everything being 

 equal, the danger from infection is greatest when the 

 number of bacteria in the air is greatest. Persons lead- 

 ing an indoor existence, and those living in large cities, 

 inhale more bacteria and are more exposed to infection 

 than people living in the country. 



It does not follow, at the same time, that the danger 

 of contracting a disease, say tuberculosis, is greater in 

 the summer than it is in the winter, simply because 

 there are more germs abroad. After all, most of the 

 bacteria in the air are harmless, and it is very Hkely 

 that the actual number of the germs of tuberculosis and 

 pneumonia in the winter air is greater than that in the 

 summer air. 



