I4O AIR 



abound in fresh air. The bacilli of the B. coli group obtained 

 from drain air correspond essentially in their characters with 

 those of sewage, while such bacilli are absent, or nearly absent, 

 in fresh air. The presence of these bacilli was more readily 

 detected in the air of drains. When a test microbe (B. pro- 

 digiosus) was added in bulk to the contents of the drains, it 

 could be recovered, under suitable conditions of splashing, from 

 the drain air in great numbers and at considerable distances 

 from the points at which it was added to the sewage in the 

 drain. 



In a second investigation, using for examination the drain 

 air of a large public institution and of a private dwelling, 

 Andrewes showed that sewage bacteria could readily be demon- 

 strated in the drain air in almost any situation, but that their 

 presence is of a highly intermittent character. He concluded 

 that, in the drainage system of a house or institution in ordinary 

 occupation, the determining cause of the presence of sewage 

 bacteria in the drain air is droplet contamination from splashing, 

 such droplets being of extremely minute size. As regards the 

 content of the drain air in faecal bacteria, the most important 

 determining factor is the faecal content of the sewage. The 

 relative number of lactose fermenting coliform bacilli in the 

 drain air bears a direct relation to the abundance of faecal 

 material in the sewage. 



In a further report Andrewes showed that when the air of 

 the drain or sewer becomes infected from sewage splashing 

 natural air currents may transport sewage microbes for con- 

 siderable distances before they settle. 



The scantiness of the bacteria characteristic of sewage in the 

 air of sewers is also shown by Delepine's 1 systematic observa- 

 tions upon the air of a Manchester main sewer. He found 

 sewage bacteria to be very scanty, and out of several thousand 

 bacteria collected by him from sewer air only six proved to be 

 typical B. coli. 



1 Loc. cit. 



