AIR 141 



The bacteriological examination of air. 



Of methods available for the bacteriological examination of 

 air a large number have been advocated and practised, but the 

 majority of them are but little used at the present day. Of those 

 now used the following methods may be mentioned : 



I. Simple Plate Exposure. Tubes of sterile agar and gelatine 

 are poured into ordinary sterile Petri dishes and allowed to 

 solidify. These plates are then exposed to the air under investi- 

 gation for definite periods, by simply removing their covers and 

 replacing them after the required time has elapsed, Fifteen to 

 thirty minutes is a convenient time of exposure. In removing 



Fig. 14. Agar plates exposed for 30 minutes in a school 



(a) under ordinary working conditions ; 



(b) after the children had marched out of school. 



the covers care must be taken that the air is -not unduly agitated, 

 and that dust e.g. from the clothes of the investigator is not 

 discharged in the neighbourhood of the plates. The plates are 

 then incubated at 20 to 22 C. for the gelatine, and at 37 C. 

 for the agar. By using both gelatine and agar plates and by 

 giving definite times of exposure valuable data may be obtained 

 as to the relative bacterial state of the atmosphere. The number 

 of organisms developing should be counted, and, if necessary, 

 the different kinds investigated by the usual microscopic and 

 cultural methods. For comparative work the area of the Petri- 

 dish should be calculated [area = (radius) 2 x -^ 2 -], and the results 



