28 



HUMAN BIOLOGY 



If brooms are used, small pieces of crumpled newspapers or 

 tea leaves should be moistened and scattered on the floor 

 before the sweeping is done. 1 



In cleaning public buildings, the floors should first be 

 sprinkled with moist sawdust and then the coarser dirt 

 collected by brushing with hair brooms. The floors should 

 then be washed each day if possible. 2 Dirty streets, too, 

 are a constant source of dust infection. Most of the irrita- 

 tion and possible diseases from this source would be avoided, 



1 Figure ,11 shows, so far as bacteria are concerned, the compara- 

 tive results obtained by four methods of sweeping. Four rugs of 

 the same size and approximately the same amount of use were 

 selected, and placed at night in four different rooms. Early the 

 next morning a Petri dish was uncovered in each room, and thus 

 the nutrient agar of each dish was exposed to the air of the room 

 for five minutes ; after which the dishes were covered. 



A second set of four dishes was then opened in turn for five min- 

 utes while the four rugs were being cleaned as follows. Rug D was 

 swept with a dry broom; rug C was covered with pieces of wet 

 newspaper and then swept with a broom ; rug B was cleaned with 

 a carpet sweeper ; and rug A was cleaned with a vacuum cleaner. 



All of the eight dishes were then closed and kept in a warm room 

 for five days and at the end of that time were photographed. (See 

 Fig. 11.) The number of bacteria colonies in each dish were counted, 

 and the results are expressed in the following table : 



Hence over four times as many bacteria were stirred up by a car- 

 pet sweeper as by a vacuum cleaner, eighteen times as many when 

 the sweeping was done with a broom and wet paper, and over seventy 

 times as many when a dry broom was used. 



2 We are indebted to Mr. John H. Federer, Superintendent of the 

 New York Public Library building, for valuable information con- 

 tained in the preceding paragraphs. 



