CHAPTER XXVIII. 

 BACTERIA IN AIR. 



BACTERIA require for their growth moisture, food, a suitable 

 temperature and usually the absence of direct sunlight. The 

 moisture conditions of the atmosphere at times may be optimum 

 for the growth and reproduction of bacteria, but none of the other 

 conditions are. Hence, bacteria do not multiply and grow in the 

 atmosphere as they do in water, soil and milk. These substances 

 may and do have a natural bacterial flora, but we cannot consider 

 the air as having a definite one, for the number and kind continually 

 vary with many factors and there are scarcely two places having 

 the same number and species of microorganisms. 



How Bacteria Enter Air. The earth is surrounded by the atmos- 

 phere, which when "looked at as a whole, its calms are exceptional, 

 and its movements are the rule. We may find the gentle breeze, 

 the cyclonic wind or the restless tornado, but always active. These 

 movements do not confine themselves to horizontal paths, but the 

 gases rise and plunge, pursue broad curves and narrow spirals, and 

 would present to an eye that could see them from above a 

 tumult like the sea in storm." In all this activity it is picking 

 up bits of sand, organic matter and oftentimes even water. These 

 contain microorganisms which are carried into the air and may 

 subside with the particle to which they adhere or become free and 

 float about for a period. 



As the waters of the ocean, lakes, rivers and smaller streams beat 

 against some barrier the fine spray so formed carries into the air 

 bacteria, as do 'also the hurrying feet and rattling wheels in a 

 crowded street. Furthermore, individuals speaking or coughing 

 force from the mouth numerous bacteria which for a time help to 

 make up the microbial inhabitants of the atmosphere. 



Number and Kind. The number and kind of organisms found 

 in the air are governed largely by the locality. They are most 

 plentiful in densely populated areas and within buildings such as 

 churches, theaters and other places where a large number of people 

 congregate. Miquel found that as an average 1 cubic meter of air from 

 the streets of Paris contained 3480 bacteria, laboratory air, 7420, 

 the air of old houses 36,000, whereas the air of the Paris Hospital 

 contained 79,000 bacteria in 1 cubic meter. It is quite evident from 

 these figures that air of inhabited districts contains many bacteria. 

 These are carried on the dust particles. It does not, however, 

 always follow that the number of bacteria in the air is an exact 



