The Air We Breathe 253 



find. Powerful updrafts of air currents heavilv laden with street 

 dust, however, can carry organisms to unbelievable altitudes. 



In addition to being transported on particles of dust, bacteria 

 can find their way into the air via moisture droplets expelled by 

 man during talking, coughing, and sneezing. Even the bark of a 

 dog adds microbes to the air. The number and the size of droplets 

 varies with the intensity of the expulsion from the nose or mouth. 

 Larger particles settle out relatively quickly, while smaller moisture 

 droplets remain in suspension in the air for periods of time depend- 

 ing upon the temperature, humidity, speed of air currents, and 

 other factors. The potential possibility for the dissemination of 

 upper respiratory diseases is graphically shown in high-speed pho- 

 tographs of an unstifled sneeze. It does not require too much 

 imagination to realize how simple it would be for a heavily 

 infected individual to spread his misery to large numbers of sus- 

 ceptible persons with whom he might have close contact in 

 crowded, poorly ventilated rooms. 



Air in itself will not support growth of microorganisms; it is not 

 suitable as a bacteriological medium. Air is merely a passive 

 transfer agent for organisms being borne on dust particles or in 

 moisture droplets. The types of organisms one isolates from the 

 atmosphere depend upon the source of the samples. 



AIR POLLUTION AND HEALTH 



Besides carrying "ordinary" dust, air can be the vehicle for the 

 spread of silica dust in stone-cutting and grinding operations. 

 Unless the operator wears a protective mask, he might inhale 

 dangerous amounts of sharp-edged particles of silica which are 

 capable of setting up a condition in the lungs known as silicosis. 

 The shearing action of silica can predispose an individual to sub- 

 sequent microbial infection of the lungs. 



Since house dust is notorious for its high bacterial count, manu- 

 facturers of vacuum cleaners have capitalized on this point in their 

 sales promotion schemes. There is no doubt that cleaning the 

 floor, rugs, and furniture is more efficient when the operation is 

 performed with a vacuum cleaner than it would be with "old- 



