The Air We Breathe 257 



against the walls of the apparatus where agar media are able to 

 trap the organisms. 



It was estimated by one investigator that a person living in 

 London breathes in about 300,000 microbes each day. Some of 

 these organisms are undoubtedly potentially pathogenic, but the 

 efficient filtering mechanisms of the human nose and throat, 

 coupled with other internal defense mechanisms to be discussed 

 later in this book, help to explain how man is able to remain 

 healthy much of the time. 



Persons working in air-conditioned offices probably breathe in 

 much fewer than the number of organisms reported in the London 

 survey, while other individuals engaged in "dusty occupations" 

 may inhale millions or billions of organisms during a twenty-four- 

 hour period. The number of bacteria in the air has no deep sani- 

 tary significance, but the kinds of organisms may be important. 

 Inhaling moisture droplets expelled from persons suffering from 

 respiratory infections is considered to be almost a direct contact, 

 and the public health significance is greater than the mere inhala- 

 tion of large numbers of saprophytic bacteria. 



AIR AS A VEHICLE FOR DISEASE TRANSMISSION 



The air exhaled through the nose during ordinary respiration 

 contains no organisms. But talking, even in low tones, requires 

 forceful expulsion of air. Hundreds, thousands, or even millions 

 of droplets are expelled as we increase the explosive force in loud 

 talking, coughing, and sneezing. 



The use of face masks is an attempt to minimize the spread of 

 organisms by individuals working in such places as operating 

 rooms, maternity wards, and the bottle-filling and capping sec- 

 tions of biological supply houses. Unless a mask is properly pre- 

 pared, however, unwarranted reliance may be placed upon the 

 effectiveness of the technic. A mask made of six layers of special 

 gauze is considered effective if it is laundered before use and 

 snugly fitted over the nose and mouth of the wearer. Since only 

 dry shields are efficient, masks which become moist with perspira- 

 tion should be replaced. It must be remembered that forced ex- 



