AIR-BORNE INFECTION 339 



Bacteria in Inspired and Expired Air. Inasmuch as the atmosphere 

 contains numerous bacteria it is to be expected that many will be 

 inhaled with the inspired air. It is estimated that a person living 

 in London breathes about 300,000 bacteria each day and individuals 

 living in other districts may take many times this number. Most 

 of these are harmless and are caught on the moist mucous mem- 

 branes of the upper respiratory passages, very few finding their 

 way into the deeper alveoli. 



The expired air, during normal respiration, is practically free 

 from bacteria. But during the acts of coughing, sneezing and 

 speaking the air is forced out and with it bacteria, some of which 

 may be pathogens and if inhaled by a second individual may give 

 rise to the specific disease. 



Air-borne Infection. The air has long been considered as the 

 chief vehicle for the spread of communicable diseases. This 

 was but natural, for until recently the virus of these diseases was 

 believed to be gaseous or at least readily diffusible and borne by 

 air currents. After the bacterial nature of disease was discovered 

 and it was found that the discharges from the nose and mouth of the 

 diseased body often contains the causative organisms, and hence 

 could readily find their way into the air, this was a favorite 

 method for explaining infection. Recent work, however, has 

 demonstrated that the pathogens do not long retain their vitality 

 when free in air, and where infection is conveyed by air it is due to 

 dust or droplet infection. 



Dust infection occurs only in the case of those diseases caused 

 by organisms which can survive considerable periods of drying. 

 The most important is that of tuberculosis and is here confined to 

 rooms and dusty places which have been occupied by careless 

 consumptives. The extent to which dust is a factor in the trans- 

 mitting of disease is not well known, but it probably is not great. 



Flugge and his students were the first to demonstrate that 

 minute droplets may be emitted from the mouth during talking, 

 coughing and sneezing. The droplets may be carried in a quiet 

 room as far as twenty or thirty feet. The large ones soon settle 

 out, whereas in the smaller ones there is a great tendency for many 

 pathogens to perish. Hence, droplet infection is conveyed only a 

 few feet. 



