76 DUST AND ITS DANGERS. 



dust-clouds are encountered by those passing 

 through them ; if the streets are not properly 

 sprinkled before sweeping, either by machine 

 or hand ; if ignorant or careless street-cleaners 

 are allowed to scatter clouds of dust about 

 them as they sweep or shovel or transport the 

 pulverized filth, the chances of inhalation of 

 dangerous dust particles are proportionally 

 increased. But, on the whole, the risk of 

 infection out-of-doors from dust, even in 

 crowded towns, unless they are notably filthy, 

 is not actually very great. 



Indoors, however, the conditions are en- 

 tirely different. Let us first consider private 

 houses and living rooms. Here, as we have 

 already seen, the sources of micro-organisms 

 are various, but we need consider here only 

 those which cause consumption. These may 

 be brought in on feet and garments from the 

 streets or other places, or be blown in through 

 open windows or drawn in by other modes 

 of ventilation. If there be no consumptive 

 persons in the house or rooms, these chance 

 sources of infection are all that need be re- 

 garded. If there be consumptives in the 



