DUST AND ITS DANGERS. 75 



against danger, even in very dirty and very 

 infectious places. Many disease-producing bac- 

 teria soon die, in greater or less numbers, soon 

 after they are expelled from the bodies of sick 

 persons ; many are swept away by the winds 

 into uninhabited regions ; many fail to come 

 in contact under favorable conditions with 

 susceptible human beings. But these natural 

 safeguards cannot be implicitly relied upon 

 for safety by any one at all times, nor can 

 any one with impunity overtask their capaci- 

 ties by unnecessary and constant exposure of 

 his person to infectious dust. 



It is certain that in the out-of-doors air in the 

 country, and also in cities whose streets are 

 kept decently clean, there is little danger of 

 harm from the inhalation of germs of con- 

 sumption or of any other disease, because the 

 constant purifying agency of wind and air 

 currents will either soon sweep away the dust 

 or so largely dilute it that it will be practically 

 free from disease germs, the sources of which 

 are so comparatively limited. If, however, the 

 streets of cities be or are allowed to remain 

 filthy, so that abundant and pretty constant 



