DUST INFECTION. 51 



exceptionally in dwellings (typhoid and less often 

 diphtheria). 



This leaves out of consideration the unknown conditions 



., ., T , , in Dwellings. 



organisms of the exanthemata, referred to above. 

 The conditions as they ordinarily exist in dwell- 

 ings are taken as a standard, and the weight and 

 size of the particles, and the velocity of the air 

 in an upward direction, as well as the viability of 

 the different organisms in a dried condition, are 

 the essential factors which govern the likelihood 

 of dust infection. "Naturally, only those infected 

 particles of dried dust which may be carried up- 

 ward for a considerable distance by a very low 

 velocity of the air are able to remain suspended 

 for some time in the atmosphere of a room and, 

 consequently, exist as a protracted danger of air 

 infection. M. Neisser considers as the limit that 

 degree of "Verstaubbarkeit"* in which the in- 

 fected particles may be carried to a height of 80 

 cm. by an air velocity (upward) of 1 cm. (per 

 second)." (Quoted from Gotschlich in Kolle & 

 Wassermann's Handbuch der Pathogenen Mikro- 

 organisms, Vol. 1, p. 168). After the dust in a 

 room is once thoroughly stirred up, from one to 

 eight hours are required for it to settle completely 

 under the most favorable conditions (Stern, 

 Fliigge, cited by Gotschlich) . Dust infection may 

 play a part in public buildings and conveyances 

 where the currents of air are stronger than they 

 ordinarily are in dwellings. Even in dwellings, 

 however, the conditions are by no means uniform. 

 More violent currents are excited by a breeze 

 through an open window, by the movements of 

 persons, and the chances of infection are increased 



* Pulverization. 



