Abstract
The question of infection of air by droplets given off in coughing and sneezing has received considerable attention during the last few years. In particular, investigation has been directed towards determining the role of the air-borne droplet nuclei which result from the evaporation of droplets proper. The bacteriological and epidemiolog-ical aspects have recently been discussed by Wells, Wells, and Mudd. 1 Experimentally, little is known of certain of the characteristics of such droplets—their number, size, velocity, settling rate, and rate of evaporation—although these factors are concerned in their dissemination and the production of droplet nuclei. Wells 2 has calculated theoretical settling times and distances of fall before evaporation, for plain water droplets of various sizes suspended in still air of different humidities. These figures are only roughly applicable to actual conditions of air infection, since mouth spray and naso-pharynx droplets will contain some dissolved solids or be mucus-like, and the air will not be still. Rooks 3 has shown experimentally that droplet size is an important factor in nasal filtration, although he knew only the theoretical differences in average size of the droplets with which he worked. Weyrauch and Rzymkowski 4 have photographed the tracks of moving droplets given off in sneezing and talking. The paths shown indicate that most of the actual droplets travel only relatively short horizontal distances.
We have been able, by means of high-speed photography, to “stop” the motion of droplets given off in coughing and sneezing, thereby permitting measurements of droplet size, velocity, etc. The technic utilizes the light source and control instruments developed by Edger-ton, et a/., 5 for stroboscopic illumination and high-speed photography. The light source, which is placed at one side of the subject's face, consists of a 9-inch specular reflector with a spiral argon-filled tube through which a 56 microfarad condenser (charged to 2500 volts) discharges.
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