Droplet expulsion from the respiratory tract.

The aerial transmission of infection involves three separate stages: production, transport, and reception of infective particles. The site of reception of a particle in the respiratory tract depends largely upon the size of the particle ( 1) . In the case of tuberculosis, for example, the airborne particles responsible for the initiation of infection are most probably of minute dimensions, of the order of a few microns (2). Aerial transport is also dependent upon particle size; only very small particles are maintained in aerial suspension for any length of time (3). The production of particles with infective potential is less well understood than is transport or reception. While Hare ( 4) has suggested that most infective particles are dispersed from the skin or clothing of the infector, it seems probable that for certain diseases, particularly tuberculosis, the infective particle of most importance is the droplet nucleus, derived by evaporation from droplets expelled directly into the air from the respiratory tract (5). A number of methods have been used in the past to determine the numbers and sizes of droplets expelled from the respiratory tract during various respiratory maneuvers (6--9). Methods involving light scattering have yielded useful information, but the resolution available using this method 1s somewhat limited. Duguid (7) conducted a series of experiments in which a marker microorganism or a dye was introduced into the mouth, and respiratory maneuvers were made into a closed chamber. Expelled particles were recovered by impaction, by settling, or by air sampling, and were recognized by the dye stain or by the growth of organisms. Buckland and Tyrrell (9) studied the dispersal of