Size distribution and sites of origin of droplets expelled from the human respiratory tract during expiratory activities

A new expiratory droplet investigation system (EDIS) was used to conduct the most comprehensive program of study to date, of the dilution corrected droplet size distributions produced during different respiratory activities. Distinct physiological processes were responsible for specific size distribution modes. The majority of particles for all activities were produced in one or more modes, with diameters below 0.8 μm at average concentrations up to 0.75 cm^-3. These particles occurred at varying concentrations, during all respiratory activities, including normal breathing. A second mode at 1.8 μm was produced during all activities, but at lower concentrations of up to 0.14 cm^-3. Speech produced additional particles in modes near 3.5 and 5 μm. These two modes became most pronounced during sustained vocalization, producing average concentrations of 0.04 and 0.16 cm^-3, respectively, suggesting that the aerosolization of secretions lubricating the vocal chords is a major source of droplets in terms of number. For the entire size range examined of 0.3-20 μm, average particle number concentrations produced during exhalation ranged from 0.1 cm^-3 for breathing to 1.1 cm^-3 for sustained vocalization. Non-equilibrium droplet evaporation was not detectable for particles between 0.5 and 20 μm, implying that evaporation to the equilibrium droplet size occurred within 0.8 s.