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 m 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 m 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 m. 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 m, 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 m, implying that evaporation to the equilibrium droplet size occurred within 0.8 s.

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Size distributions of expiratory droplets expelled during coughing and speaking and the velocities of the expiration air jets of healthy volunteers were measured. Droplet size was measured using the interferometric Mie imaging (IMI) technique while the particle image velocimetry (PIV) technique was used for measuring air velocity. These techniques allowed measurements in close proximity to the mouth and avoided air sampling losses. The average expiration air velocity was 11.7 m/s for coughing and 3.9 m/s for speaking. Under the experimental setting, evaporation and condensation effects had negligible impact on the measured droplet size. The geometric mean diameter of droplets from coughing was 13.5 m and it was 16.0 m for speaking (counting 1-100). The estimated total number of droplets expelled ranged from 947 to 2085 per cough and 112-6720 for speaking. The estimated droplet concentrations for coughing ranged from 2.4 to 5.2 cm −3 per cough and 0.004-0.223 cm −3 for speaking.