Characterization and Dispersion of Human Expiratory Droplets – a Review

Abstract

The paper reviews studies conducted on human expiratory droplets for the purpose of defining the characteristics of expiratory droplets, their maximum dispersion and the forces influencing that in an unventilated environment. The review shows coughing, sneezing and speaking droplets to have comparable size ranges, while breathing droplets have the narrowest size range. Sneezing droplets have the largest average size and highest velocity among expiratory droplets. Compiled data reveal droplet Froude number offers a plausible quantitative measure of the droplet maximum spread. The fate of the airborne droplets is seen to be dictated by an interplay between their inertial force and gravitational force. The higher the Froude number, the greater is the droplet spread. Small droplets with high flow inertia, such as dry sputum droplets, are capable of reaching longer horizontal distances in comparison to large droplets. The review shows the maximum horizontal distance coughing droplets can reach exceeds 2 m, while sneezing droplets can reach distances above 6 m, greater than the 2 m physical distancing currently adopted to avoid virus contamination.