Small droplets can maintain suspended in indoor air for a quite long period and possess great possibilities to be inhaled and then cause infection. Herein, passive gas method is employed to investigate the transmission and dispersion of human respiratory droplet residuals, and the exposure risk of the co-occupant is then evaluated under three air distribution methods, specifically mixing ventilation (MV), under-floor air distribution (UFAD) and displacement ventilation (DV). One-way coupling driftflux model is also adopted to investigate the dispersion and the co-occupant’s health risk for droplets residuals with varying diameters less than 10µm under MV, with the considerations of gravitational settling and deposition mechanisms of these droplets.

The results show that the strong effect of coughing jet in the first several seconds and the prolonged prevailing influence of indoor air movement constitute two stages of pathogen-containing droplets dispersion. The lower inhaled mass in the first stage and more inhalation in the second stage under UFAD result in about the same inhaled dose as that under MV, while DV can achieve the lowest exposure risk for the whole process when carbon dioxide is used as tracer gas. When the influence of droplet size is considered under MV, it is found that the coughing airflow can lead to almost the same inhalation during the first 100 seconds, and the total inhaled dose decreases with the particle size because of the increasing deposition of larger droplets. Due to the specific characteristic of MV, all the droplets residuals investigated show nearly the same transmission and dispersion patterns.