Co-occupant's exposure of expiratory dropletsEffects of mouth coverings

Infectious disease transmission in enclosed environments has become one of the major public concerns since the outbreak of severe acute respiratory syndrome. Mouth covering, acting as a non-pharmaceutical intervention, is one feasible measure to mitigate viral infection and has been extensively studied. However, its performance in reducing airborne infections under different ventilation conditions was seldom investigated. In this article, the effects of mouth covering on the co-occupant's exposure are numerically studied under three commonly employed ventilation systems, and the dispersion of exhaled droplet residuals at different coughed airflow velocities is also evaluated under a mixing ventilation system. The results show that the use of mouth covering can interrupt the horizontal transportation of exhaled air to protect the co-occupant from direct exposure to the coughed droplets for all systems, but the effects on the later-stage indirect exposure of the co-occupant are very different for the three ventilation systems. The displacement ventilation system can achieve the lowest exposure for all droplets investigated when the mouth of the polluting person is covered. The total inhaled dose can be reduced by more than 70% under displacement ventilation by wearing a mouth covering and by about 40%-60% under mixing ventilation and under-floor air distribution systems. Meanwhile, the research on varying coughing velocities indicates that the expiratory velocity can only influence the co-occupant's direct exposure, and the inhaled dose at the later stage is primarily associated with the specific ventilation method.