Influence of indoor relative humidity on the number concentration, size distribution, and trajectory of sneeze droplets: Effects on social distancing guidelines

The spread of the COVID-19 pandemic is mainly due to the direct transmission routes of SARS-CoV-2 virus-carrying aerosols in indoor environments. In this study, the effect of indoor relative humidity (RH(∞)) on the number concentration, size distribution, and trajectory of sneeze droplets was studied in a confined space experimentally and numerically. Computational fluid dynamics (CFD) simulations using the renormalization group k-ε turbulence model by considering the one-way and two-way (humidity) coupling models were performed to assess the effects of humidity fields on the propagation of droplets. Number concentration profiles indicated that the RH(∞) affected the dispersion modes of droplets differently for the puff, droplet cloud, fully-dispersed, and dilute-dispersed droplets phases identified by the shadowgraph imaging technique. The two-way (humidity) coupling model led to a close agreement with the experimental data in all phases. In particular, the two-way coupling provided better agreement with the data in the puff phase compared to the one-way coupling model. However, the one-way coupling model was sufficient for studying the motion of airborne droplets in the other phases. The velocity fields in the droplet cloud were more sensitive to RH(∞) than the puff and fully-dispersed droplets phases. Also, the effect of RH(∞) on the maximum spreading distance of droplets, d(max,sp), in the puff was insignificant, while its effect became dominant in the dilute-dispersed droplets phase. A dynamic change in the velocity profile of the sneeze jet was seen at a critical relative humidity RH(∞,crit) of about 48%. At RH(∞)< RH(∞,crit), the number concentration of aerosolized droplets increases, significantly affecting the size distribution and the velocity of droplets. At RH(∞)≥ RH(∞,crit), the effect of evaporation time on the number concentration, and diameter of droplets was negligible. At RH(∞) of 24 and 64%, d(max,sp) was 2.14 m (7 feet) and 3.05 m (10 feet), respectively. However, a dry indoor environment led to an increase in evaporation rate and more than four times number concentration of aerosolized droplets compared to a humid environment. Thus, the risk of direct transmission of Covid-19 in a humid indoor environment was higher than the dry conditions, which suggested the requirements for incorporating the RH(∞) effect in the social distancing guideline.