A SARS-CoV-2-Containing Aerosol Settling Time Model Supports the Significant Role of Indirect Airborne Transmission

Abstract:

A better understanding on the role of the aerosol transmission route in the COVID-19 pandemic is critical for optimal control practices to mitigate its spread. Speech droplets have been found to remain in the air for minutes, and recent studies suggest SARS-CoV-2 virus may stay infectious in experimentally produced aerosols for 3 hours. However, the absence of a large-scale association between regional outbreaks and local characteristics (e.g. settling time and viral viability) of speech aerosols makes it challenging for policy-making on appropriate infection control measures. Here we investigate the correlation between the timeseries of daily infections and that of virus-containing particles’ settling time typically produced by speaking. Characteristic droplet settling time, influenced by daily weather conditions, was estimated based on local meteorological data using an empirical model developed for Reynolds Number Re ~ 1. Daily infection data were calibrated from local reported cases based on established infection progressing time frame. Correlations were observed for major counties studied, including King County, Miami-Dade County, Los Angeles County, etc., during February and March before shelter-in-place orders. As an example, for King County, WA, a Spearman’s correlation (r = 0.949, p = 0.05) was observed between daily cases percentage increase and the settling time of an expelled virus-laden, biological fluid droplet of diameter D_p ~ 6 μm generated during speaking, with a delay of 6 days typical of the asymptomatic stage. Our results suggest that indirect aerosol transmission may be an important pathway for the spread of SARS-CoV-2. For future efforts on effectively slowing down the transmission, these findings support implementation of control measures including social distancing, enforcing mask wearing, and systematic preventive measures such as improved indoor ventilation in both community and healthcare settings.