Superposition of Droplet and Aerosol risk in the transmission of SARS-CoV-2

ABSTRACT

ObjectivesConsidering three viral transmission routes fomite contact, aerial transmission by droplets, and aerial transmission by aerosols, the aerial routes have been the focus of debate about the relative role of droplets and aerosols in SARS-CoV-2 infection. We seek to quantify infection risk in an enclosed space via short-range airborne transmission from droplets and long-range risk from aerosols toward focusing public health measures. MethodsData from three published studies were analyzed to predict relative exposure at distances of 1 m and farther, mediated by droplet size divided into two bins larger than 8 {micro}m and smaller than 75 {micro}m (medium droplets) and smaller than 8 {micro}m (small droplets or aerosols). The results at 1 m from an infectious individual were treated as a boundary condition to model infection risk at greater distance. At all distances, infection risk was treated as the sum of exposure to small and medium droplets. It was assumed that number of virions is proportional to droplet volume. ResultsThe largest infection risk (as exposure to droplet volume) came from medium droplets, close to the infectious individual out to approximately 1 m. Farther away, the largest risk was due to aerosols. For one model, medium droplet exposure disappeared at 1.8 m. ConclusionsPolicy concerning social distancing for meaningful infection reduction relies on droplet exposure as a function of distance, yet within this construct droplet size determines respiratory deposition. This two-fold distance effect can be used to evaluate additional measures such as plexiglass barriers and masking.