A Superposition Model of Droplet Exposure to SARS-CoV-2

The relative role of droplets and aerosols in SARS-CoV-2 infection has been debated. We seek to quantify virion exposure in an enclosed space via short- range and long-range airborne transmission to inform public health decision making. Data from five published studies were analyzed to predict relative exposure at distances of 1 m and farther. A droplet size of 8 μm was used to compare data from published studies, while not defining particle transport behavior in terms of size. Results at 1 m from an infectious individual were a boundary condition to model infection risk at shorter and longer distance. At all distances, exposure was treated as the sum of all air routes. Number of virions was assumed proportional to particle volume. The largest exposure occurred close to the infectious individual, and out to approximately 1 m, direct deposition and airborne routes both contributed. Farther away, the largest exposure was airborne. For one model, short-range exposure disappeared at 1.8 m. Policy concerning physical distancing for infection reduction relies on exposure as a function of distance, yet within this construct deposition varies. This two-fold distance effect can be used to evaluate control technology such as plexiglass barriers, masking, and ventilation.