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A Superposition Model of Droplet and Aerosol Risk in the Transmission of SARS-CoV-2
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  • John McCarthy,
  • Barry Dewitt,
  • Bob Dumas,
  • James Bennett
John McCarthy
Washington University in St Louis
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Barry Dewitt
Carnegie Mellon University
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Bob Dumas
Omnium CPG
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James Bennett
Centers for Disease Control and Prevention

Corresponding Author:[email protected]

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Considering three viral transmission routes— fomites, droplets, and aerosols— two 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 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, mediated by droplet size divided into two bins: ≧ 8 μm (medium and large droplets that we call “droplets”) and < 8 μm (small droplets that we call “aerosols”). The results at 1 m from an infectious individual were treated as a boundary condition to model infection risk at shorter and longer distance. At all distances, infection risk was treated as the sum of exposure to aerosols and droplets. It was assumed that number of virions is proportional to particle volume. The largest infection risk occurred close to the infectious individual, and out to approximately 1m, droplets and aerosols both contributed. Farther away, the largest risk was due to aerosols. For one model, droplet exposure disappeared at 1.8 m. Policy concerning physical distancing for meaningful infection reduction relies on exposure as a function of distance, yet within this construct particle size determines respiratory deposition. This two-fold distance effect can be used to evaluate measures such as plexiglass barriers, masking, and ventilation.