A Superposition Model of Droplet and Aerosol Risk in the Transmission of
SARS-CoV-2
Abstract
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.