The Cigarette Smoke Analogy
Cigarette smoke provides an analogue for spreading of the SARS-CoV-2
virus, as its particle size distribution is in a similar range of
0.1-0.4 µm(4). So virus particles can travel in a
similar way when emitted from an infected person by sneezing or
coughing. The current two-metre social distancing is presumably based on
droplet size and principles of droplet spreading. However, droplets
evaporate quickly and it is evident that we are facing a most
penetrating aerosol. The particle size distribution of droplets coming
out of the nose and mouth has been extensively characterised. Both
unimodal and bimodal distributions have been reported with the modes
around 500 µm for the former and 90 µm and 600 µm for the latter, and
with geometric mean values of 72 and 386 µm(3). An
early report indicates an even much finer distribution in the range 4-8
µm(5). So it is of interest to estimate how long it
takes for the droplets to shrink to their ultimate size encasing the
virus.
Considering the uncertainty about the viral load which causes infection,
and the number of copies of virus prevailing in the droplets, we
consider a number of cases and investigate the size of virus clusters on
drying: (1) a load of 1000 virus particles inside each droplets; (2) a
concentration of 107.2/ml of virus particles in
respiratory liquid which is sprayed out on sneezing or
coughing(6); (iii) an infectious dose in the range of
1.95 × 103 and 3.0 × 103 viral
particles, based on Influenza A virus(7).
Results