(ii) Constant concentration of 107.2/ml of virus
particles in respiratory liquid
As the droplets originate from a pool
of liquid inside the body, this concentration corresponds to viral load
of an infected person with common flu. In this case a constant volume
fraction of the virus is considered for all droplets. The number of
viruses in each droplet then depends on the droplet size. Taking the
above virus concentration (corresponding to a virus volume fraction of
4.2×10-9) and the bimodal droplet size distribution
reported by Han et al.(3), as shown in Figure 2, this
represents about 1 virus in the lower droplet size (50 µm) and 6900 in
the largest droplet size. Following the drying time shown in Figure 1,
the size distribution of the dried virus clusters shifts drastically to
the left and is in the range 0.1 to 1.8 µm, as shown in Figure 2.
Reports on the droplet size distribution produced by sneezing, coughing,
shouting and speaking show a great variation. If we consider an earlier
study by Duguid(5) in which he reports the most common
droplet size is in the range 4 to 8 µm, then the time taken to reach the
same ultimate size is even much shorter, around 0.05 to 0.22 s. So the
calculated size range of the virus clusters formed by the smaller
droplets is very similar to that of the size distribution of cigarette
smoke particles.