(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.