Fig. 4.

Virus exposure (violet, low; green, high) for the four ambient conditions simulated: $T=5$ °C and $RH=50\text{to}90\%$ (A and B) and $T=20$ °C and $RH=50\text{to}90\%$ (C and D). Exposure is defined as the number of virus copies (virions) that go past a control area in different locations of the domain. The results are shown normalized by the total number of virus copies ejected during a sneeze. The dimensional concentration of virus copies can be obtained by multiplying the normalized exposure data for the viral load and the ejected liquid volume ($\simeq 0.01\hspace{0.17em}\text{mL}$ in the present simulations). We can observe the presence of a core region characterized by a high level of virus exposure, which is mainly determined by the large droplets (100 microns or more). These droplets follow almost ballistic paths and settle to the ground within $\simeq 1.25\hspace{0.17em}\text{m}$. This core region is surrounded by a wider region characterized by a lower level of virus exposure. Although, in this outer region, the value of exposure is smaller, a susceptible individual is still exposed to thousands of virus copies (here we consider an average viral load for SARS-CoV-2 of $7\times 10^6$ copies per mL). According to the independent action hypothesis, the presence of thousands of virus copies in the small droplets and droplet nuclei poses a significant threat on both the short- and long-range airborne transmission routes of SARS-CoV-2.