Mean time to infection by small diffusing droplets containing SARS-CoV-2 during close social contacts

Airborne viruses such as SARS-CoV-2 are partly spread through aerosols containing viral particles. Inhalation of infectious airborne particles can lead to infection, a route that can be even more predominant than droplet or contact transmission. To study the transmission between a susceptible and an infected person, we estimated the distribution of arrival times of small diffusing aerosol particles to the inhaled region located below the nose until the number of particles reaches a critical threshold. Our results suggested that although contamination by continuous respiration can take approximately 90 min at a distance of 0.5 m, it is reduced to a few minutes when coughing or sneezing. Interestingly, there is not much difference between outdoors and indoors when the air is still. When a window is open inside an office, the infection time is reduced. Finally, wearing a mask leads to a delay in the time to infection. To conclude, diffusion analysis provides several key timescales of viral airborne transmission.