Distinct regimes of particle and virus abundance explain face mask efficacy for COVID-19

Cheng, Y.; Ma, N.; Witt, C.; Rapp, S.; Wild, P.; Andreae, M. O.; Pöschl, U.; Su, H.

Airborne transmission is an important transmission pathway for viruses, including SARS-CoV-2. Regions with a higher proportion of people wearing masks show better control of COVID-19, but the effectiveness of masks is still under debate due to their limited and variable efficiencies in removing respiratory particles. Here, we analyze experimental data and perform model calculations to show that this contrast can be explained by the different abundance regimes between particles and viruses. Upon short-term exposure, respiratory particles are usually in a particle-rich regime, but respiratory viruses are often in a virus-limited regime where the numbers of viruses inhaled by susceptible people are below or close to the infectious dose. This virus-limited regime ensures mask efficacy and synergy of multiple preventive measures in reducing the infection risk.