6533b7d9fe1ef96bd126ceaf

RESEARCH PRODUCT

High efficacy of face masks explained by characteristic regimes of airborne SARS-CoV-2 virus abundance

subject

description

Abstract 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 regimes of abundance of particles and viruses. Because of the large number of particles exhaled during human respiration and vocalization, indoor environments are usually in a particle-rich regime which means that masks cannot prevent the inhalation of large numbers of respiratory particles. Usually, however, only a small fraction of these particles contain viruses, which implies a virus-limited regime where masks can help to keep the number of inhaled viruses below the infectious dose. For SARS-CoV-2, the virus load in the respiratory tract of infectious individuals can vary by 3 to 4 orders of magnitude (5th to 95th percentile), leading to substantial variations in the abundance of airborne virus concentrations and infection risks. Nevertheless, we find that most environments are in a virus-limited regime where masks have a high efficacy in preventing the spread of COVID-19 by aerosol or droplet transmission during short-term exposure. The characteristic contrast between particle-rich and virus-limited regimes explains why face masks are highly efficient in most but not all environments, and why the largest benefits can be achieved by non-linear synergetic effects of combining masks with other preventive measures such as ventilation and social distancing to reduce airborne virus concentrations and the overall risk of infection.