As the COVID-19 pandemic progressed across the world, governments, international agencies, policymakers, and public health officials began recommending widespread use of nonmedical cloth masks to reduce the transmission of SARS-CoV-2. The authors from McMaster University and St. Joseph's hospital suggest that there is convincing evidence to support this recommendation.

As Clase and Fu, et al. (2020) explain, "Physical distancing, hand hygiene, and disinfection of surfaces are the cornerstones of infection control during the coronavirus disease 2019 (COVID-19) pandemic. At the same time, governments, international agencies, policymakers, and public health officials have been debating the validity of recommending use of nonmedical masks by the general public to reduce the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We believe that these decisions should be informed by evidence. Although no direct evidence indicates that cloth masks are effective in reducing transmission of SARS-CoV-2, the evidence that they reduce contamination of air and surfaces is convincing and should suffice to inform policy decisions on their use in this pandemic pending further research."

The researchers continue, "Cloth does not stop isolated virions. However, most virus transmission occurs via larger particles in secretions, whether aerosol (<5 µm) or droplets (>5 µm), which are generated directly by speaking, eating, coughing, and sneezing; aerosols are also created when water evaporates from smaller droplets, which become aerosol-sized droplet nuclei. The point is not that some particles can penetrate but that some particles are stopped, particularly in the outward direction. Every virus-laden particle retained in a mask is not available to hang in the air as an aerosol or fall to a surface to be later picked up by touch. Filtration efficiency is the ability of a material to block transmission; it is expressed as a percentage and assessed using surrogate markers, including biological aerosols. Mask standards set by ASTM International require tests with latex spheres and aerosolized Staphylococcus aureus, but masks are not assessed separately for every pathogen: Filtration efficiency depends on the physical retention of particles of different sizes, regardless of which pathogen the particle contains."

To read the full text: https://www.acpjournals.org/doi/10.7326/M20-2567.

Reference: Clase CM, Fu EL, et al. Cloth Masks May Prevent Transmission of COVID-19: An Evidence-Based, Risk-Based Approach. Ann Intern Med. May 23, 2020.