Efficacy of face masks as source control and respiratory protection against transmission of SARS-CoV-2

During the COVID-19 pandemic, medical and community face masks have been used extensively by health care workers and the general public to reduce transmission of the SARSCoV-2 virus. This report reviews the available literature on the efficacy of medical and community face masks, both as a means for source control and respiratory protection. The literature spans a multitude of scientific disciplines. The review has, however, mainly focused on aerosol physics, aerobiology, microbiology and occupational health and safety. The literature shows that medical and community face masks remove almost all large and medium-sized respiratory droplets from the exhaled air. To some extent, they also reduce the number of emitted small droplets. All respiratory activities generate turbulent jet plumes in the direction of exhalation. These directed jet plumes are able to augment the travel distance of droplets of all sizes and lead to localized regions with elevated droplet concentrations. An important function of face coverings, such as medical and community face masks, is their ability to obstruct these jets, and thereby diffuse the droplet concentration. Due to the combined effect of the above factors, medical and community face masks appear to be highly efficient when used as a means for source control. On the other hand, medical and community face masks do not typically fulfill the performance requirements that apply to respiratory protection devices. The reason for this is twofold. First, medical and community face masks are loose-fit devices prone to gap leakages around the perimeter. Therefore, a substantial fraction of the respiratory droplets may bypass the filtering layer of the mask. Second, the filtration efficiency for very small droplets is highly variable for medical and community face masks. This finding is not surprising, since the intended use for medical face masks as a personal protective device is limited to direct transmission via splashes and sprays. The performance requirements and test methods applied to such devices, according to the relevant harmonized standards, do therefore not consider their function as a respiratory protection device. The role of very small droplets in the transmission of SARS-COV-2 is currently not well understood. Therefore, it is difficult to ascertain whether the strict performance requirements for respiratory protection devices are necessary to achieve a sufficient protection level. Some studies show that the use of medical face masks may offer some degree of respiratory protection to the wearer, in particular if additional measures such as double masking or knotting and tucking are used to improve the fit and reduce gap leakages.