https://www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/masking-science-sars-cov2.html Jan 2, 2022
Background
SARS-CoV-2 infection is transmitted predominantly by inhalation of respiratory droplets generated when people cough, sneeze, sing, talk, or breathe. CDC recommends community use of masks to prevent transmission of SARS-CoV-2. Masks are primarily intended to reduce the emission of virus-laden droplets by the wearer (“source control”), which is especially relevant for asymptomatic or presymptomatic infected wearers who feel well and may be unaware of their infectiousness to others (estimated to account for more than 50% of SARS-CoV-2 transmissions).1, 2 Masks also help reduce inhalation of these droplets by the wearer (“filtration for wearer protection”). The community benefit of masking for SARS-CoV-2 control is due to the combination of these two effects (source control and filtration for wearer protection); individual prevention benefit increases with increasing numbers of people using masks consistently and correctly.
Source Control to Block Exhaled Virus
Multi-layer cloth masks block release of exhaled respiratory particles into the environment,3-6 along with any microorganisms associated with these particles.7, 8 Cloth masks not only effectively block most large droplets (i.e., 20-30 microns and larger),9 but they can also block the exhalation of fine droplets and particles (also often referred to as aerosols) smaller than 10 microns3, 5 which increase in number with the volume of speech10-12 and specific types of phonation.13 Multi-layer cloth masks can both block 50-70% of these fine droplets and particles3, 14 and limit the forward spread of those that are not captured.5, 6, 15, 16 Upwards of 80% blockage has been achieved in human experiments,4 with cloth masks in some studies performing on par with surgical masks as barriers for source control.3, 9, 14, 17 In one study, conducted prior to widespread circulation of the Delta variant, masks worked equally well for blocking aerosolized particles containing both “wild-type” virus and the Alpha variant (a more infectious variant).17
Filtration for Wearer Protection
Studies demonstrate that cloth mask materials can also reduce wearers’ exposure to infectious droplets through filtration, including filtration of fine droplets and particles less than 10 microns. The relative filtration effectiveness of various masks has varied widely across studies, in large part due to variation in experimental design and particle sizes analyzed. Multiple layers of cloth with higher thread counts have demonstrated superior performance compared to single layers of cloth with lower thread counts, in some cases filtering nearly 50% of fine particles less than 1 micron.14, 18-30 Some materials (e.g., polypropylene) may enhance filtering effectiveness by generating triboelectric charge (a form of static electricity) that enhances capture of charged particles20 while others (e.g., silk) may help repel moist droplets31 and reduce fabric wetting and thus maintain breathability and comfort. In addition to the number of layers and choice of materials, other techniques can improve wearer protection by improving fit and thereby filtration capacity. Examples include but are not limited to mask fitters, knotting-and-tucking the ear loops of medical procedures masks, using a cloth mask placed over a medical procedure mask, and nylon hosiery sleeves.32-36
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