PMC:7784787 / 11423-14969 JSONTXT

{"target":"https://pubannotation.org/docs/sourcedb/PMC/sourceid/7784787","sourcedb":"PMC","sourceid":"7784787","source_url":"https://www.ncbi.nlm.nih.gov/pmc/7784787","text":"Discussion\nThe FFP1 mask is the only one which must pass a fit test to be awarded a CE mark, and indeed it showed the lowest leakage and undetected penetration. For the other mask types, the fabrics with lower penetration showed higher leakage, indicating that these masks displayed a trade-off between fabric and fit.\nThe scattering of light by small particles is described by Mie theory [27], and is quite non linear. However, using the experimental setup described herein, these nonlinearities will smooth over to a large extent due to the breadth and angle of the laser cone, and wide distribution of particle sizes. The brightness can, therefore, be taken as approximately proportional to the volume of material. Although it is possible to extract pseudo-quantitative results from these images by merely summing up the brightness of each pixel, we caution against doing so for two reasons. As seen in the no-mask image (Figure 3A), a narrow, fast-moving jet appears dim due to the small area of laser sheet it covers, despite carrying a large volume. This weakness could be addressed by more complex image analysis. Another caveat is that the results are dependent on the exact position of the laser sheet. In the configuration used in the current study, for example, any leakage from under the chin would not be detected. However, when using additional laser sheets, it is apparent that almost all leakage occurs around the eyes and cheeks for the masks we tested. These additional laser sheets complicate the interpretation of the images, even if using different laser colors. Directing the cone straight onto the face to form a laser ring around the masks results in a large background scatter from the dummy skin. Although this background can be reduced by black paint and image processing, this configuration would then require another laser sheet to detect penetration.\nIt is also important to note that the aerosol used in the current experimental setup has a mass median aerodynamic diameter of 3 μm but is also a broad distribution with 10% of the mass in particles larger than 10 μm. This covers the range of particle sizes emitted from speech [28], but whether this also correlates to the infectious particles is not currently known. Most of the fabric testing to date has focused on submicron particles [29,30]. However, a study of influenza in ferrets found the disease was transmitted in particles larger than 1.5 μm, but not smaller particles [31]. For SARS-CoV-2, we do not yet know which size of the aerosol is likely to carry the most infectious viral load. Larger particles are more likely to be captured by the mask material. To an extent this is also true for leaks, as larger particles are more likely to deposit due to impaction when passing around corners.\nIt is also unknown how different leakage patterns could affect infectivity. Even the worst performing single-layer mask served to slow down the flow from the mouth, which would reduce the spread of particles even if they were not captured. The fate of leakage jets directed to the side, or upward, would depend on the local airflow.\nFurthermore, the dummy head used in this study is larger than 95% of adult male heads (tip of the nose to the back of head 237 mm), meaning that the masks were a tighter fit than the average human head. The exact shape of the nose would also have a potentially considerable influence on the leakage. Nonetheless, despite these limitations, our preliminary findings are clear – mask efficacy is dependent on both fabric and 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