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Properties of materials considered for improvised masks

During a pandemic in which aerosol and droplet transmission is possible, the demand for masks that meet medical or workplace standards can prevent most individuals or organizations from obtaining suitable protection. Cloth masks are widely believed to impede droplet and aerosol transmission but most are constructed from materials with unknown filtration efficiency, airflow resistance and water resistance. Further, there has been no clear guidance on the most important performance metrics for the materials used by the general public (as opposed to high-risk healthcare settings). Here we provide data on a range of common fabrics that might be used to construct masks. None of the materials were suitable for masks meeting the N95 NIOSH standard, but many could provide useful filtration (>90%) of 3 micron particles (a plausible challenge size for human generated aerosols), with low pressure drop. These were: nonwoven sterile wraps, dried baby wipes and some double-knit cotton materials. Decontamination of N95 masks using isopropyl alcohol produces the expected increase in particle penetration, but for 3 micron particles, filtration efficiency is still well above 95%. Tightly woven thin fabrics, despite having the visual appearance of a good particle barrier, had remarkably low filtration efficiency and high pressure drop. These differences in filtration performance can be partly explained by the material structure; the better structures expose individual fibers to the flow while the poor materials may have small fundamental fibers but these are in tightly bundled yarns. The fit and use of the whole mask are critical factors not addressed in this work. Despite the complexity of the design of a very good mask, it is clear that for the larger aerosol particles, any mask will provide substantial protection to the wearer and those around them.