  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.
