  In the COVID--19 pandemic, among the more controversial issues is the use of
masks and face coverings. Much of the concern boils down to the question --
just how effective are face coverings? One means to address this question is to
review our understanding of the physical mechanisms by which masks and
coverings operate -- steric interception, inertial impaction, diffusion and
electrostatic capture. We enquire as to what extent these can be used to
predict the efficacy of coverings. We combine the predictions of the models of
these mechanisms which exist in the filtration literature and compare the
predictions with recent experiments and lattice Boltzmann simulations, and find
reasonable agreement with the former and good agreement with the latter.
Building on these results, we explore the parameter space for woven cotton
fabrics to show that three-layered cloth masks can be constructed with
comparable filtration performance to surgical masks under ideal conditions.
Reusable cloth masks thus present an environmentally friendly alternative to
surgical masks so long as the face seal is adequate enough to minimise leakage.
