Efficacy of face coverings in reducing transmission of COVID-19: calculations based on models of droplet capture.

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. We build on these results to predict the utility of various materials from which masks are comprised, and predict their efficiency for removing particles of varying size. We make assumptions about the relative viral load of the respirable droplet size distribution to show that even simple cloth-based face coverings have the potential to significantly reduce the number of secondary infections per infected individual.