  The goal of this work is to consider widespread use of face masks as a
non-pharmaceutical control strategy for the Covid-19 pandemic. A SEIR model
that divides the population into individuals that wear masks and those that do
not is considered. After calculating the basic reproductive number by a next
generation approach, a criterion for determining when an epidemic can be
prevented by the use of masks only and the critical percentage of mask users
for disease prevention in the population are derived. The results are then
applied to real world data from the United States, Brazil and Italy.
