Here l (non-mask group) and l m (mask group) are the forces of infection and lS and l m S m are the transfer rates from the susceptible classes, S and S m , to the exposed classes, E and E m . The infection rates, l and l m , incorporate the probability of transmission per contact, b, the reduced infectiousness due to incubation, a, the reduced number of contacts because of symptomatic infection, h, and 1{g j , (j = s or i), which accounts for the effectiveness of the mask in reducing either susceptibility (g s ) or infectivity (g i ). The transmissibility, b, is defined as the Effective Reproduction Number < con for N95 respirators. Notice that < con decreases as a higher percentage of people wear masks as well as when masks are more effective. < con is greatly reduced when 50% of the population wears masks and masks are 50% effective. doi:10.1371/journal.pone.0009018.t005