This article says one study estimates 5 fold reduction: https://www.google.com/amp/s/amp.theguardian.com/world/2020/mar/21/can-face-mask-protect-me-coronavirus-

This article says one study estimates 5 fold reduction: https://www.google.com/amp/s/amp.theguardian.com/world/2020/mar/21/can-face-mask-protect-me-coronavirus-covid-19-myths-busted https://www.google.com/amp/s/amp.theguardian.com/world/2020/... If that reduces the R infection ratio 5-fold, it would drop below 1 and change exponential growth into exponential decay.