Among other things, Gawande writes on the effectiveness of masks. My summary of what he writes about filtration effectiveness:
Mask type Outbound Inbound
(protect (protect
others) wearer)
Surgical 99% 75% ("about three quarters")
Two layer 60% 37% ("half as much on average")
cotton
N95 no valve 95% 95%
N95 with valve low 95%