[lolc]

As far as I know a mask is more effective to contain when worn by the infected. But that doesn't mean the not-infected (including those who mistakenly think they are not infected!) to not wear one. Because it does protect. Not only from airborne droplets but also from smear infections.

Reducing the likelihood of infection by ten percent can make the difference between eradication and an epidemic.

[sokoloff]

It absolutely helps. I was only describing how asymmetry in effect can arise from symmetry in a passive filter.