In message prlkef5vnm4qb5pg81dq2u1sr1fh7vjho6@None, at 12:49:14 on
Wed, 17 Jun 2020, Arthur Conan Doyle remarked:

No they aren't. The majority are DIY dusk-masks, and the vent is to let
the moist air out.

Which kind of puts to rest the argument that masks protect others.

That's just one type of mask, and even so it will help capture the worst
of any coughs or sneezes, and also help stop the wearer touching their
face and transferring virus to the hands, to then spread on hard
surfaces.
--
Roland Perry