Arthur Conan Doyle wrote:
Roland Perry wrote:

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.

The moist exhaled air will obviously get out anyway, but any form of face
covering that makes the droplets follow a more tortuous route will reduce
the distance they are projected. That's all that these face coverings for
the general public are expected to do. They are not medical grade PPE.