This is a different thing to particulate masks and not to be confused with the above. For a start, particulate masks have a release valve, which AFAICT is not ideal for protecting other people, which is the main point.
Jeremy Howard, Cloth masks can protect the wearer:
As we all know now, the science shows that DIY masks are particularly good at protecting those around you, in case you’re infected with COVID-19. But that does’t mean that you ca’t do a lot to protect yourself too.
Unfortunately, many public health bodies still incorrectly claim that there is no evidence that DIY masks are useful at protecting the wearer. There’s actually plenty of evidence they can. Effective protection for the wearer of a mask depends on three critical things:
Much more useful summary and advice there.
There is a link to fixthemask which includes a rubber band to keep masks snug. If you do’t want to hunt for weird filter types on ebay, there is the smart air guide which ranks normal fabrics and household filters.
i.e. against pollution. See particulate masks.