It certainly depends on the type of mask. N95 and higher (N99, N100, and similar) provide, in my view, adequate protection for the wearer. There are plenty of other parameters though, eg. proximity and time near others, the size of the room you're in and its circulation. But as example, I feel comfortable wearing N95 while I'm shopping at a super market, and would still feel comfortable if no one else was wearing any mask at all.
