In general, you're not supposed to wear a beard with a respirator mask (N95, P100, etc), at least not in a way where you have facial hair under the seal:

But how much worse is the fit? A P100 with a beard is going to filter less well than a P100 without a beard, but does it do as well as an N95? Or is it hopelessly compromised? And is stubble as bad as a full beard? Does length matter? In an emergency where I really need my mask for protection should I shave?

Bottom line up front: with my rough DIY test setup I got 80% filtration with a long beard, 92% with a short one, and 99.7% with stubble. But see my note at the end about a weird effect I saw.