LJ,
Deliberately being pedantic - but that link takes me to a page that simply says (at the bottom) that it is impossible to achieve a good seal with facial hair - not how this was found out, how good/ bad a seal could be achieved, when a bad seal mattered at all with the supply as it is...
I can understand why folk may suppose a beard leads to problems - I just don’t think it’s ever been proved scientifically what they are.
If we limit to FJ O2 masks for now does anyone know differently?