I am no expert, but most full face respirators do seal well with a beard. They just do not advertise it for legal reasons. So if you respirator need is in the "dust mask" category and not OSHA required, they should work. If you are working with hazardous chemicals, I wouldn't try it.

My Beard Journey:  http://jefffsbeardboard.yuku.com/topic/25137/Finally-I-have-a-Beard
Beard and Bald Thread:  http://jefffsbeardboard.yuku.com/topic/27831/Beard-bald-head-shaved-head-or-buzzcut
Patchy Beard Success Stories:  http://jefffsbeardboard.yuku.com/topic/33742/Patchy-Beard-success-stories-Before-and-After-photos#.ViujbH7nuM8
Taming the Wild Mustache:  http://jefffsbeardboard.yuku.com/topic/30822/Taming-the-Wild-Stache#.Vj_Mh7erTIU