In this answer, it is noted that a woodworker with a beard should consider a positive airflow respirator due to the beard preventing a good seal. I have also seen (on metalworking forums) the suggestion of a full face respirator.
How much does having a beard affect the effectiveness of a "normal" respirator like this one?
A beard will cause leakage around a standard half-mask "passive" respirator. The respirator's effectiveness will vary depending on how thick your beard is and how tightly the respirator fits over your beard, but keep in mind that air will follow the path of least resistance. Your beard won't provide anywhere near the filtration of a P100 cartridge which removes 99.97% of particles 0.3 microns and larger. Some air will probably still pass through the filter, but if it's easier for the air to flow through your beard than through the respirator's filter, the respirator won't be very effective.
You should consider using a positive pressure respirator, also called a Powered Air Purifying Respirator (PAPR). This type of respirator pulls air in through a filter and exhausts the filtered air inside the respirator, producing continuous outward leakage.
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One disadvantage of the all-in-one units as pictured above is that they are heavy. Some other variations use a belt- or backpack-mounted motor.
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One additional advantage of powered full-face respirators over a typical full-face shield is that the continuously-circulating air prevents the face shield from fogging up.
Standard test for respirator seal; put it on your face without the straps. Cover the inlets with your palms.. Suck in a breath and hold it. It should maintain the resulting partial vacuum for a reasonable time. If it doesn't, some air is sneaking in around the mask every time you breathe in; the better the filters the more leakage past them.
Better than nothing, but you aren't getting the protection you should.
Positive pressure does solve that problem; any leakage is clean to dirty rather than the other way around.
(my beard comes and goes; the moustache mostly stays.)
I use (and recommend) a product called Resp-o-Rator. It has a scuba-style mouthpiece that routes over your shoulders to two HEPA filters that are positioned behind your head. It's passive (no power required), lightweight, and inexpensive.
http://www.duckworksbbs.com/tools/resp/
For what it's worth, I don't use the nose clip and haven't had any issues. If you can't breathe through your mouth without plugging your nose, though, it might be problematic.
I use the 3m PAPR which has now been updated and looks to be way better than mine. I do like what feels like a cool breeze on my face when wearing it. The belt does not work for me, I had to buy tool belt suspenders, the face shield falls apart quite often when flipping up and down, I must disassemble it and re-assemble whenever this happens. I bought mine in 2008, I still can still get probably 8 hours on each battery, I have an extra which it did not come with. It was very expensive but I have occupational related asthma and it works to protect me. It is very awesome when using a router.
