Short respirator primer:
Look for the NIOSH rating: Those letters and numbers mean something. (If it doesn't have a NIOSH rating, don't waste your money)
For dusts and mists, you have 95 -removes 95% of the particulates, 99 - removes 99 %, and 100 -removes 99.97% (commonly known as HEPA filters). For anyone with impaired lung function, go with the 99 or preferable the 100.
Preceding that number is a letter - N, R, or P. These have to do with oil resistance: N is not oil resistant, R is partially resistant, and P is oil resistant.
The other letters have to do with the vapors: The one most commonly needed by woodworkers is "OV", which stands for Organic Vapors. Other letters you may see are Cl (Chlorine), Fl (Flourine), HCl (Hydrogen Chloride), HS (Hydrogen Suflide), etc. These respirators will come with a paper explaining the letters and specify what that particular respirator/filter can protect you from if used properly.
Vapor respirators will have an activated charcoal element to them. They should remained sealed when not being used as the humidity in the air will degrade the charcoal over a period of time. (A zip-loc bag works fine)
If you have a breathing impairment such as asthma or emphysema, and are planning to use a tight-fitting respirator (ie, the rubber or silicone ones with replaceable filters) I strongly recommend you have your doctor perform a pulmonary function test prior to use to make sure you can get enough air through it.
Disclaimer: I am not a doctor. I was the certified respirator use trainer/fit tester for an acft industrial paint shop for 8 years, required to meet the OSHA standards. We used disposables, tight-fittings cannister types, and air-supplied full-face respirators, depending on the task.
There's a lot more to it, including determining if the respirator is fitting and operating correctly and knowing when to toss it, so if anyone wants more info, PM me or I will answer in open forum, as you wish.
Hope this helps
Go
PS: Rodger, the beard will impair the fit, but wearing one will greatly reduce the intake of bad stuff. A little vaseline or water soluble gel around the edge will improve the effectiveness for a temporary improvement if you feel it is necessary. If its a disposable, you will probably have more leakage around the bridge of the nose than through the beard, anyway. If you are spraying finishes or have poor ventilation, get one rated for OV as well as the dust rating.