I also was an certified OSHA respirator instructor/fit tester. The OSHA requirements are designed for the industrial setting, and quite often a company will greatly exceed the minimum OSHA requirement at the behest of their insurers. That said, some more info:
To check how well a filter repirator fits and is adjusted, put it on and wear it for at least five minutes. Then put your hand over the exhaust valve and exhale. If the repirator does not balloon up, you have leakage. Then cover the inlet filters (latex gloves work well for this) and inhale and hold your breath. The mask should stay deflated (sucked against your face) If it leaks you can usually feel where the air is coming in. In any event, make sure the exhaust valve works. Other wise you will just trap fumes and the CO2 you exhale in the mask, (which can cause your body to ignore your brain!!)
When using it, if you end up with paint residue alongside your nose by your nostrils, the respirator is too small or adjusted wrong. If the residue is on the bridge of your nose, it is too large or the straps too loose. Some people with a high bridge or narrow nose will need to try different styles to get a good fit.
Good ventilation can negate the need for one. If you have your own spray area, realize the best airflow is across your body, not from behind. If across, it is taking the fumes/dust away from you. If from behind, the eddies caused by your body being in the air flow will bring the contaminants up to your face.
All that said, any respirator is better than none. Look for ones that have a NIOSH label. They should come with a pamphlet that tells you what they are rated to protect you against. For woodworking coatings, you want one rated for OV (organic vapors). If you are into metallized dying (potassium permanganate, sodium hydroxide, etc), you will want one that is also rated for NaOH, acids, and metal fumes. In that case the ventilated face shields would be better protection anyway as they also provide eye protection. For sanding dust, you want the P95 or P100 (Hepa) particulate filter.
Also: the blood vessels in your eyes will absorb 10 times the fumes that skin exposure will. Full face protection is better than half face if the fumes/dust are heavy.
If you have a breathing or cardiac disorder, you may want to wear the respirator doing something moderately physical like walking up and down a set of stairs first to see if it causes breathing problems. If so, see a doctor before you attempt to wear one for any period of time. The long term hazard of the fumes/dust will be moot if you have a heart attack from wearing restrictive breathing protection.
For those with facial hair, vaseline, KY Jelly, mustache wax. etc, can improve the seal for the short term for a particularly nasty job.
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