1. Having worked before with part of my duties as a respirator fit tester and certifier, I would add a some comments.
2. For dust filters there is usually no set shelf life, so they can be used until you notice a restriction. However, if your mask does not fit correctly, there will be leaks so you won't notice restrictions but will notice dust around your nose/mouth when you take it off. If the dust is on the bridge of your nose, the mask is usually too large. If around the nostrils, usually the mask is too small or the straps not adjusted correctly.
3. Organic vapor filters (have activated charcoal as the main active ingredient) start to degrade as soon as the package seal is opened. This is mainly due to humidity in the air and other contaminants starting to load up the charcoal. When I was working in the aircraft industry, we were required to keep the mask and filters in a sealed plastic bag when not in use, and to change filters at least weekly, whether or not the mask had been used. Some manufacturers give up to 1 month.
4. If you have or think you have any lung impairment at all, and are planning to buy a negative pressure regulator (any one that restricts the air flow), please have your doctor give you a pulmonary function test first, and okay you for using one. Its foolish to worry about lung problems from what you breathe if the safety device triggers a heart attack or lung problems itself.
5. If the doc doesn't clear you for a negative pressure regulator, then you will need one that forces air into the mask or blows it into a face covering hood. (The one that GeorgeM referenced is in this category) These are called free-flow or positive pressure respirators. Some are a sealed face mask that have a fan motor and filter that attaches to your belt, and can be used to protect against dust and paint fumes. Others take fresh air from outside the work area and blow it through a hose that connects to the hood. There are also face shield protectors that are stuck on the outside of the face plate and are just peeled off and replaced when they get scratched or paint overspray build-up. A lot cheaper than replacing the masks which are expensive.
6. Under no circumstance use your air compressor to supply breathing air. The hazards are carbon monoxide poisoning, as well as microscopic oil particles that will fill your lungs causing lymphatic pneumonia (which cannot be cured because they have no way to remove the oil from your lungs.) A regular oil/water separator will not remove them.
Kudoes to those of you who are protecting yourselves.
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