Disclaimer applies here as well: I am not a certified Industrial Hygienist and you should choose to use or not use this advice at your own risk.  [Sad that America has come to the point where everyone has to put these disclaimers on everything, don't you think?]  The government web sites for NIOSH (and more precisely, the Respirator Selection and Use Guide,)and OSHA (especially the sections on personal protective equipment (including respirators) Reg. 1910 Subpart I, sections 134) are excellent resources if you have more specific questions, and I encourage you to visit them more more detailed info.

The purpose of this page is to help inform the hobbyist woodworker as to what sort of respiratory hazards are typically present in your shop and the types and classes of products that are available to protect you against those hazards.  It is not intended as legal advice nor should it be taken as such.  If you run a woodworking business, you need to consult a certified Industrial Hygienist (CIH) or other Safety Professional as to your responsibilities and options.

Introduction
Contaminants and the Regulations:

There are a few key terms to understand:

    Dust Mask:  This is a general term that is widely used but can lead to a lot of confusion.  It is not a regulated term so anyone can call anything a "dust mask" and be legitimate.  However, that doesn't mean a "dust mask" has to do much of anything.  We in the industry often refer to a non-approved dust mask as something that keeps the birds and low-flying planes out of your mouth because they do not provide any real respiratory protection.  You will see me use the term "mask" a lot as shorthand.  When I do, I mean a certified respirator, not a dust mask, unless I specifically say otherwise.

    Respirator:  To be called a respirator in the US, the product must be tested and approved by the National Institutes of Occupational Safety and Health (NIOSH), which is a federal agency that is part of the Centers for Disease Control under the Department of Labor.  The product must meet minimum standards and the manufacturer must also meet minimum quality control standards.  A certified respirator will protect your lungs according to the approval classification it has and provided it is properly fitted to your face.  A respirator can protect against particulates hazards, gases and vapors, or a combination of the two.  NIOSH is a separate and distinct agency from the more familiar OSHA.  NIOSH writes the standards that respirators must meet and certifies products to those standards.  OSHA writes the regulations that business must follow, including the circumstances where a person must wear respiratory protection and what type, and also enforces the regulations.  OSHA will use NIOSH standards as the basis for what types of products are required for a given hazard.

Negative Pressure/Positive Pressure/Supplied Air:  Three broad categories of respiratory protection.  Negative pressure means that the mask is a passive filter and your lungs provide the airflow through the filter.  Positive pressure means that an outside power source (air compressor, blower/battery pack, air tank) supplies the clean air to the wearer and the pressure inside the facepiece or other protective gear is slightly above atmospheric pressure to help keep contaminants from finding their way in through leaks.  Supplied air is a little more specific version of positive pressure and refers to air supplied by a compressor or air tank (not a battery pack/blower combo).  These definitions are not all-encompassing but hit the key points that most people will care about.  This doesn't mean you can plug an airline into your shop compressor and use it as a source of breathing air. See below for more info on supplied air.
 

Legalese
When are you required by law to wear a respirator?  Well, there are two key factors. First you must fall under OSHA jurisdiction, and secondly the concentration of the hazard in the air you are breathing must be above the Permissible Exposure Level (PEL).  Does this apply to you?  Well, I don't know you or your situation, but in all likelihood it doesn't. OSHA regulates businesses and commercial enterprises, not your basement hobby shop.  So what does that mean to the hobbyist?  Should you bother to wear a respirator?  Just because the government doesn't require you to wear a mask isn't a good reason to forgo protection.  You shouldn't just wear a respirator because the government tells you to - you should do it to protect yourself.  Over time you could become sensitized to the dust of some species of wood dust, or to all wood dust, and certain woods can be particularly bad.  There used to be a nice wood toxicity chart somewhere on the web, but I can't find it lately.  These two sites have some info, but it isn't always in the best format: wood toxicity chart1 and wood toxicity chart 2  Wearing a mask is often a wise precaution, both for your short-term comfort (sneezing, etc.) and for your long-term health (lung damage and sensitization).  The government is trying to set minimums to cover the most serious hazards, but that doesn't mean there is no hazard if you are below OSHA thresholds.  If you run a business and are concerned as to your legal responsibilities (as you should be), this is not the site that will tell you everything you need to know:  You should either contact a certified Industrial Hygienist or OSHA - this web page is not a substitute for legal advice.
 
 

This is the category that is most important to woodworkers.  A particulate can be either a solid or liquid aerosolized into the air you breathe.  Wood dust from sanding or sawing is the obvious example but also included in this category is the overspray that occurs from spraying a finish (that may also include a gas & vapor hazard, see Paint Spray).  The basic rule of Industrial Hygiene is "Get it at the source".  In woodworking that means good dust collection or a spray booth.  Unfortunately, that isn't always possible, and if you are working with some particularly nasty wood or have wood dust allergies, it may not be enough to have good dust collection.  That is where respiratory protection comes into play.

The Regulations
NIOSH changed the regulations in July, 1995 so there are two sets of approved products that you may run into for the next few years until inventories of the old products are depleted.  The old regulations have the catchy name 30CFR11 and the new ones fall under the equally dazzling heading of 42CFR84.

30CFR11

There were three main product approval classifications under 30CFR11 from lowest to highest filtration efficiency:  Dust/Mist (DM), Dust/Fume/Mist (DFM), and HEPA (High-Efficiency).  Fumes are a special case that is handled below, and aren't very important to the woodworker (in this context, the term "fume" doesn't mean what you may expect).  The Dust/Mist masks are going to be the ones you most commonly find in stores.  They have one or two straps and a "paper" or cloth-like cup-shaped body that is also the filter.  Most HEPA masks are not this type (though there are some) but rather are filters that attach to molded rubber facepieces.  The testing procedures under 30CFR11 were, in my opinion, not terribly consistent from one approval class to another, which was probably one of the driving factors behind the change to 42CFR84 (also 30CFR11 had been around for some 30 years).  A DM mask was the basic standard under 30CFR11 and provides good protection to the wearer, when properly fitted.  If you buy a molded rubber facepiece, you will probably use HEPA filters unless you are dealing with a paint spray setup, described below.  For your typical woodworker, either DM or HEPA is sufficient protection against particulate wood dusts.  The DFM mask (often called a "welding respirator") is the moral equivalent of the DM for our purposes, and so provides adequate protection as well.

As noted above, 30CFR11 products are no longer allowed to be manufactured or sold by the manufacturers in the US.  There was a 3 year transition period, so these products were still made up to as late as July 1998 in some cases.  The 30CFR11 product that was in retail stores or distributors as of 7/98 may still be sold until it is gone, so there is still a lot of 30CFR11 product in the pipeline and likely will be for the next couple of years.

42CFR84

The change in the regulations added some complexity to a user's selections, but made testing and approvals much more consistent and straightforward, in my opinion.  I should note that the regulation change primarily changed particulate respirators and filters and not gas & vapor protection, or other specialized categories like supplied air or Power Air Purifying Respirator (PAPR) systems.  The old DM/DFM/HEPA categories are gone and replaced by a 3x3 matrix of approval categories.  There are 3 classes of filter types: N, R, and P.  These refer to the contaminants that the mask is certified to protect against.  The other axis of the matrix has 3 filtration efficiencies: 95%, 99%, and 99.97% (called 100% for simplicity).  Masks sold under 42CFR84 are required to be labeled according to the following grid:
 

Now, while the 3 efficiency levels are pretty straight forward, the classes need some explaining.  "N" class masks are intended for use against solid or water-based liquid aerosols (wood dust is a good example of an N-class particulate), "R" class masks are intended for use against oil-based liquid aerosols, and "P" class masks are for both oily- and non-oily aerosols (the universal class, if you will).  There is more technical detail behind the R and P classes that I won't go into since they typically don't matter for woodworking.  True oily-mist hazards are uncommon in industry, so if you have a situation where you think you might have oily-mists, you are beyond the scope of this summary for the most part (exception again is paint spray).

Under 42CFR84, the user has 9 categories to select from, and that can be confusing.  This is perhaps the one point where 42CFR84 is overly complicated, but it turns out to be simpler than it looks.  The reason for this is that most filters and masks made and sold are either N95 or P100 (there are some in other classes like N100 and a couple of R95s and P95s, but not too many).  The reason why is the mask selection is limited this way is fairly simple (and perhaps should have been anticipated by NIOSH, but that is water under the bridge) in that most people either need/want basic protection against solid particulates (N95) or need/want the best protection they can get (P100).  If you're thinking of getting a 99% mask, why not just get a 100% and be safe?  And if you have an oily-mist hazard, you probably also have solid particulates so you need a "P" anyway.  That seems to be the logic of most users.  The N95 class is a better filter than the DM class which it ostensibly replaces, so even the lowest 42CFR84 mask can be considered to be a step up from 30CFR11 products.  There are some specific contaminants that OSHA requires certain filters for (lead is a good example) but those are beyond the scope of hobbyist woodworking.

So what do you need?  It comes down to two choices which depends primarily on the type of mask you get.  If you decide on a filtering facepiece mask (the basic disposable type that people often - incorrectly - call "paper" masks) then you will probably get an N95.  If you pick a molded rubber facepiece, you will probably get replaceable P100 filters.  Again, it is up to to user to determine suitability, but if you are only looking to protect against wood dust either one will serve you well in almost all hobbyist/small shop situations.

What do you do if you have both particulate and G&V hazards?  Check out Multiple Hazards below.
 
 

I am more of a particulates guy than a G&V guy by experience but, just as for particulates, the choices for G&V protection for woodworkers is pretty straight forward.  A gas or vapor is the volatile part of a substance, generally coming from a liquid, that exists in the gas phase at the conditions you are working.  What you smell from a volatile organic solvent such as Acetone or Mineral Spirits is the vapor and is the type of hazard addressed in this section.  Typical solvents that a woodworker should be protected against include alcohol, mineral spirits, turpentine, paint thinner, acetone, lacquer thinner, any wood finish that contains one or more of these solvents, and basically anything else you can get at the paint store.

Negative pressure G&V protection involves a cartridge (or pair of them) containing activated carbon to soak up the chemical vapors in the air you breathe.  There are many specialty cartridges designed to protect against a whole gamut of specific hazards but the most important one for woodworkers is the general Organic Vapor (OV) cartridge.  The only special case that jumps out at me is that ammonia cartridges are good to have if you do any ammonia fuming in your wood finishing.  Note that concentrated ammonia will burn your eyes and requires gas-tight eye protection in addition to the respiratory protection.

G&V cartridges do not filter out particulate contaminants (see multiple hazards).  Again, if you have some special hazards or questions, you need to contact someone certified to answer your question or OSHA.

Multiple hazards

What if you have both particulate and organic vapor contaminants present in your environment?  A G&V filter is not effective against particulates, and vice versa.  You need both simultaneously: a stacked filter.  A stacked filter is at it sounds, simply an organic vapor cartridge with a particulate filter attached to it to cover both hazards.  The particulate filter sees the contaminated air first, and then passes the particulate-filtered air (that still has vapors in it) to the G&V cartridge that then filters out the organic vapors which in turn gives the wearer clean air to breathe.  There are two basic configurations of stacked cartridges: preassembled and adapters.  Preassembled stacked filters have the two components put together at the factory and you replace both the G&V cartridge and particulate filter at the same time.  Some manufacturers also make available adapters that allow the user to attach a particulate filter on top of a G&V cartridge for the same effect.  Either way protects the same.  The biggest difference is one of waste.  In the preassembled stacked combo, when either the G&V portion or the particulate portion is used up, you have to replace the whole stack, rather than just the component that needs replacing.  Often the G&V cartridge will wear out long before the particulate filter is plugged.  For infrequent use, this may not matter as much to you as it would if you were a constant user.
 

Paint Spray

Spray finishing is a special sub-case of multiple hazards in which  there are both particulate and organic vapor hazards.  The particulate hazard is the aerosolized finish (lacquer, paint, etc.) that remains airborne either due to missing the workpiece or do to bounce-back from the surface being sprayed.  These particles of finish may be uncured, partially cured, or fully cured and as a result can clog a standard particulate filter very rapidly.  If you think about it, the purpose of a finish is to coat and flow out to a smooth surface, and it will do this on any surface whether it is your workpiece or your facemask's filters.  Because of this characteristic of spray finishes, manufacturers often make special filters that resist clogging more so than standard filters and refer to them as either paint spray filters or paint spray prefilters.  You would use this filter as the particulate filter in front of the G&V cartridge in the above multiple hazard example.  There is not an extra filter added to the stack.

Fumes

Fumes are another special case that existed in 30CFR11 rules, but are no longer treated differently than standard particulates under the new 42CFR84.  In the language of particulate hazards, a fume is defined as a very large surface area particle, not as a vapor from an organic solvent as most people commonly use the term.  These particles are most often associated with molten metals and as such can occur in casting or smelting operations, for example.  The most common reason a hobbyist-type person would encounter a metal fume is welding.  A lot of people have small arc welders for occasional use, so I wanted to make note of it here.  The reason there was a separate class for fumes is not because the particles are a special hazard, but rather due to their very large surface area and agglomerating characteristics, they can plug up a filter much more rapidly than typical particulates.  The respirators certified to meet the  D/F/M class were designed to resist clogging.  You can typically use a standard particulate filter or mask for metal fumes, but it will likely clog up faster.  This is not much of a concern for a person doing occasional welding, but for the professional welder.  Failure to protect yourself from metal fumes via ventilation or respiratory protection can lead to a condition called "metal fume fever" or worse. Again there are special cases where OSHA has specified a certain filter class for certain hazards (again, lead is one example), so if this is really an issue for you, consult a certified Industrial Hygienist.

There is one final point on welding to keep in mind.  Certain platings or coatings can be very hazardous and you should never weld them unless you 1. Know what you are doing, 2. Have proper ventilation, and 3. Wear proper respiratory protection.  Two of the most common cases someone would encounter (but not the only ones) are galvanized steel (zinc coated) and cadmium plated steel (some automotive parts).  Don't mess with these unless you know what you are doing.