Maybe this is overkill, but I used to work in chemical plants and had to wear respirators as a required part of my daily job. Here's an engineer's take on the matter:
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If you want a
respirator for short exposure to lye mist, such as what you'd get during soaping, I would choose a high efficiency particulate respirator with an "N100" or "P100" rating. These "100" series respirators remove 99.97% of the dust/mist in the air that you breathe, assuming the respirator is fitted properly to your face.
There are respirators with slightly less stringent N95 or P95 ratings, but I recommend the higher N100 or P100 rating to remove absolutely as much lye mist as possible. If the "100" series respirators are not available to you -- and they can be harder to find -- by all means get a "95" series respirator. They will work too for all but a tiny % of the super small droplets and particles. The difference between "N" and "P" is that N type respirators are not rated to handle particulates and mists that are oily. The P respirators are. In soaping, the difference is moot -- buy whichever version is cheapest or most easily available.
You can get the disposable kind that look like white cups that fit over the mouth and nose. They will have at least two elastic bands to hold the mask firmly onto the face. Or you can get a "half face" respirator that is designed to accept replaceable cartridges and put a particulate cartridge on it. Either version is fine as long as it is OSHA, MSA, or NIOSH rated for particulate removal.
3M's disposable 8233 respirator is N100 rated at under $10 each:
http://www.amazon.com/dp/B008MCV43K/?tag=skimlinks_replacement-20
3M also makes several reusable facepieces that can be fitted with suitable cartridges. The facepiece I use sells on Amazon for under $20 already fitted with a P100 cartridge:
http://www.amazon.com/dp/B00AR63OGE/?tag=skimlinks_replacement-20 These facepieces come in sizes; many women will want the small facepiece.
There are other reputable manufacturers to choose from -- just look for a respirator with the N100 or P100 rating and, in the USA, a NIOSH/OSHA/MSA approval. I replace particulate cartridges when they get hard to breathe through or when they look dirty, whichever comes first.
Hardware stores sell a cheap, thin "nuisance dust respirator" with a single thin elastic band to hold it onto the face. This type of product is a joke -- you're better off holding your breath like some soapers do rather than trust your lungs to this type of respirator.
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If you also want
protection against inhaling FOs and EOs as well as lye mist, you will need a respirator rated for organic vapors AND for N100 or P100 particulate removal. This kind of respirator doesn't come in a disposable style; you have to go to a reusable facepiece like the one in the second link (above) and install the correct cartridge (below) on the facepiece.
One possibility if you don't have huge health issues with EOs or FOs and don't work with them a lot -- you could use an N100 or P100 particulate cartridge with the added ability to remove small amounts of "nuisance organic vapors". Here's a 3M cartridge rated for N100 particulates and for nuisance organic vapors:
http://www.amazon.com/dp/B00328IAO0/?tag=skimlinks_replacement-20
If you do have serious health issues with EOs or FOs or are working with EOs and FOs a lot, you should consider a full-fledged heavy-duty organic vapor cartridge. This is the kind of cartridge I use when spraying lacquer. This 3M cartridge removes organic vapors AND is N100 rated for particulates:
http://www.amazon.com/dp/B002D8ZR5K/?tag=skimlinks_replacement-20
If at any time you can smell the organic odor through the respirator, the cartridges are spent and you need to replace them immediately.
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If the respirator doesn't fit properly, it can't do its job. I was required to wear a respirator when I worked in chemical labs and production plants. We were checked regularly to see that our respirators were in good shape and properly fitted. I use the same fitting and testing procedures when I wear a respirator for woodworking, soap making, and such. I see a tendency in people who have never used a respirator for their job to ignore proper selection and fit. A person might as well not wear one at all if it doesn't fit right or isn't the right kind of respirator for the job.
A disposable is more difficult to fit properly, so if you have a hard-to-fit face like me, you may need to use a reusable facepiece. If you're buying a respirator at a "big box" hardware or lumber store, the respirator selection is likely to be geared toward men. That means the disposable respirators and reusable facepieces are probably going to be mostly (or all) in the medium and large sizes. Most women need a small size mask to fit their smaller cheekbones and jaw, so you may have to order from a catalog or online.
Getting back to disposables -- pay close attention to forming the nosepiece to fit snugly around the bridge of your nose and upper cheeks. There should be two bands on a disposable respirator -- one to go around the back of your head to stabilize the top part of the mask and one around the back of your neck to stabilize the lower part of the mask. Tighten each band so the mask is pulled gently against your face. Don't overtighten to avoid distorting the fit and causing headaches.
When the mask is adjusted to fit comfortably, take a deep sharp breath in and pay attention to the sensation of any cool air flowing in around the edges of the mask. The top of the nose and the jaw bone are the usual trouble spots for me. If you feel leaks, adjust the mask as needed to improve the seal. If you can't get the mask to stop leaking no matter what you do, you may need to try a different size and/or a different style. Or you may need to switch to a reusable facepiece. A properly fitted disposable should "suck down" tight against the face when you take a deep breath in -- there should be no leaks around the edges, so all of the air is coming through the mask material.
Fitting a resuable facepiece is basically the same -- your goal is to have no leaks around the edges of the mask when you suck in a deep breath. To do this, first remove the cartridges from the facepiece. To remove a cartridge, twist it a quarter or half a turn and pull it away from the facepiece. Put the facepiece on and adjust the elastic bands to give a snug, but comfortable fit around your face. Put the palms of your hands over the holes where the cartridges normally go to seal those openings. Take a sharp breath in. Because you're sealing off the air intake, the facepiece should seal tightly to your face, letting no air in. If you feel leaks, adjust the fit as needed. The process of fitting a facepiece often goes a bit easier than a disposable because a facepiece is made of soft rubbery material that seals against the face better. If you have leaks especially around the jawline that you just cannot get rid of no matter what you try, you might need a different size facepiece to get a proper fit.
If you're a guy with a beard or mustache, be aware that almost all disposables and most reusable "half face" facepieces can't handle facial fuzz. You may need to go to a "full face" respirator for proper fit and effective protection.