I did some more reading.
If you want protection only for lye mist, I would choose an "N100" or "P100" respirator. This respirator removes 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 less stringent N95 ratings, but you really, really do want to get the higher N100 or P100 rating to properly handle lye mist.
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 approval. I replace particulate cartridges when they get hard to breathe through or when they look dirty, whichever comes first.
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If you also want protection against inhaling FOs and EOs as well as lye mist, you will need an organic vapor respirator as well as 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.
Hope this helps!