Avoid surplus masks for any NBC use like Poison Gas or Nuclear Contaminates or anything else that might kill you.

If you believe you may need one then get a good current military type mask.

I have a M95 Mask with good filters.

A older mask like an M17 should be good for CS Tear Gas. Although I never tried one with Tear Gas, I did use one for protection against those Gopher Gas Bombs that my friend used inside a Horse Barn to smoke out the Rats, and it did work OK and I never even smelled the Gas and more importantly I never got sick.

There is one very important thing to consider if you intend to use a Mask in a Combat situation and that is the location of the Filter and how it effects your ability to use a Rifle properly.

With the M95 the filter can be placed on either side.

Also a Mask prevents you from getting in proper position to use the sights and I found that a Red Dot on top of an AR15s carry handle mostly solved that problem.