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Adapted and edited slightly from this thread by @AliceAvizandum on twitter. Archived here and here.

When we talk about tear gas we generally mean the class known as "riot control agents", the two most common of which are CS gas and OC ("pepper spray"). Neither of these are gases, they're aerosols made up of large molecules that get stuck easily. Gas masks filters have a particulate layer with a lot of surface area for molecules to stick to, and sometimes also a layer of carbon to absorb them.

A filter that only has a particulate layer is rated as P3 in Europe or P100 in the US and this *should* stop riot control agents. If you want an extra margin of safety, you can get one that's rated against gases as well. Just make sure it ALSO has the P3 or P100 rating. If you're looking for a gas rated filter, it's an A2P3 in Europe and in the US color-coded black or brown because there's no overriding standard. You probably do not need a gas rated filter.

You can choose between half face or full face masks. Full face masks save having to get extra eye protection, and the whole thing seals so you won't be crying. On the other hand it will be more claustrophobic, you probably won't be able to wear glasses, and it may not be impact rated. If you wear glasses, get a half mask and some goggles. MAKE SURE THAT YOUR MASK WORKS WITH THE FILTERS YOU GET. Civilian masks are usually proprietary (e.g., 3M masks only work with 3M filters). If you get a military surplus mask from like, Ebay, there are four types of filters:

• ✅ 40mm NATO. Most modern masks use this, get this one because you can get new filters for it.
• ❌ Soviet GOST. Can fit NATO filters with tape but is a pain [and probably more likely to fail if you do a bad job with the tape].
• ❌ 60mm NATO. Old. If you find it somehow, avoid it.
• ❌ Cheek filters (from masks with no visible canister hanging off, just big hamster cheeks). Avoid, are always expired.

Best bet is to ignore whatever filter your mask comes with and get a new sealed one but, if it comes down to it basically any mask filter will probably stop riot agents.

There is probably asbestos in the filter.

The filter will suck to breathe through, especially if it's old. It'll be exhausting to run around with it on, so try to (safely) minimize how much time you spend wearing the mask. Try using a bandana until you actually start getting gassed. Wearing a gas mask might get you targeted by the cops, whoops

Boring but important:

• if your mask has a manual, read it. Practice putting it on and taking it off. Change the filter if it gets noticeably hard to breathe through. Having a beard will massively lower the effectiveness of the mask so probably shave, sorry.
• If you get gassed the outside of the mask will be contaminated. You can wash the mask but not the filter, so it's one-and-done if the filter is exposed to anything.
• A decent test of whether a gas mask is working well enough for this context is putting it on, spraying some deodorant around and seeing if you can smell it.
• You can test whether or not the mask is sealing by, with the filter off, putting th epalm of your hand over the valve the filter goes onto and inhaling. If the masks sucks in around your face, it's sealing.
• if the mask you ordered off EBAY came covered in white dust, don't worry, it's just the talcum powder they use to keep the rubber from degrading.

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