++ [ originally posted by Rickenbacker2 ] ++
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You can not get a working gas mask for under $200 american. The ones that you see for $50 have poor filters, and do not even come close to filtering all of the common airborne weapons out there.
Don't go to an Army surplus store, because those masks all have shot filters.
In conclusion, I can't imagine what you would want a gas mask for, because most of the really nasty nerve gas will melt the plastic that they're made out of anyway.