Many fumes and gasses generated from recovery and refining of gold, silver, and other precious metals will not be stopped by a gas mask. Look into fume hoods, fume scrubbers, and the like.
Another option, in addition to a good fume hood is an positive air pressure hood. The device has several names in different industries. Local auto paint shop has what they call “paint hoods” with an external air supply so employees will not breathe paint fumes and particles.
If you are looking to stop CS and similar gasses used by police at riots, any gas mask with proper filters will do.
Time for more coffee.
