The police generally use CN gas, a milder variant used with civilians. Never having been “hit” with CN, I can offer no information as to the effectiveness of a COVID-19 mask, other than to say that I assume that it would be of absolutely zero effect.
The military uses CS gas, another “critter” altogether.
CS is NASTY! Clinton/Janet Reno used CS at Waco. CS in a confined space, with no way of exit or access to fresh air, will kill you. CS attacks the mucous membranes causing saliva, tears and snot to flow like fire hoses.
I pulled a fellow tunnel rat from a Viet Cong tunnel in 1969. We’d pumped CS into these tunnel complexes hoping to drive out the VC. Unfortunately, due to heavy rains for several days, the other Marine had a gas mask with wet, ineffective filters. It took me several minutes to find a dry mask for myself. By the time I got to him , he was not breathing. When I tried to turn him around so that I could put a loop of rope under his arms, he just gurgled.
With men outside the tunnel pulling and me pushing, we got him outside where the Navy Corpsmen started CPR. Believe it or not, he returned from the hospital a month later with no more evidence of brain damage due to oxygen deprivation than that displayed by the rest of us knuckle-dragging Grunts. :-)
When you are hit with CS, you will run as far and as fast as you can to get fresh air. You will run over anyone in your way. You will run until you bounce off of boulders and large trees. Trying to wash you face only makes it worse, as it is activated by moisture.
I say, give the rioters, looters, arsonists and mobs a good dose of CS!