I'm a volunteer firefighter. I've wondered what my county's response would be if I borrowed one of our SCBAs (self-contained breathing apparatus) to wear to an anti-ICE demo or ICE observation incident. It's not a gas mask in the sense of this article, because as the name suggests it's an entirely self-contained air supply - it doesn't matter what is in the air at all. The one issue is that a single air bottle has a finite and not particularly long use time (if you're in good shape and don't panic, 20-30 minutes tops).