I believe mustard gas is created from a mixture of bleach and ammonia, not bleach and a chlorine component of some type, thus that's why I would tend to agree that it was chlorine gas being released. But yes there was no way in hell I was going to even attempt an acid cleaning without the windows open and the fan set on high speed blowing air out. I know what that stuff is capable of and it isn't pretty. Once or twice a couple of strong fumes got in my nose fast enough before I realized it and forced myself to exhale. Absolutely nasty shit. I truly feel for those guys in WWI who were killed or otherwise permanently injured by that stuff. The mucus glands in your nose immediately kick into overdrive as sort of a self-defense mechanism as best I can tell, nonetheless it's a terrible way to go. I think I'd rather take a bullet to the head.
Anyway, back on topic, I'll pick up some neutralizer tomorrow and give that a try with a thorough cleaning.