I was wondering about this actually.
Chlorine, if used in high concentrations especially in confined areas can be a deadly choking agent. It is legal, cheap and readily available. For military purposes it is no longer considered a viable chemical weapon but civilians would still be vulnerable to this sort of stuff.
Another weapon to beware of is possibly ammonia. There's a lot of high concentrate floor cleaners out there and with a little lab work it can be made even more potent. Ammonia has some qualities about it that make it favorable to other gases in terms of penetration.
Honestly I'm surprised terrorists haven't tried this yet.
