How dangerous do you want it to be? Can the characters avoid it, or mitigate it?
Treating it as poison certainly makes sense. You could give bonuses on RRs if the players take appropriate actions (e.g. cover their nose and mouth with a damp cloth), penalties if they are in there unprotected for extended periods. On the low end, a toxic gas could result in activity penalties (from coughing, choking, eyes burning, etc). On the high end of course it could be much worse. The more severe results could kick in only if they fail by a certain amount, or the penalties could scale (e.g. -5/5fail, for 1 minute per 5fail).
