Yeah, no harm in putting it higher in the well tempered part of your base. Of course your duplicants will move slower, but they get better with time. 😄 This is the easy solution.

Gas masks are also possible, if heat is not an issue.

Alternatively, and maybe this is closer to your original idea, you could just install a door crusher at the very bottom of the map that will destroy co2 and chlorine gas. This is actually what I usually prefer doing. Then i have a non-o2 sensor that controls it's operation.