Mythic Sustenance (Su): Your mythic power is enough
to sustain you. You no longer need to eat, drink, or
breathe to live. You’re immune to inhaled poisons and
any spell or effect that requires breathing, though you are
still affected by any food or drink you intake.
Does this effect your passive perception for smell?
Smells are handled via your nasal cavity, breathing through your lungs. This mythic path ability states that you're immune to things that hit the latter, not the former. With it, it seems pretty clear you'll be able to stand in a cloud of chlorine gas and be fine.
Now, for things like Cloudkill and such, that's more of an air-borne contact poison, similar to nerve gas. You'd need a Life Bubble to get around that.