I created a gas based off carbon monoxide but in this case non-flammable for Pathfinder. It could be identified with either a Knowledge Nature, Survival or Knowledge Dungeoneering as it was a natural gas on Golarion. It also could be identified with a Craft Alchemy check as it was a byproduct of certain alchemical concoctions.
The first time it was encountered I made it a Fortitude Save DC 15 or fall unconscious without any Constitution damage. Not totally realistic, but it was meant to be more of a warning of an environmental condition then a punishment. They were level 2, after all.
Just like carbon monoxide, it's an odorless and colourless gas so be sure to give prior warning if you're using it. Also, be aware that this was meant to be more of an environmental hazard and was thus more of a house rule / mechanic for an aesthetic rather than a hard core mechanic so it best suits a more story-driven game than a mechanics-based game as it doubtless isn't balanced.
In this instance, I raised the Fortitude saves and changed the possible outcomes of a failed roll depending on the saturation of the gas in the air.
At a high saturation it causes Unconsciousness and 1 point of Constitution damage each round on a failed Fortitude Save DC 15. It would also extinguish open or contained flames at this level (gives them a way of noticing it).
At a lower saturation it simply deals 1 point of Constitution damage on a failed Fortitude save each round without risk of Unconsciousness at a Fortitude Save DC 12. It weakens open or contained flames at this level which basically leads to a halved light radius.
Either way, since you can't smell it or see it, you can only recognise it by seeing dead animals, failing two rolls and then making a successful Heal check (as the symptoms feel like lethargy and weakness rather than choking), or making a Perception DC 40. In other words, lay clues around them before they stumble onto it unless they're of significantly high level.
This gas is also heavier than air and thus tends to stick close to the ground, allowing pockets of safe air to form.
Check out the wikipedia entry for the symptoms.
You can find the session write up where I used this mechanic over here.