Dwarves really get the short end of the stick. They're not "greedy" for digging too deep, it's literally what they were made to do. Their whole economy depends on mining. What do you want them to do, grow crops? With what farmland? The elves and halflings took it all. And elves never awaken ancient evils of the forest by growing trees too tall, and halflings never face the consequences of their greed in eating too much. Dwarves spend all day working so they can survive, and the stories call them evil. Elves spend all day lazing around in their vacation homes, and if you complain, somehow you're the bad guy.
One of the things I liked in the recent season of Mushoku Tensei. They're casting fire spells in a labyrinth and they warn the MC that they'll suffocate if they use too many.
CO is 97% the weight of typical atmospheric mix, so yes it's lighter, but by such a small difference that it spreads and fills a space evenly over a short period of time rather then starting at the ceiling and filling in the space downward.
Basically, going down to escape won't work, you need to get away from the area of high concentration.