You don’t need CO to die from asphyxiation. You just need enough of a gas to displace the oxygen. It’s the same reason people die from inhaling nitrous oxide. Carbon Monoxide is just especially good at it and used to be a lot more common.

Had to Google this, cause I thought that was the weirdest euphemism for sex, and I was curious how it came to be. Turns out, it’s a co-op platformer.