It's not like your average dungeon or lich cave has OSHA approved vents, fans or electric lighting. Your average party, without specific magical aid, is burning torches and lanterns in unventilated spaces.
ETA: if you're going to run carbon monoxide, etc rules, your supply shops should stock and recommend caged canaries to anyone buying large quantities of torches and lamp oil, dungeoneering/mining gear, etc. People would know about it and have figured out a primitive solution that merchants would be happy to capitalize on if this is an in-world issue.