Why don’t adventurers suffocate in lower levels of dungeons?

Why don’t adventurers (and monsters) suffocate in lower levels of dungeons?

I asked a question about this on Physics to check my reasoning. Why does Nitrogen not sink in air if other dense gases do? An answer states that CO2, which is a product of exhalation, will sink to lower levels in enclosed spaces. There is also a hazard in mines generally from gases seeping out of the rocks.

Question

How do PCs and NPCs survive in such conditions? If there is some form of ventilation to the outside world, can explorers escape through the ducts?