The amount of oxygen in the cell is extremely low, that's why it's Unbreathable (btw, polluted oxygen is breathable, "breathability" is not about type of gas). Dupes can consume oxygen only when it's around 50mg+ but will gain extra stress, and it'll be okay for them to breathe in environment with 300mg+ of oxygen, see here: https://oxygennotincluded.gamepedia.com/Oxygen