Given that the planet is not crammed in all dimensions so tightly there'd be no room for humans, and yet we have fresh air outside, I'm doubting your statement a bit.
1) the planet is a big, BIG place and there are rainforests and oceans. Like, all of the air that we breathe originally started from ocean life.
and 2) there have been studies on this done by NASA, et al, and the general consensus was you need a lot of plants
The planet is a bit larger than a house though.
We have the ocean
https://scripps.ucsd.edu/news/research-highlight-ocean-air-f...