It's a straightforward question. The entire planet was covered by city. You don't see very many trees in the movies. This doesn't mean there aren't any of course, but certainly nothing on the scale of the Amazon rainforests on Earth. What does everyone breathe?
Wookiepedia just says
Citizens who lived in the upper levels were able to breathe air that was filtered and clean. Sunlight never reached the lower levels which had to be lit by artificial light and the inhabitants were forced to breathe the air of toxic fumes from factory and vehicular waste.
Does this mean there was a clean-air utility that piped clean air (or atmosphere appropriate to your biology) to homes and offices? And the planet's own atmosphere is irredeemably toxic to humans?