Question

Why doesn't the air run out of oxygen or fill up with carbon dioxide

Answer 1

the air won't ever "run out" of oxogen unless all of the tress and plants are killed and can not give out oxogen or we aren't here to give out dioxide, which the trees take in and do a process called photosynthesis which makes oxogen

Answer 2

Trees breath carbon dioxide in and breath out oxygen where humans do the opposite so we keep each other alive