It is always maintained at least at the level that we breathe in room air, which is 21We also breathe in nitrogen, mostly nitrogen and oxygen.
(It; is maintained; L:at least at the level; T:always) (the level; breathe; L:in room air) (room air; is; 21We)
It the level room air
is maintained breathe is
L:at least at the level T:always
L:in room air
21We