Humans can't bring the level near 0% because they would die long before that. Humans need between 19% and 24% O2. Below 19%, humans can't exert much energy wi

Humans can't bring the level near 0% because they would die long before that. Humans need between 19% and 24% O2. Below 19%, humans can't exert much energy without passing out. Above 24% increases damage from free radicals.