We've evolved for millions of years in an atmosphere of about 21 percent oxygen

" Evidence from prehistoric times indicates that the oxygen content of pristine nature was above the 21% of total volume that it is today. It has decreased in recent times due mainly to the burning of coal in the middle of the last century. Currently the oxygen content of the Earth’s atmosphere dips to 19% over impacted areas, and it is down to 12 to 17% over the major cities. At these levels it is difficult for people to get sufficient oxygen to maintain bodily health: it takes a proper intake of oxygen to keep body cells and organs, and the entire immune system, functioning at full efficiency. At the levels we have reached today cancers and other degenerative diseases are likely to develop. And at 6 to 7% life can no longer be sustained. "