Ever wondered why you breathe harder and faster when you’re working out at a higher altitude than that to which you’re accustomed? In colloquial terms, we often say that at elevation, the air is thin, or that there’s less oxygen in the air. What does this mean, though, particularly in light of the fact that whether you’re at sea level or on top of Mount Everest, every breath you take is 21% oxygen?
The difference between a lungful of air at sea level and a lungful of air on top of Everest is that the atmosphere is more dense at lower elevation, meaning that a given volume contains more molecules of air. Since your lungs have a (relatively) fixed capacity, you take in the same volume at lower elevation as you do at higher elevation. At higher elevation, though, that volume doesn’t contain as many total molecules…and since 21% of the molecules you take in are oxygen (regardless of elevation), you’re getting fewer oxygen molecules per lungful at higher elevation.