100 years ago had twice that of today's. We become slightly stagnant, and serve as a growth medium for anaerobic parasites. But all pathogens are inherently much weaker than our own cells, which are a lot more highly evolved than viruses, bacteria, fungus and such. So we can always get rid of them by just raising our internal oxygen concentration above what they can stand. Your body's supposed to be at least eighty percent water, which is eight ninths oxygen.
Water is two hydrogens for each oxygen, but the oxygen atom is sixteen times bigger.
You're supposed to be composed of over two thirds oxygen, twice as much as everything else in you combined. The bigger a proportion of something in a formula, the more margin there is in the high to low range of how much you can put in and still make that formula work. That also applies to the formula for an organism. So you can see how a person could exist anywhere along a wide range of oxygen saturation that can support his cells, though the lower levels are not much fun. But people can still function, more or less, even at oxygen percentages so low that anaerobic microbes can inhabit them quite comfortably.