Science has shown us that our air contains 20.9% oxygen. Other studies have shown that fire combustion can be initiated at approximately 16% oxygen, but cannot develop into and actually sustain fire, until the oxygen level rises to 18.9%. However, optimal usable combustion does not occur until oxygen level rises to, you guessed it, 20.9%. Another amazing scientific discovery is the if you raise the oxygen level to between 21.9% and 30%, combustion becomes unstable and explosive.
The other interesting thing is that the minimum oxygen concentration for human breathing is 19.5 percent. Below that brain and body cells began to die. For humans and many animals to sustain normal functions, the percentage of oxygen in our breathing environment must be within a small range. The Occupational Safety and Health Administration, OSHA, determined the optimal breathing range to be between 19.5 and 23.5 percent oxygen. As a matter of fact, once a human begins breathing oxygen levels above 23.5%, numerous health issues arise. With all that said, there are human activities, such as jet pilots, divers, etc. that breath higher levels of oxygen, but they require special training to do so.