Answer :
oxygen is a constant 20 percent throughout the atmosphere. Even in space but the air is so thin in space that the oxygen would have to be compressed to be able to breathe it. Here on earth at sea level the atmospheric pressure is 14.7 pounds per square inch (PSI) making it possible for your body to inhale enough usable oxygen for you to breathe.
Oxygen resides in the atmosphere for about 4,500 years. It usually takes about 2,000 years to cycle through
