From Wikipedia at https://en.wikipedia.org/wiki/Atmosphere_of_Earth :
"By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere."
I assume that if you kept on filling with air, you'll just be gradually increasing the percentage of Oxygen and other gasses until you reach a state equivalent to pure air.
Or not. Just spit-balling...