How much oxygen is present in air?

Oxygen (O2) makes up about 21% of dry air. Nitrogen (N2) makes up about 78% and Argon (Ar) less than 1%. There are a few trace gases mixed in the atmosphere such as Carbon Dioxide, Nitrous Oxide, Methane, Helium, Neon, in very small amounts represented in parts per million (ppm). Moist air is a mixture of water vapor (H2O) and the components of dry air, and the amount of water vapor is variable.