While the composition of dry air differs slightly from place to place, it is generally made up of about 78 percent nitrogen and 21 percent oxygen. Argon makes up most of the remaining 1 percent, and carbon dioxide and other trace elements make up the rest.
If air is not completely dry, water vapor may be up to 5 percent of its composition. The presence of water does not change the relative proportions of other gases.
Different layers of the atmosphere contain differing air compositions. For example, water vapor is mostly contained in the lowest layers of air, while about 90 percent of the Earth's ozone is contained within the stratosphere. The very light elements helium and hydrogen are mostly found in the outer layer, the exosphere.