The mixing ratios for these gases do not vary in our lifetimes; there may be some variation over geological time (billions of years). The "other" permanent gases are present in such low quantities, they are inconsequential to human life.

Alternate names for permanent gases are non-variable gases and fixed gases. In this sense, the "fixed" gases are not the same as the "fixed" air that was named by Joseph Black (which was actually carbon dioxide).

Water vapor and carbon dioxide are the most important of the variable gases because (1) they are the most abundant variable gases and (2) they both act as greenhouse gases, due to their ability to absorb infrared (IR) radiation, which causes the Earth's surface to be warmer than if there was no atmosphere.

There are non-varying trace gases; their mixing ratios are given in the screen at the top of this page.

We are most interested in the variable trace gases because many of them are pollutant gases. A few examples are: O₃ (ozone), NO_x (nitrogen oxides, where x = 1, 2, . . .), SO_x (sulfur oxides), CO (carbon monoxide), and so on.