Parts per million (ppm)
A concentration ratio meaning one part of a substance per million parts of the mixture, abbreviated ppm. The exact interpretation depends on what is being measured.
Parts per million describes a proportion, not a total amount. For gases, ppm usually means molecules or volume units of one gas per million equivalent units of the gas mixture. A reading of 1,000 ppm carbon dioxide is 0.1 percent by volume.
The same abbreviation is used for dilute substances in liquids, often on a mass basis such as milligrams per liter in water. Gas-phase ppm and solution ppm are different measurements and should not be substituted for each other.
A concentration alone does not state how much material a room or reservoir contains. Total quantity also depends on volume, temperature, pressure, density, and the basis used for the ratio.