A means of expressing extremely minute concentrations of substances in water or air. In the United States, the concentration is the number of units of the substance found in 1x1012 units of water or air; an equivalent unit is nanograms per kilogram....

A means of expressing minute concentrations, often of an element, compound, or particle that contaminates soil, water, air, or blood. The expression may be on either a mass or a volume basis, or a combination of the two, such as micrograms of a...

A means of expressing small concentrations, usually of an element, compound, contaminant, or particle in water or a mixed medium such as soil and normally on a mass basis. Units that are equivalent to ppm include micrograms per gram, milligrams per...

Parts-per notation is often used in the measure of dilutions (concentrations) in chemistry; for instance, to measure the relative abundance of dissolved minerals or pollutants in water. The expression “1 ppm” means a given property exists at a...