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 chemical per kilogram of soil, or micrograms of chemical per liter of water. In the United States, one billion equals 1x10⁹. Dioxin, a contaminant associated with the herbicide 2,4,5-T and implicated in the formation of birth defects, can cause concern at concentrations measured in ppb.