A weight to weight ratio used to describe concentrations. Parts per billion (ppb) is the number of units of mass of a contaminant per 1000 million units of total mass.
Also Вµg/L or micrograms per liter.
ppb (or ppbm) is used to measure the concentration of a contaminant in soils and sediments. In that case 1 ppb equals 1 Вµg of substance per kg of solid (Вµg/kg).
ppb (or ppbm) is also sometimes used to describe small concentrations in water. in which case 1 ppb is equivalent to 1 Вµg/l because a liter of water weighs approximately a 1000 000 Вµg. This use of ppb tends to be phased out in favour of Вµg/l.
ppb (or ppbV ) is often used to describe concentrations of contaminants in air (as a volume fraction). In this case the conversion of ppb to Вµg/m 3 depends on the molecular weight of the contaminant.
For example, 1 ppb chlorine represents one part of chlorine in one thousand million parts of air by weight, which is 1.45 Вµg/m 3 .
The ppb unit is mainly used in North America.