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 µg/L or micrograms per liter.
Source: GreenFacts
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ppb (or ppbm) is used to measure the concentration of a contaminant in soils
and sediments. In that case 1 ppb equals 1 µg of substance per kg of solid
(µg/kg).
ppb (or ppbm) is also sometimes used to describe small concentrations in
water, in which case 1 ppb is equivalent to 1 µg/l because a liter of water
weighs approximately a 1000 000 µg. This use of ppb tends to be phased out in
favour of µg/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 µg/m3 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 µg/m3.
