A weight to weight ratio used to describe concentrations. Parts per million
(ppm) is the number of units of mass of a contaminant per million units of total
mass.
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ppm (or ppmm) is used to measure the concentration of a
contaminant in soils and sediments. In that case 1 ppm
equals 1 mg of substance per kg of solid (mg/kg).
ppm (or ppmm) is also sometimes used to describe small concentrations
in water, in which case 1 ppm is equivalent to 1 mg/l because a
liter of water weighs approximately a 1000 g. This use of ppm tends to be phased
out in favour of mg/l.
ppm (or ppmV) is often used to describe concentrations of
contaminants in air (as a volume fraction). In this case
the conversion of ppm to mg/m3 depends on the molecular weight of the
contaminant.
For example, 1 ppm chlorine represents one part of chlorine in one million
parts of air by weight, which is 1.45 mg/m3.
