Parts per million

Parts per million is a unit which expresses a small amount of dimensionless quantities. Proportionally, parts per million represents one thousandth of a millilitre of something per litre of water, or one milligram of something per kilogram of soil. Just as per cent means out of a hundred, so parts per million or ppm means out of a million. Usually describes the concentration of something in water or soil. One ppm is equivalent to 1 milli-litre of something per litre of water (ml/l) or 1 milli-gram of something per kilogram soil (mg/kg).