Parts per million (ppm) is a measure of concentration that is used where low levels of concentration are significant. The ppm value is equivalent to the absolute fractional amount multiplied by one million (106).
The metric system is the most convenient way to express this since metric units go by steps of ten, hundred and thousand. For example, a milligram is a thousandth of a gram and a gram is a thousandth of a kilogram. Thus, a milligram is a thousandth of a thousandth, or a millionth of a kilogram. A milligram is one part per million of a kilogram thus, one part per million (ppm) is the same as one milligram per kilogram. Just as part per million is abbreviated as ppm, a milligram per kilogram has its own symbolic form -- mg/kg. Using our abbreviations, one ppm equals one mg/kg.