Parts Per Million — PPM
This is a way of expressing very dilute concentrations of substances. Just as per cent means out of a hundred, so parts per million means out of a million. e.g. One ppm is equivalent to 1 milligram of something per liter of water (mg/l). or 1 milligram of something per kilogram solid (mg/kg). One part per million (ppm) denotes one part per 1,000,000 parts, one part in 106.