Parts per million

Parts per million is a concentration unit expressing how many units of a substance are present in one million units of the whole sample.

One part per million equals one milligram per kilogram, and for a liquid roughly one milligram per liter. It is the usual unit for residual solvents, most heavy metals and many pesticide limits, where the levels of concern are small but measurable. A percentage can be converted directly: one percent is ten thousand parts per million.

On a certificate of analysis, the unit sits beside each contaminant result and its action limit, and the same unit is sometimes used for minor cannabinoids in an extract. A reader compares the measured value with the limit in the same unit; a result of two parts per million against a limit of five is within limits. Cannabinoid potency in finished products is more often given in milligrams per unit or as a percentage, which is the same information expressed on a different scale.