Parts per billion

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

One part per billion equals one microgram per kilogram, or a single drop in a very large swimming pool. Laboratories use the unit for contaminants that matter at extremely low levels, such as mycotoxins and some heavy metals and pesticides. It is one thousand times smaller than a part per million.

On a certificate of analysis, the unit is written next to each result and its action limit, and the two must be in the same unit before they are compared. A measured value of a few parts per billion against a limit of twenty is a pass; the same figure in parts per million would be a thousandfold difference. Mixing the units is the most common reading error on the contaminant sections. The unit describes concentration, not total quantity, so it does not change with container size.