Parts Per Billion (ppb)/Parts per Million (ppm):

Published February 2000

Units commonly used to express contamination ratios, as in establishing the maximum permissible amount of a contaminant in water or soil. In soil: ppm = milligrams of contaminant to one kilogram of soil, ppb = micrograms of contaminant per kilogram of soil. In water: ppm = for water is milligrams of contaminant per liter of water, ppb = micrograms of contaminant per liter of water.