Definitions for "Parts per billion"
Number of parts of a chemical found in 1 billion parts of a particular gas, liquid, or solid.
Ratio of amounts expressed as parts pesticide per 10 sample. Strictly the quantities should be the same i.e. weight to weight (solids) or volume to volume (liquids or gases) e.g. 1ppb = 1æ g/kg. A common usage is for weight to volume but to avoid confusion it is recommended that the SI units are used rather than ppb; e.g. æ g/L (Mills et al., 1993)
One pound per billion pounds of water.