Units commonly used to express contamination ratios, as in establishing the maximum permissible amount of a contaminant in water, land, or air.
PARTS PER BILLION (PPB)/PARTS PER MILLION (PPM)
Meaning of PARTS PER BILLION (PPB)/PARTS PER MILLION (PPM) in English
Environmental engineering English vocabulary. Английский словарь экологического инжиниринга. 2012