ppb - part per billion - a unit for measuring the concentration of a gas or vapour in air -- parts (by volume) of the gas or vapour in a billion parts of air. Usually used to express measurements of extremely low concentrations of unusually toxic gases or vapours.
Näiteks merevees, milles Hg foon on alla 0,1 ppb (ppb="parts per billion" - miljardik osa) - s. o. looduslik, elutsevate kalade koes võib kontsentratsioon ületada 0,5 ppm (ppm - miljondik osa).
