P.P.M.
In science and engineering, the parts-per notation is a set of pseudo-units to describe the small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction.
Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement. Commonly used are
parts-per-million – ppm, 10−6
parts-per-billion – ppb, 10−9
parts-per-trillion – ppt, 10−12
parts-per-quadrillion – ppq, 10−15
This notation is not part of the International System of Units – SI system and its meaning is ambiguous.
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