P.P.M.
About P.P.M.
In science and engineering parts-per notation is 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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