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Parts per million (ppm) is a commonly used unit of concentration for small values. One part per million is one part of solute per one million parts solvent or 10-6. Parts per million and other "parts per" notations (e.g., parts per billion or parts per trillion) are dimensionless quantities with no units. Preferred methods for expressing parts per million include µV/V (microvolume per volume), µL/L (microliters per liter), mg/kg (milligram per kilogram), µmol/mol (micromole per mole), and µm/m (micrometer per meter).
The "parts per" notation is used to describe dilute solutions in chemistry and engineering, but its meaning is ambiguous and it is not part of the SI system of measurement. The reason the system is ambiguous is because the concentration depends on the original unit fraction that is used. For example, comparing one milliliter of a sample to a million milliliters is different from comparing one mole to a million moles or one gram to one million grams.
- Milton R. Beychok (2005). "Air Dispersion Modeling Conversions and Formulas". Fundamentals of Stack Gas Dispersion (4th ed.). Milton R. Beychok. ISBN 0964458802.
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