**Parts per million** (**ppm**) is a measure of concentration that is used where low levels of concentration are significant. The ppm value is equivalent to the absolute fractional amount multiplied by one million (10^{6}). It is a term with several variants in meaning, so the meaning should be made clear if this term is used.

**Parts per million** is related to percent (parts per hundred) as follows:

- 1% = 10,000 ppm or 1/100 = 10,000/1,000,000.

## Senses of parts per million

The metric system is the most convenient way to express this since metric units go by steps of ten, hundred and thousand. For example, a milligram is a thousandth of a gram and a gram is a thousandth of a kilogram. Thus, a milligram is a thousandth of a thousandth, or a millionth of a kilogram. A milligram is one part per million of a kilogram thus, one part per million (ppm) by mass is the same as one milligram per kilogram. Just as part per million is abbreviated as ppm, a milligram per kilogram has its own symbolic form -- mg/kg, which unlike ppm is unambiguous.

- By mass:
- By volume: (ppmv)
- one millilitre (or cubic centimetre) in a cubic metre (or kilolitre) is 1 ppm by volume. For most gases (those behaving much like an ideal gas) this is numerically equivalent to µmol/mol on the basis of molecules (not atoms). See Avogadro's law.

- By mass/volume ratio for dilute aqueous solutions (ppm
*w*/*v* or ppm *m*/*v*):
- 1 litre (L) of water has mass of approximately 1 kg
^{1}, so 1 mg/L is, loosely speaking, 1 ppm, for small concentrations in a water solution^{2}.

- By number of particles or moles:
- one micromole per mole can also be called 1 ppm.

## Use

Examples of situations where parts per million are an appropriate measure include:

## Inexact analogues

- one square centimeter in 1000 square feet is about .95 ppm
- one two-parent, two-child family in a city of about 4 million people is roughly 1 ppm
- one CD in the 1.57-million disc
^{3} FreeDB catalogue is nearly 2/3 ppm

## Related units

## NIST caution

According to the U.S. National Institute of Standards and Technology (NIST) *Guide for the Use of the International System of Units (SI),* "the language-dependent terms part per million, part per billion, and part per trillion ... are not acceptable for use with the SI to express the values of quantities." NIST's *Guide for the Use of the International System of Units (SI)* has examples of alternative expressions.

## Notes

- Exactly one kg of pure water at maximum density (~4°C) and standard pressure was the definition of a litre from 1901 to 1964; today the litre is defined as exactly 1 dm³, the distinction being whether it is calibrated to the international standard kilogram or the international standard meter, which are defined without reference to one another.
- Properly speaking it is approximately 1 ppm
*by mass* or *by weight* in solution. When solids dissolve, they can increase or decrease the total volume they occupy, and even increase or decrease the total volume of the solution. Adding 1 ppm by weight will rarely produce a solution that is 1 ppm by volume to the same precision. The notation *ppm w/v* or *ppm m/v* demonstrates the exact nature of the ratio and is therefore the most precise.
- As of January 2005.

Last updated: 06-02-2005 04:37:05

Last updated: 08-18-2005 10:38:38