Parts-per notation

"Parts-per" notation is used in science and engineering, to denote proportionalities in measured quantities such as proportions at the parts-per-million (ppm), parts-per-billion (ppb), and parts-per-trillion (ppt) level. Since parts-per notations are quantity-per-quantity measures, they are known as dimensionless quantities; that is, they are pure numbers with no associated units of measurement.

Parts per million (ppm)

In ordinary prose, 1 ppm is "one part per million parts" and has a numerical value of 1×10^-6 in most (if not all countries).

Parts-per notation is often used in the measure of dilutions (concentrations) in chemistry; for instance, for measuring the relative abundance of dissolved minerals or pollutants in water. The expression 1 ppm means a given property exists at a proportion of one part per million parts examined, as would occur if a water-borne pollutant was present at a concentration of one-millionth of a gram per gram of sample solution.

Similarly, parts-per notation is used also in physics and engineering to express the value of various proportions. For example, a metal might expand 1.2 micrometre per metre of length for every degree Celsius and this would be expressed as a coefficient of thermal expansion of 1.2 ppm/°C. As another example, the accuracy of land-survey distance measurements when using a laser rangefinder might be 1 millimetre per kilometre of distance and this could be expressed as an accuracy of 1 ppm.

Parts per billion (ppb)

In ordinary prose, 1 ppb is "one part per billion parts" and has a value of 1×10^-9 in the United States only. This terminology should be used with great caution because:

• In France and frequently in continental Europe, 1×10⁹ is 1 milliard
• In the United Kingdom and in other nations using British English, 1×10⁹ is 1 thousand million and 1 billion is 1×10¹²

Parts per trillion (ppt)

In ordinary prose, 1 ppt is "one part per trillion parts" and has a value of 1×10^-12 again in the United States only. This terminology should also be used with great caution because:

• In the United Kingdom and other nations using British English, France and continental Europe, 1×10^-12 is 1 billion and 1 trillion is 1×10^-18

• Concentrations are sometimes expressed as ppt meaning parts per thousand which conflicts with ppt meaning parts per trillion.

Differentiation between volume and weight proportionalities

The notation ppmv is often used to designate parts per million parts by volume and ppmw is often used to designate parts per million by weight.

Similarly, ppbv, ppbw, pptv and pptw have the same connotations.

Summary

The parts-per notation is not formally part of the International System of Units (SI).

The International Bureau of Weights and Measures (an international standards organization known also by its French-language initials BIPM) recognizes the use of parts-per notation. However, the BIPM suggests avoiding the use of ppb and ppt to avoid misunderstandings.

Perhaps the best practice is to use the scientific notations 1×10^-6, 1×10^-9 and 1×10^-12 to avoid misunderstandings.