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, 1×10-9 is 1 milliard
- In the United Kingdom and in other nations using British English, 1×10-9 is 1 thousand million and 1 billion is 1×10-12
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.
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.[1] 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.