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Parts Per Billion (ppb)

What Does Parts Per Billion (ppb) Mean?

Parts per billion (ppb) is a term that expresses the number of units (parts) of a given substance that exist as a portion of a greater substance comprised of one billion parts. “Parts-per” notations such as ppb are referred to as “volume in volume” measurements.

One part per billion equates to one particle existing within a substance comprised of 999,999,999 other particles. This measurement can be expressed as 1ppb, which is equal to 1 x 10−9 or 0.000 0001% of the total substance.

Many occupational exposure limits (OELs) provided by the Occupational Safety and Health Administration (OSHA), the National Institute of Occupational Safety and Health (NIOSH), and the American Conference of Governmental Industrial Hygienists (ACGIH) are expressed in parts per billion or parts per million.

Safeopedia Explains Parts Per Billion (ppb)

The occupational exposure limits for fluids (gases and liquids) are commonly expressed using ppb notation. In contrast, OELs for aerosols are expressed as weight in volume measurements such as milligrams per cubic meter (mg/m3). Industrial substances with OELs that are phrased in ppb notation include Methylene diphenyl diisocyanate (MDI) and 2,4-Toluene diisocyanate (TDI). The ACGIH time-weighted exposure limit (TLV-TWA) for both chemicals is 5 ppb.

There are no specific scientific units associated with ppb or any other parts-per measurement. Parts-per notation provides a pure ratio of the amount of one quantity of substance to another quantity of substance. As such, parts per billion ratios can be directly compared to ratios given in any specific unit of measurement. For instance, one part per billion is roughly equivalent to one second per thirty two years. Additionally, because parts-per notation is phrased in powers of ten, anything expressed in parts per billion can also be expressed in terms of parts per million or any other parts-per notation.

Although ppb is widely used to express OELs, its use may sometimes be discouraged in order to avoid language-based misunderstandings. This is because the word “billion” has two potential meanings (either a million millions or a thousand millions) which vary by country. The U.S. National Institute of Standards and Technology (NIST) takes the position that use of parts per billion and all other parts-per notation is not acceptable for expressing the quantity of a substance.


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