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EPA-Expo-Box (A Toolbox for Exposure Assessors)

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Persistent Organic Pollutants (POPs)

Persistent organic pollutants (POPs), which are also referred to as persistent, bioaccumulative, and toxic compounds (PBTs, or sometimes just PB chemicals), are a subcategory of organic compounds that are highly persistent due to their physical and chemical properties (Stockholm Convention Exit Disclaimer).

The Stockholm Convention on Persistent Organic Pollutants (POPs) is a global treaty to eliminate or reduce the release of POPs into the environment. In the Stockholm Convention, POPs are defined as organic compounds that become widely distributed throughout the environment and remain intact for long periods of time, accumulate in the fatty tissue of living organisms and are found at higher concentrations at higher trophic levels, and are toxic to humans and wildlife. There are currently 22 POPs listed in the Stockholm Convention, including those included in the initial convention (2001) or new ones added during subsequent meetings (2009, 2011) (Stockholm Convention Exit Disclaimer).

Persistent Organic Pollutants (POPs) listed in the Stockholm Convention
Substance Category
Aldrin Pesticide
Chlordane Pesticide
DDT Pesticide
Dieldrin Pesticide
Endrin Pesticide
Heptachlor Pesticide
Hexachlorobenzene Pesticide, Industrial Chemical, Byproduct
Mirex Pesticide
Toxaphene Pesticide
Polychlorinated biphenyls (PCBs) Industrial Chemical, Byproduct
Polychlorinated dibenzo-p-dioxins (PCDD) Byproduct
Polychlorinated dibenzofurans (PCDF) Byproduct
Alpha hexachlorocyclohexane Pesticide, Byproduct
Beta hexachlorocyclohexane Pesticide, Byproduct
Chlordecone Pesticide
Hexabromobiphenyl Industrial Chemical
Hexabromodiphenyl ether and heptabromodiphenyl ether (commercial octabromodiphenyl ether) Industrial Chemical
Lindane Pesticide
Pentachlorobenzene Pesticide, Industrial Chemical, Byproduct
Perfluorooctane sulfonic acid, its salts and perfluorooctane sulfonyl fluoride Industrial Chemical
Technical endosulfan and its related isomers  Pesticide
Tetrabromodiphenyl ether and pentabromodiphenyl ether (commercial pentabromodiphenyl ether) Industrial Chemical

Source: Stockholm Convention Exit Disclaimer

Both organic and inorganic compounds can be persistent, bioaccumulative, and toxic (or any combination of the three). U.S. EPA (1999) defines these terms as follows:

  • Persistent compounds have a transformation half-life of > 2 months
  • Bioaccumulative compounds have a fish bioaccumulation factor or bioconcentration factor of ≥ 1,000
  • Toxic compounds are identified using accepted toxicity studies

A substance’s persistence is of interest when assessing exposures:

  • Some persistent compounds have the potential for long-range transport from sources. For example, elemental mercury emitted to the atmosphere as a vapor can remain in the atmosphere for over a year. This period of time allows the mercury to be transported great distances from the point of emissions by weather patterns before it is deposited to soil, lakes, or other surfaces.
  • Some persistent compounds also have the potential to pose exposures many years after they are released to the environment. For example, PCBs are degraded very slowly in the environment; as a result, a spill of PCBs (such as from an electric transformer) can result in elevated levels of PCBs in the soil at that location for years (or even decades) to come.

A substance’s ability to bioaccumulate is also relevant to exposure assessment, especially in combination with longer persistence:

  • When a substance that bioaccumulates is also persistent in an organism (and therefore is not metabolized or degraded rapidly), concentrations of that substance can increase over time and may result in greater potential for exposure.
  • Substances that bioaccumulate can also result in exposure through the food chain. For example, dioxins and methyl mercury in an aquatic ecosystem can be absorbed by algae, which are then consumed by small fish, which subsequently can be consumed by larger fish.
  • If the levels of a persistent and bioaccumulative substance increase with successive levels of a food chain, the process is sometimes referred to as biomagnification. In this case, organisms at the top of a food chain will experience exposures that are greater than if they were only exposed directly to the chemical in the environment.
  • Persistent and bioaccumulative substances may be of concern to humans if people consume contaminated organisms from the top of the food chain, such as large, piscivorous fish.

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