Persistent, bioaccumulative and toxic substances (PBTs) are a class of compounds that have high resistance to degradation from abiotic and biotic factors, high mobility in the environment and high toxicity. Because of these factors PBTs have been observed to have a high order of bioaccumulation and biomagnification, very long retention times in various media, and widespread distribution across the globe. Most PBTs in the environment are either created through industry or are unintentional byproducts.Blais J. 2005. Biogeochemistry of persistent bioaccumulative toxicants: processes affecting the transport of contaminants to remote areas. Canadian Journal of Fisheries and Aquatic Sciences 62: 236-243.
History Persistent organic pollutants (POPs) were the focal point of the Stockholm Convention 2001 due to their persistence, ability to biomagnify and the threat posed to both human health and the environment. The goal of the Stockholm Convention was to determine the classification of POPs, create measures to eliminate production/use of POPs, and establish proper disposal of the compounds in an environmentally friendly manner.Ridding the world of POPs: A guide to the Stockholm Convention on Persistent Organic Pollutants". United Nations Environment Programme. April 2005. Retrieved 2008-06-06. Currently the majority of the global community is actively involved with this program but a few still resist, most notably the US.
Similar to POPs classification, the PBT classification of chemicals was developed in 1997 by the Great Lakes Binational Toxic Strategy (GLBNS). Signed by both the US and Canada, the GLBNS classified PBTs in one of two categories, level I and level II. Level I PBTs are top priority which currently, as of 2005, contained 12 compounds or classes of compounds.USEPA. The Great Lakes Water Quality Agreement U.S Eighth Response to International Joint Commission. Retrieved June 6, 2012
Level I PBTs (GLBNS) Mercury Polychlorinated biphenyls (PCBs) Dioxins/furans Benzo(a)pyrene (BaP) Hexachlorobenzene (HCB) Alkyl-lead Pesticides Mirex Dieldrin/aldrin Chlordane Toxaphene Octachlorostyrene
The GLBNS is administered by the U.S Environmental Protection Agency (USEPA) and Environment Canada. Following the GLBNS, the Multimedia Strategy for Priority Persistent, Bioaccumulative and Toxic Pollutants (PBT Strategy) was drafted by the USEPA. The PBT Strategy led to the implementation of PBT criteria in several regulational policies. Two main policies that were changed by the PBT strategy were the Toxics Release Inventory (TRI), which required more rigid chemical reporting, and the New Chemical Program (NCP) under the Toxics Substances Control Act (TSCA), which required screening for PBTs and PBT properties.
Compounds
General PBTs are a unique classification of chemicals that have and will continue to impact human health and the environment worldwide. The three main attributes of a PBT (persistence, bioaccumulative and toxic) each have a huge role in the risk posed by these compounds.
Persistence PBTs may have a high environmental mobility relative to other contaminants mainly due to their resistance to degradation (persistence). This allows PBTs to travel far and wide in both the atmosphere and in aqueous environments. The low degradation rates of PBTs allow these chemicals to be exposed to both biotic and abiotic factors while maintaining a relatively stable concentration. Another factor that makes PBTs especially dangerous are the degradation products which are often relatively as toxic as the parent compound. These factors have resulted in global contamination, most notably in remote areas such as the arctic and high elevation areas, which are far from any source of PBTs.
Bioaccumulation and biomagnification The bioaccumulative ability of PBTs follows suit with the persistence attribute by the high resistance to degradation by biotic factors, especially with in organisms. Bioaccumulation is the result of a toxic substance being taken up at a higher rate than being removed from an organism. For PBTs this is caused mainly by a resistance to degradation, biotic and abiotic. PBTs usually are highly insoluble in water which allows them to enter organisms at faster rates through fats and other nonpolar regions on an organism. Bioaccumulation of a toxicant can lead to biomagnification through a trophic web which has resulted in massive concern in areas with especially low trophic diversity. Biomagnification results in higher trophic organisms accumulating more PBTs than those of lower trophic levels through consumption of the PBT contaminated lower trophic organisms.
Toxicity The toxicity of this class of compounds is high, with very low concentrations of a PBT required to enact an effect on an organism compared to most other contaminants. This high toxicity along with the persistence allows for the PBT to have detrimental effects in remote areas around the globe where there is not a local source of PBTs. The bioaccumulation and magnification along with the high toxicity and persistence has the ability to destroy and/or irreparably damage trophic systems, especially the higher trophic levels, globally. For this reason, PBTs have become an area of focus in global politics.
Specific toxicants
PCBs Historically, PCBs were used extensively for industrial purposes such as coolants, insulating fluids, and as a plasticizer. These contaminants enter the environment through both use and disposal. Due to extensive concern from the public, legal, and scientific sectors indicating that PCBs are likely carcinogens and potential to adversely impact the environment, these compounds were banned in 1979 in the United States.USEPA. Basic Information on PCBs. Accessed June 1, 2012 The ban included the use of PCBs in uncontained sources, such as adhesives, fire retardant fabric treatments, and plasticizers in paints and cements. Containers that are completely enclosed, such as transformers and capacitors are exempt from the ban.
The inclusion of PCBs as a PBT may be attributed to their low water solubility, high stability, and semi-volatility facilitating their long range transport and accumulation in organisms.Ritter L; Solomon KR, Forget J, Stemeroff M, O'Leary C.. "Persistent organic pollutants". United Nations Environment Programme. Retrieved 2007-09-16. The persistence of these compounds is due to the high resistance to oxidation, reduction, addition, elimination and electrophilic substitution.Amy Boate, Greg Deleersnyder, Jill Howarth, Anita Mirabelli, and Leanne Peck (2004). "Chemistry of PCBs". Retrieved 2007-11-07. The toxicological interactions of PCBs are affected by the number and position of the chlorine atoms, without ortho substitution are referred as coplanar and all others as non-coplanar. Non-coplanar PCBs may cause neurotoxicity by interfering with intracellular signal transduction dependent on calcium.Simon T, Britt JK, James RC (2007). "Development of a neurotoxic equivalence scheme of relative potency for assessing the risk of PCB mixtures". Regulatory Toxicology and Pharmacology 48(2): 148–70. DOI:10.1016/j.yrtph.2007.03.005. Ortho-PCBs may alter hormone regulation through disruption of the thyroid hormone transport by binding to transthyretin.Chauhan KR, Kodavanti PR, McKinney JD (2000). "Assessing the role of ortho-substitution on polychlorinated biphenyl binding to transthyretin, a thyroxine transport protein". Toxicol. Appl. Pharmacol.162 (1): 10–21. DOI:10.1006/taap.1999.8826. Coplanar PCBs are similar to dioxins and furans, both bind to the aryl hydrocarbon receptor (AhR) in organisms and may exert dioxin-like effects, in addition to the effects shared with non-coplanar PCBs.Safe, S. and Hutzinger, O. (1984). "Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs): biochemistry, toxicology, and mechanism of action". Crit. Rev. Toxicol. 13 (4): 319–95.Safe S, Bandiera S, Sawyer T, Robertson L, Safe L, Parkinson A, Thomas PE, Ryan DE, Reik LM, Levin W. (1985). "PCBs: structure-function relationships and mechanism of action". Environ. Health Perspect. (38) 60: 47–56. The AhR is a transcription factor, therefore, abnormal activation may disrupt cellular function by altering gene transcription.
Effects of PBTs may include increase in disease, lesions in benthic feeders, spawning loss, change in age-structured populations of fish, and tissue contamination in fish and shellfish.Lehmann DW, Levine JF, Law JM. 2007. Polychlorinated Biphenyl Exposure Causes Gonadal Atrophy and Oxidative Stress in Corbicula fluminea Clams. Toxicol Pathol. 35:356.Debruyn AMH, Meloche LM, Lowe CJ. 2009. Patterns of Bioaccumulation of Polybrominated Diphenyl Ether and Polychlorinated Biphenyl Congeners in Marine Mussels. Environ. Sci. Technol. 43:3700–3704. Humans and other organisms, which consume shellfish and/or fish contaminated with persistent bioaccumulative pollutants, have the potential to bioaccumulate these chemicals. This may put these organisms at risk of mutagenic, teratogenic, and/or carcinogenic effects. Correlations have been found between elevated exposure to PCB mixtures and alterations in liver enzymes, hepatomegaly, and dermatological effects such as rashes have been reported.
DDT One PBT of concern is DDT (dichlorodiphenyltrichloroethane), an organochlorine that was widely used as an insecticide during World War II to protect soldiers from malaria carried by mosquitoes. Due to the low cost and low toxicity to mammals, the widespread use of DDT for agricultural and commercial motives started around 1940. However, the overuse of DDT lead to insect tolerance to the chemical. It was also discovered that DDT had a high toxicity to fish. DDT was banned in the US by 1973 because of building evidence that DDT's stable structure, high fat solubility, and low rate of metabolism caused it to bioaccumulate in animals.Harrison, Karl. 1997. DDT a Banned Insecticide. Molecules of the Month. http://www.chem.ox.ac.uk/mom/ddt/ddt.html While DDT is banned in the US, other countries such as China and Turkey still produce and use it quite regularly through Dicofol, an insecticide that has DDT as an impurity.Turgut, C., Cengiz, G., Cutright, T. 2009. Contents and sources of DDT impurities in dicofol formulations in Turkey. Environmental science and pollution research international. Volume 16. issue 2. page 214 This continued use in other parts of the world is still a global problem due to the mobility and persistence of DDT.