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.
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. 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.
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
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