How many persistent organic pollutants should we expect?

Under the Stockholm Convention on Persistent Organic Pollutants (POPs), currently 22 chemicals or groups of chemicals are regulated as POPs. However, various screening exercises performed on large sets of chemicals indicate that the number of substances fulfilling the screening criteria defined in Annex D of the Stockholm Convention might be much higher. Most of these screening studies searched for highly persistent and bioaccumulative chemicals, but did not include the long-range transport potential, which is a key criterion under the Stockholm Convention. We apply the screening criteria for persistence, bioaccumulation and long-range transport potential of the Stockholm Convention to a set of 93 144 organic chemicals. Because no toxicity threshold is defined under the Stockholm Convention, we use the toxicity threshold of REACH, the chemicals regulation of the European Union. For the vast majority of the chemicals, the property data required for the assessment had to be estimated from the chemical structure. Assessment results for the acknowledged POPs and for POP candidates currently under review are discussed. Beyond these well-known substances, we find 510 chemicals that exceed all four critieria and can be considered potential POPs. Ninety eight percent of these chemicals are halogenated; frequent types of chemicals are halogenated aromatic compounds, including polychlorinated diphenylethers, tetrachloro benzyltoluenes, brominated and fluorinated naphthalenes and biphenyls; and highly or fully chlorinated and fluorinated alkanes (cyclic, linear, branched). Non-halogenated substances are highly branched alkanes and nitroaromatic compounds. Ten substances are high-production volume chemicals and 249 are pre-registered in the EU. We used uncertainty ranges of the chemical property data to estimate a lower and upper bound of the number of potential POPs; these bounds are at 190 and 1 200 chemicals. These results imply that several tens of potential POPs may have to be expected for future evaluation under the Stockholm Convention. (C) Author(s) 2012. This work is distributed under the Creative Commons Attribution 3.0 License.