by Dr. Bruce D. Rodan, Dr. Noelle Eckley and Dr. Robert S. Boethling

Presented by Dr. Bruce D. Rodan

Persistent organic pollutants (POPs) are a small subset of organic chemicals whose characteristics of persistence in the environment, accumulation in biological organisms and toxicity make them priority pollutants and environmental risks to humans and ecosystems. Risks may occur far from the site of initial entry of the POP into the environment, and include effects in remote, polar and oceanic regions of the planet. A number of international negotiations have been instituted in response to concerns about the transborder risks posed by POPs and the inability of affected nations to regulate far-off emission sources. Most recent is the 1997 decision by the Governing Council of the United Nations Environment Programme (UNEP) to begin negotiations in 1998 on a binding global agreement to prohibit, restrict or reduce the production, use or release of certain POPs. This decision is the global counterpart to similar, regional, negotiations recently completed on a POPs protocol under the United Nations Economic Commission for Europe (UNECE) Convention on Long-Range Transboundary Air Pollution (LRTAP). In North America, POPs are being addressed under the NAFTA Commission for Environmental Cooperation (CEC) and the bilateral United States/Canada agreement to control the discharge or release of POPs in the Great Lakes Basin.

Of central importance to international negotiations is the clarification of what constitutes a POP warranting international action and how best to screen and select these substances. Along with a list of twelve POPs under initial consideration for global action, emphasis will be placed in the UNEP negotiation on the development of criteria to add substances. It is, thus, timely to investigate the scientific foundation for POPs screening criteria, building on preceding international agreements (Table 1) that highlight the properties of bioaccumulation, persistence, toxicity and long-range transport.

Both theoretical and empirical methods were used to analyze the scientific foundation for POPs screening criteria. The theoretical approach relied on modeling to determine if any parameters could be inferred to identify critical POP screening values. In all cases it was assumed that the chemicals obeyed first-order degradation kinetics and that no other dispersion pathways were operating. The analysis demonstrates that there are no scientific "bright lines" for determining POPs screening or listing criteria. The more persistent, bioaccumulative, and toxic the substance, the worse the potential problem.

Figure 1 graphs the accumulation, over time, of two hypothetical chemicals in either soil or water. One chemical has a half-life of 1 month, the other 12 months. Two modes of application are shown for each chemical. The first mode assumes the single application of one hypothetical unit of chemical at the start of each year. The decline in chemical residue between applications, followed by the spike at the start of each new year, leads to the peak and trough appearance. The second mode assumes continuous application totaling one unit per year. Figure 1 illustrates that a different steady state concentration is approached for every chemical half-life (T1/2). For the continuous application scenario, the accumulated concentration at time T follows the mathematical formula: