CHEMICAL PROFILES|Toxicity and Persistence Scores

Several human health and ecological ranking systems are based on indicators of toxicity and environmental persistence. By including persistence, these systems provide a better measure of the potential for a chemical's toxicity to become manifest. If a chemical has a long half-life in the environment, target organisms are more likely to be exposed to it and accumulate a dose that is hazardous. If a chemical has a very short half-life, there will generally be fewer exposures that could result in significant environmental health risks. Persistence is a more important criterion for assessing risks of long-term exposures than short-term exposures.

Environmental persistence is not always an accurate indicator of exposure potential. Chemicals may have extremely long half-lives, but be sequestered in environmental compartments (like the seafloor) where there is little likelihood of human or other target organism exposure. Persistence and fate measures can generally provide reliable indicators of the potential concentration of a contaminant in different environmental media, but they do not capture the frequency or extent of an organism's contact with these media, so they may not provide reliable indicators of the dose of toxic chemicals that organisms can receive.

A chemical's environmental persistence score can be based on several widely available indicators of half-life in different environmental compartments, and may also include other indicators of exposure potential. The WMPT Human Health Risk Screening Score)and the WMPT Ecological Risk Screening Scores consider both persistence and bioaccumulation. The UTN Total Hazard Value Score is based on indicators that determine fate in aquatic environments.

Ranking systems that take into account toxicity and persistence provide a better indicator of the potential environmental hazards of a chemical than toxicity alone. While they may not completely capture differences in exposure potential across chemicals, these ranking systems do penalize chemicals that resist degradation or bioaccumulate through the food chain. Such chemicals are likely to present long term chemical management problems (and unwanted environmental surprises), so they should be priorities for use reduction or elimination. While better indicators of exposure potential would improve priority-setting even more, such indicators require significant amounts of data and environmental modeling and are currently unavailable for most chemicals in widespread use.