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.
WHY RANK CHEMICALS BASED ON TOXICITY AND PERSISTENCE?
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.