CHEMICAL PROFILES|Ecological Risk Screening Score (WMPT)

U.S. EPA developed the Waste Minimization Prioritization Tool (WMPT) to help guide the national effort to reduce the quantity and toxicity of hazardous wastes. WMPT is a software system that provides relative rankings of more than 1,300 chemicals according to their environmental persistence, bioaccumulation potential, and human and ecological toxicity. The WMPT ranks chemicals separately according to their human health risks, their ecological health risks, and their overall environmental health risk.

WHAT DO THE SCORES MEAN?
WMPT ecological scores indicate how a chemical compares with others in terms of its toxicity to aquatic ecosystems and its exposure potential. The graphic shows where a chemical's ecological score falls relative to all chemicals that have been ranked using the WMPT system, and indicates whether a chemical is more or less toxic than most chemicals. Chemicals that score at the far right end of the scale are significantly more hazardous(in the worst 11% according to this scoring system).

All chemicals scored by a system are placed in "bins" defined by percentiles (e.g., a chemical's score is in the least toxic 25% of chemicals scored by a system). The graphic illustrates which bin a chemical falls in according to each scoring system in Scorecard. Looking across these different systems, it is possible to identify chemicals that consistently score as high or low hazards, as well as chemicals that score high on some measures (such as human health hazards) but low on others (such as ecological hazards).

STRENGTHS AND WEAKNESSES OF THIS TYPE OF SCORING SYSTEM?
WMPT scores are based on measures of toxicity, persistence in the environment and bioaccumulation potential. The system has important limitations because it does not actually estimate environmental exposures, but relies on more readily available surrogate measures of exposure potential. Because of the need to include many data-poor chemicals, the system is also based on a relatively small set of toxicity indicators. Only data on acute and chronic aquatic toxicity are used to assign a general ecological toxicity score. If aquatic organisms are not the most susceptible wildlife affected by a toxicant, WMPT scores may understate the potential risk to ecosystems.

The WMPT scoring system assigns twice as much importance to a chemical's exposure potential as it does to its inherent ecological toxicity. A chemical's rank is driven by the scores assigned to its persistence and bioaccumulation potential, and substantial variations in toxicity potential may not be reflected in a chemical's ecological risk screening score.