CHEMICAL PROFILES|Human Health Hazards

CAS Number: 96-18-4
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.

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).

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.


Ecological Risk Screening Score (WMPT) = 7

The WMPT ranking system is designed to provide a screening-level assessment of chemical hazards, based on the conceptual framework of more data-intensive risk assessments. Standard ecological risk assessment quantifies ecological risk potential as a function of both ecological toxicity and ecological exposure:

Ecological Risk Potential = (Toxicity) x (Exposure)

Because the data used in scoring calculations often varies by several orders of magnitude, the WMPT system uses a log scale, turning the multiplicative relationship into an additive relationship:

Ecological Risk Screening Score = (Toxicity Score) + (Exposure Score)

Ideally, the ecological toxicity factor in these equations would be an integrated measure of all adverse ecological health effects associated with chronic environmental exposures to a chemical. However, to provide screening-level assessments of many data-poor chemicals, WMPT is forced to rely on more generally available, surrogate measures of ecological toxicity. The WMPT ranking system uses aquatic toxicity as a proxy indicator of general ecological toxicity, which includes both aquatic and terrestrial ecosystems.

The WMPT ranking system assigns toxicity scores of 1 - 3 based on available quantitative measures of a chemical's aquatic potency. Chronic measures of aquatic toxicity are preferred over acute measures, which are only utilized to fill data gaps. Measures of aquatic potency utilized by WMPT are ordered by data quality and include sediment quality final chronic values, ambient water quality criteria, aquatic toxicity reportable quantities and predicted acute toxicity values. Scores are assigned based on "fencelines" that divide the range of each of these measures into high, medium and low toxicity groups.

Ideally, the ecological exposure factor would be based on monitored or estimated environmental concentrations associated with various chemical use or release scenarios, as well as data on the resulting exposures experienced by various target organisms. Since such information is rarely available, WMPT focuses instead on exposure potential, as indicated by two more widely available surrogate measures: persistence and bioaccumulation potential.

The WMPT ranking system assigns exposure potential scores of 2-6 by adding sub-scores given to surrogate measures of a chemical's ability to persist and bioaccumulate in aquatic environments. Measures of environmental persistence utilized by WMPT are ordered by data quality and include estimated biodegradation time in water and soil and estimated hydrolysis half-life. Measures of bioaccumulation utilized by WMPT are ordered by data quality and include n-octanol-water partition coefficients, bioaccumulation factors and bioconcentration factors. Scores are assigned based on "fencelines" that divide the range of each of these measures into high, medium and low exposure potential groups.

Finally, the ecological toxicity factor is added to the ecological exposure potential factor to create the chemical's overall ecological risk potential. In sum:

Ecological Risk Potential (3-9) = Ecological Toxicity (1-3) + Ecological Exposure Potential (2-6)

WMPT: US EPA, Office of Solid Waste and Office of Pollution Prevention and Toxics. Waste Minimization Prioritization Tool: Background Document For The Tier III PBT Chemical List. Appendix A: WMPT Summary Spreadsheet. EPA, Washington, DC. July, 2000.