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 human health scores indicate how a chemical compares
with others in terms of its capacity to harm human health. The graphic shows where a chemical's human health 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 5% 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 human exposures to toxic chemicals, but relies on more readily available surrogate measures of exposure potential.
The WMPT scoring system assigns twice as much importance to a chemical's
exposure potential as it does to its inherent human 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 human risk screening
HUMAN HEALTH RISK SCREENING SCORE (WMPT) FOR DIAZINON
Human Health Risk Screening Score (WMPT) = 6
TECHNICAL DETAILS ON HOW SCORES ARE DERIVED
The WMPT ranking system, although only a screening-level
method, follows the same conceptual framework commonly used
in standard risk assessment. Standard human health risk
assessment methodology quantifies human health risk
potential as a function of both human toxicity and human
Human Health Risk Potential = (Human Toxicity) * (Human
However, because the data used in the scoring calculations
often varies by several orders of magnitude, the scoring
scales are roughly approximated by a log scale, turning the
multiplicative relationship into an additive relationship:
Human Health Risk Potential = (Human Toxicity) + (Human
The human toxicity factor evaluates adverse effects to human health from chronic exposures, and is calculated from subscores for cancer and noncancer effects. The WMPT ranking system assigns toxicity scores of 1 - 3 based on quantitative measures of a chemical's human toxicity and on qualitative considerations. For cancer effects, scores are assigned by comparing cancer potency factors to "fencelines" that divide the range of potency measures into high, medium and low toxicity groups, and include consideration of the weight of evidence indicating a chemical could cause cancer in humans. For noncancer effects, scores are assigned by comparing available risk assessment values (or experimental NOAEL/LOAELs) to "fencelines" that divide the range of potency measures into high, medium and low toxicity groups, and take into account data quality. In cases where the chemical has both cancer and noncancer effects, the highest of the two scores is used as the human toxicity
Ideally, the human 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 human populations. 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 are measured or estimated bioaccumulation and bioconcentration factors.
This reflects a significant change from the WMPT Beta Test Version 1.0, in which bioaccumulation potential scores for most organic chemicals were based on estimated values of the n-octanol-water partition coefficient (Kow). 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 human toxicity factor is added to the human
exposure potential factor to calculate the chemical's overall
human health risk potential. In sum:
Human Health Risk Potential (3-9) = Human Toxicity (1-3) +
Human 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.