U.S. EPA has developed a system that ranks chemicals based on their
chronic toxicity to human health as part of its Risk-Screening Environmental Indicators (RSEI) project, which is used to evaluate the potential health impacts of chemical releases reported to the Toxics Release Inventory (TRI). The RSEI system assigns a toxicity weight to a chemical depending on where its risk assessment value falls relative to other chemicals with risk assessment values for cancer and noncancer health effects. Separate toxicity weights are assigned for ingestion and inhalation exposures.
WHAT DO THE SCORES MEAN?
EPA's toxicity weights indicate how a chemical compares with others in terms of its capacity to cause chronic human health effects (either cancer or noncancer effects). The graphic shows where a chemical's toxicity weight falls relative to all chemicals that have been ranked using the RSEI 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).
Scorecard places all chemicals scored by a system into "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
Chronic exposure human health toxicity weights are based only on toxicity considerations, and have important limitations because they do not consider variations in exposure potential across chemicals. (The complete RSEI system considers exposure by generating site-specific dose estimates for facilities that report to TRI. Scorecard uses this facility-level exposure data to conduct screening-level risk assessments in its reports on chemical releases from manufacturing facilities). EPA's chronic exposure human health toxicity weights also do not take acute toxicity into account.
EPA's RSEI system scores chemical health hazards by route of exposure. The higher of a chemical's cancer and noncancer toxicity weights is selected as the final score for a specific exposure route. The RSEI system does not sort carcinogens by their relative potency in causing cancer, nor does it sort non-carcinogens by their relative ability to cause other adverse health effects. RSEI scores integrate cancer and noncancer health risk concerns to yield separate scores for ingestion and inhalation exposures (although the model can be run to produce cancer risk indicators only).
INHALATION TOXICITY WEIGHT (RSEI) FOR CARBOXIN
Inhalation Toxicity Weight (RSEI) = 5
TECHNICAL DETAILS ON HOW SCORES ARE DERIVED
The RSEI system assigns chronic exposure human health toxicity weights based on a chemical's EPA risk assessment value. A chemical's cancer and noncancer risk assessment values are compared to values for other TRI chemicals and assigned an endpoint-specific weighting factor between 1 to 1,000,000 depending on where the substance falls on the potency spectrum (adjustments for weight of evidence concerns regarding hazard identification are also sometimes made). The highest endpoint-specific weight determines that chemical's chronic exposure human health toxicity weight. Separate toxicity weights are assigned to ingestion and inhalation risk assessment values.
The RSEI system is based on the conventional scientific practice of characterizing the relative toxicity of chemicals by comparing
the type and extent of adverse effects associated with a unit dose (such
as one milligram of a chemical per kilogram of bodyweight). A
chemical is inherently a greater hazard to human health if it causes a
higher incidence of adverse effects or more serious adverse effects per
unit dose. Regulatory agencies have developed separate measures of
chemical toxicity for carcinogens (called potency estimates, or slope
factors) and noncarcinogens (called reference doses). Cancer potency
estimates and noncancer reference doses are specific to one route of
exposure (e.g., inhalation reference concentrations for air exposures
and ingestion reference doses for oral exposures to water or food).
Scorecard reports the chronic exposure human health toxicity weights assigned to chemicals by the U.S. Environmental Protection Agency.
RSEI: US EPA. Office of Pollution Prevention and Toxics. Risk-Screening Environmental Indicators Model: Version 2.0 (1988-2000 TRI reporting data).. January 2001.
Methodology described in Chapter 1: Introduction to EPAs Risk-Screening Environmental Indicators of the RSEI User's Manual. Values from Technical Appendix A - Available Toxicity Data for TRI Chemicals. (Appendix A last updated February 2002).