RSEI Toxicity Weights
Watch a video explaining the basics of the toxicity weights used by EPA's Risk-Screening Environmental Indicators (RSEI) model.
What are RSEI Toxicity Weights?
RSEI uses relative toxicity weights that describe each chemical’s toxicity relative to other TRI-reported chemicals. The RSEI method for assigning toxicity weights is clear, transparent, and reproducible, based upon easily accessible and publicly available information, and uses expert EPA-wide judgments to the greatest extent possible.
RSEI toxicity weights are based solely on human health effects associated with long-term exposure to chemicals. Short-term exposure and ecological effects are not considered in RSEI. Each RSEI toxicity weight is based on the single most sensitive effect for the given exposure route (oral or inhalation)—that is, the effect that happens at the lowest dose. RSEI toxicity weights are not adjusted for severity of the effect, or for multiple effects.
RSEI considers both cancer and noncancer (such as reproductive, developmental, and respiratory) health effects. For some kinds of RSEI results (RSEI Hazard, toxicity-weighted concentration, and RSEI Score), the higher of the cancer/noncancer toxicity weight is used, so some results may be based on a combination of cancer and noncancer effects. Separated results for cancer and for noncancer effects are also produced for hazard-based and risk-related results. (See Understanding RSEI Results for more information.)
RSEI toxicity weights range from 0.02 to 1,400,000,000. Toxicity weights and RSEI results are available for over 400 of the more than 800 chemicals and chemical categories on the TRI list. TRI chemicals and chemical categories that have assigned toxicity data in RSEI account for 99 percent of the reported pounds for the kinds of releases and transfers that RSEI models.
Toxicity for Chemical Categories
The TRI-reportable list includes both individual chemicals as well as chemical categories, such as certain glycol ethers, diisocyanates, and metal compounds (e.g., lead, chromium, and nickel compounds). For most TRI-listed chemical categories, subject reporting facilities are not required to disclose the specific category member’s identity in their TRI reporting forms. For most chemical categories, RSEI assumes the chemical released is the most toxic form of the chemical. Exceptions to this rule include:
- Chromium and chromium compounds: RSEI uses data from the National Emissions Inventory to estimate the fraction of chromium compounds released as hexavalent chromium (the most toxic form). The remainder is assumed to be trivalent chromium, which has a low toxicity, and is not further modeled in RSEI.
- Polycyclic aromatic compounds (PACs): the toxicity is assumed to be 18 percent of the toxicity for benzo[a]pyrene, the group’s most toxic member.
- Mercury and mercury compounds: toxicity for the oral pathway is based on methyl mercury, and toxicity for the inhalation pathway is based on elemental mercury.
This is an important set of assumptions, because some metal compounds and other chemical categories are significant drivers of RSEI modeled results. RSEI uses worst-case assumptions to help identify potential situations of concern based on the data available in TRI; the actual toxicity or potential health-related impacts associated with any given release may be lower.
Toxicity Data Sources
Where possible, RSEI uses toxicity data from EPA’s Integrated Risk Information System (IRIS). For chemicals with incomplete information in IRIS, RSEI also uses information from other EPA and other governmental sources.