Risk-Screening Environmental Indicators (RSEI) Model

RSEI Toxicity Weights

Watch a short video explaining the basics of the toxicity weights used by EPA's Risk-Screening Environmental Indicators (RSEI) model:

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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 and reproducible, based on easily accessible and publicly available information, and uses expert EPA 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 pathway (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 Score and RSEI Hazard), the higher of the cancer/noncancer toxicity weight is used, so some RSEI Scores and RSEI Hazard results may be based on cancer effects and some on noncancer effects. Cancer and noncancer Hazard and Scores are also reported separately. (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 600 chemicals and chemical categories on the TRI list. Chemicals with toxicity weights account for 99 percent of the reported pounds for the kinds of releases and transfers that RSEI models.

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Toxicity for chemical categories

The list of TRI-covered chemicals includes both individual chemicals as well as chemical compound categories, such as glycol ethers, diisocyanates and metal compounds (e.g., lead, chromium, and nickel compounds). For these categories, TRI reporting forms do not indicate which form of the chemical a facility released. For most chemical categories, RSEI assumes the facility released 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 modeled.
  • 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 Scores. RSEI uses worst-case assumptions to identify potential situations of concern based on the data available in TRI; the actual toxicity 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 government sources.

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