This page provides detailed information on the kind of toxicity data used by EPA's Risk-Screening Environmental Indicators (RSEI) model and how the toxicity weights are calculated and selected for use in RSEI results, including RSEI Hazard and RSEI Scores.

On this page:

• Toxicity data sources
• Toxicity calculations
• Selecting the appropriate toxicity weight
• Important caveats regarding toxicity

Additional resources on this site include:

• Section 4 of the RSEI methodology document provides detailed information on how RSEI incorporates toxicity information.
• RSEI Technical Appendix A provides additional information on the toxicity data used.
• The RSEI Toxicity Weighting Spreadsheet v.2.3.9(106 K, December 2020)  provides the latest toxicity data used in the model.

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 uses the following sources (in order of preference):

• EPA’s National Air Toxics Assessment (NATA).
• EPA’s Office of Pesticide Programs (OPP) Acute Chronic and Reference Doses Table lists.
• The Agency for Toxic Substances and Disease Registry (ATSDR) Minimum Risk Levels (MRLs).
• California Environmental Protection Agency (CalEPA) Approved Risk Assessment Health Values.
• EPA's Provisional Peer Reviewed Toxicity Values (PPRTVs).
• EPA’s Health Effects Assessment Tables (HEAST).
• Derived Values. For a prioritized group of chemicals for which sufficient data was not found in the above sources, a group of EPA expert health scientists reviewed other available data to derive appropriate toxicity weights.

For each chemical, RSEI determines the following values, where possible:

• Oral slope factorOral Slope FactorThe Oral Slope Factor represents the upper-bound (approximating a 95 percent confidence limit) estimate of the slope of the dose-response curve in the low-dose region for carcinogens. The units of the slope factor are usually expressed as (mg/kg-day)⁻¹. (OSF) in risk per mg/kg-day.
• Inhalation unit riskInhalation Unit RiskThe upper-bound excess lifetime cancer risk estimated to result from continuous exposure to an agent at a concentration of 1 ug/m³ in air. (IUR) in risk per mg/m³.
• Reference doseReference DoseThe Reference Dose (RfD) is an estimate (with uncertainty spanning perhaps an order of magnitude) of daily exposure [RfD] to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious noncancer effects during a lifetime. (RfD) in mg/kg-day.
• Reference concentrationReference ConcentrationThe Reference Concentration (RfC) is an estimate (with uncertainty spanning perhaps an order of magnitude) of continuous inhalation exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious noncancer effects during a lifetime. (RfC) in mg/m³.

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Toxicity calculations

Each value is transformed into a toxicity weight using the following algorithms:

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Selecting the appropriate toxicity weight

Each chemical is assigned up to four toxicity weights, according to the availability of the RfC, RfD, IUR and OSF. The RSEI results may use different toxicity weights, depending on the data.

RSEI Score and RSEI Hazard use the higher cancer/noncancer toxicity weight for each (oral/inhalation) exposure route. If one route is missing both toxicity weights, the other route’s weight is used. Cancer Score and Cancer Hazard results use only the cancer toxicity weights (IUR for inhalation route or OSF for oral route), and do not use the RfC- or RfD-based weights even if the IUR or OSF is missing. Similarly, the NonCancer Score and NonCancer Hazard results only use the RfC- or RfD-based weights.

In addition, the RSEI Score, Cancer Score and NonCancer Score all use the inhalation route toxicity weight (RfC or IUR as appropriate) for the portion of the transfer to Publicly-Owned Treatment Works (POTWs) that volatilizes during treatment and the oral route toxicity weight (RfD or OSF as appropriate) for the remainder that is released with the POTW effluent. The three Hazard-based results do not account for partitioning, and use the oral route toxicity weight (RfD or OSF as appropriate) for the entire chemical transfer. The table below summarizes the selection of toxicity weights for each kind of RSEI result. More information on understanding RSEI results.

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Important caveats regarding toxicity

• Toxicity weights are not designed to (and may not) correlate with statutory criteria used for listing and delisting chemicals in TRI. RSEI risk-related model results account for estimated exposure and may not correlate with listing/de-listing decisions.
• The RSEI model only addresses chronic human toxicity (cancer and noncancer effects, such as developmental toxicity, reproductive toxicity, neurotoxicity, etc.) associated with long-term exposure and does not address concerns for either acute human toxicity or environmental toxicity.
• Toxicity weights are based upon the single most sensitive chronic human health endpoint for inhalation or oral exposure pathways, and do not reflect severity of effects or multiple health effects.
• Estimated Reference Doses and Reference Concentrations for noncancer effects incorporate uncertainty factors which are reflected in toxicity weights that are based upon these values.
• Toxicity weights for chemicals that are reported to TRI as categories are based on the toxicity for the most toxic member of the group. there are two exceptions to this:
  • The toxicity for polycyclic aromatic compounds (PACs) is assumed to be 18 percent of the toxicity for benzo(a)pyrene, its most toxic member. This is based on speciation information and follows the method used by EPA’s National Air Toxics Assessment (NATA) evaluation for polycyclic organic matter. 
  • For dioxins and dioxin-like compounds, a release-specific toxicity weight is calculated based on the release volume of the individual members of the category, which became required reporting beginning in 1998.
• Several significant assumptions are made regarding metals and metal compounds, because important data regarding these chemicals are not subject to TRI reporting. Metals and metal compounds are assumed to have the same toxicity weight, although the chronic toxicity of some metal compounds may be higher. Metals and metal compounds are assumed to be released in the valence (or oxidation state) associated with the highest chronic toxicity. There are two exceptions to this:
  • For chromium and chromium compounds, it is assumed that facilities may release some combination of hexavalent chromium and trivalent chromium. Industry-level speciation estimates from the 2011 National Emissions Inventory are used to estimate the fraction of each type. As trivalent chromium has a very low toxicity, only the hexavalent fraction is modeled, using a toxicity weight specifically for that valence state; and
  • For 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.
• While the physical form of released metals or metal compounds can affect toxicity, reasonable assumptions are made regarding the likely form of most releases. For instance, the noncancer toxicity weight for chromic acid mists and dissolved hexavalent chromium aerosols is much higher than for hexavalent chromium particulates, but these chemicals are typically released as acid aerosols, so the toxicity weight for cancer based on the inhalation of particulates is used. Analysts need to consider these assumptions, and whether the gathering of additional data is warranted, when examining model results for metals and metal compounds.

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