RSEI Toxicity-Data and Calculations
This page provides detailed information on the 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, toxicity-weighted concentration, 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.2.3.12 (xlsx) provides the latest toxicity data used in the model.
Toxicity data sources
The RSEI model relies on chemical toxicity data derived from EPA and other data sources. Data from the sources listed below are categorized in three-tiered, hierarchical fashion to give preference to EPA and consensus data sources, such as EPA's Integrated Risk Information System (IRIS), where possible when selecting toxicity values. For chemicals with incomplete information in IRIS, RSEI may use other sources in the selection of toxicity values. Toxicity data are gathered separately for chronic health effect endpoints (e.g., cancer and noncancer effects); a chemical’s oral slope factor (OSF) may be selected from IRIS, while its reference dose (RfD) may be selected from EPA's Provisional Peer-Reviewed Toxicity Values (PPRTVs), as an example.
- EPA's IRIS Program.
- EPA’s Air Toxics Screening Assessment (AirToxScreen)/National Air Toxics Assessment (NATA) Program.
- EPA’s Office of Pesticide Programs (OPP) Program.
- The Agency for Toxic Substances and Disease Registry (ATSDR) Minimal Risk Levels (MRLs).
- California Environmental Protection Agency (CalEPA) Approved Risk Assessment Health Values.
- EPA's PPRTVs.
- EPA’s Health Effects Assessment Summary Tables (HEAST).
- Derived toxicity values from other available data sources using EPA methodologies for assessing toxicity.
For each chemical, RSEI determines the following values, where possible:
- Oral 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. in risk per milligram (mg) of chemical per kilogram (kg) of body weight per day (risk per mg/kg-day).
- Inhalation 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 of chemical per cubic meter (m3) (risk per mg/m3).
- Reference 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. in mg/kg-day.
- Reference 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.
Toxicity calculations
Each value is transformed into a toxicity weight using the following algorithms:
|
Inhalation Exposure |
Oral Exposure |
|
|---|---|---|
|
Cancer Effects* |
IUR / 2.8e-7 |
OSF / 1e-6 |
|
Noncancer Effects |
3.5 / RfC |
1 / RfD |
|
*If the Weight of Evidence (WOE) Category is equal to C, each weight is divided by an additional factor of 10 to account for uncertainty. |
||
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, toxicity-weighted concentration, and RSEI Hazard use the higher cancer or noncancer toxicity weight for each exposure route (i.e., oral and inhalation), and if one exposure route is missing both toxicity weights, then the other exposure route’s toxicity weight is used. The RSEI Cancer Hazard and RSEI Cancer Score results use only the cancer toxicity weights (i.e., the IUR for the inhalation exposure route or the OSF for the oral exposure route), and do not use the RfC- or RfD-based toxicity weights even if the IUR or OSF is missing. Similarly, the RSEI Noncancer Hazard and RSEI Noncancer Score results only use the RfC- or RfD-based toxicity weights.
In addition, the toxicity-weighted concentration, RSEI Score, RSEI Cancer Score, and RSEI Noncancer Score all use the inhalation route toxicity weight (RfC or IUR as appropriate) for the portion of the publicly owned treatment works (POTW) transfer quantity that is estimated to be released to air and the oral route toxicity weight (RfD or OSF as appropriate) for the portion of the POTW transfer quantity that is estimated to be released to water. The three RSEI model hazard-based results do not account for POTW partitioning, and use the oral 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.
|
Air Releases |
Water Releases |
POTW Transfers |
Fill in Tox Data Gaps? |
|
|---|---|---|---|---|
|
RSEI Score |
Higher of IUR tox weight or RfC tox weight. |
Higher of OSF tox weight or RfD tox weight. |
For portion of transfer that is estimated to be released to air, use higher of IUR tox weight or RfC tox weight. For portion of transfer that is estimated to be released to water, use higher of OSF tox weight or RfD tox weight. |
Yes. If a chemical has no tox weight in one exposure route, use tox weight from other exposure route. For instance, if a chemical has no IUR or RfC tox weight, use higher of RfD or OSF tox weight for air releases. |
|
RSEI Cancer Score |
IUR tox weight. |
OSF tox weight. |
For air release portion, use IUR tox weight. For water release portion use OSF. |
No. If no route-specific cancer tox weight, then cancer score is zero. |
|
RSEI Noncancer Score |
RfC tox weight. |
RfD tox weight. |
For air release portion, use RfC tox weight. For water release portion, use RfD tox weight. |
No. If no route-specific noncancer tox weight, then noncancer score is zero. |
|
RSEI Hazard |
Higher of IUR tox weight or RfC tox weight. |
Higher of OSF tox weight or RfD tox weight. |
Higher of OSF tox weight or RfD tox weight. |
Yes. If a chemical has no tox weight in one exposure route, use data from other exposure route. |
|
RSEI Cancer Hazard |
IUR tox weight. |
OSF tox weight. |
OSF tox weight. |
No. If no route-specific cancer tox weight, then cancer hazard is zero. |
|
RSEI Noncancer Hazard |
RfC tox weight. |
RfD tox weight. |
RfD tox weight. |
No. If no route-specific noncancer tox weight, then noncancer hazard is zero. |
| Toxicity-weighted concentration | Higher of IUR tox weight or RfC tox weight. | Higher of OSF tox weight or RfD tox weight. | For portion of transfer that is estimated to be released to air, use higher of IUR tox weight or RfC tox weight. For portion of transfer that is estimated to be released to water, use higher of OSF tox weight or RfD tox weight | Yes. If a chemical has no tox weight in one exposure route, use tox weight from other exposure route. For instance, if a chemical has no IUR or RfC tox weight, use higher of RfD or OSF tox weight for air releases. |
Important caveats regarding toxicity
- Toxicity weights are not designed to (and may not) correlate with Emergency Planning and Community Right-to-Know Act (EPCRA) section 313 statutory criteria used for listing and delisting chemicals on the Toxics Release Inventory (TRI) chemical list. 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 adverse effect for the given exposure route and do not consider differences in the type, number, and severity of effects. For example, liver toxicity is weighted in the same way that neurotoxicity is weighted.
- Toxicity weights for TRI chemical categories are based on toxicity data for the most toxic member of the chemical category to represent the toxicity of the entire chemical category, with a few noted exceptions, including:
- 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 dioxin and dioxin-like compounds, toxic equivalence factors are developed for each congener member of the chemical category relative to the chemical category's most toxic congener member 2,3,7,8-tetrachlorodibenzo-p-dioxin.
- 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 or lower. 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. Facility-specific and industry-specific estimates derived from the EPA's National Emissions Inventory are used to estimate the speciation 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.
- Being that the physical form of released metals or metal compounds can affect toxicity, reasonable assumptions are made regarding the most likely form of releases (e.g., the noncancer toxicity weight for chromic acid mists and dissolved hexavalent chromium aerosols is much higher than for hexavalent chromium particulates, but releases of these chemicals as mists and acid aerosols are not expected to be typical so the toxicity weight for the inhalation of hexavalent chromium particulates is used).