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Technical Information

Important Characteristics of RSEI

RSEI is a screening tool used to assess the relative impacts of releases of toxic chemicals by combining estimates of toxicity, exposure level, and the exposed population to provide risk-related comparisons. RSEI provides a screening-level perspective for relative comparisons of chemical releases and other waste management activities. This tool does not evaluate risk to individuals, nor does it provide a detailed or quantitative assessment of risk (e.g., excess cases of cancer). RSEI is not designed as a substitute for more comprehensive, site-specific risk assessments.

RSEI evaluates information submitted by sources required to report to the Toxics Release Inventory (TRI); it does not account for all sources of chemical exposure. Risk-related results are available for releases and transfers to air and water. Pounds- and hazard-based results are available for all releases and transfers.

Chronic human toxicity (both cancer and noncancer) associated with long-term exposures is addressed by RSEI. Toxicity weights are chemical-specific in RSEI and are based upon the single, most sensitive chronic-health endpoint for inhalation or oral exposure, and do not reflect severity of effects or multiple health effects. Neither acute human toxicity or environmental toxicity are modeled. Toxicity weights in RSEI are not designed to reflect statutory criteria used for listing and de-listing chemicals under Section 313 of the Emergency Planning and Community Right-to-Know Act. RSEI risk-related results account for estimated exposures and may not correlate with listing/de-listing decisions.

RSEI makes several significant assumptions regarding the toxicity of metals and metal compounds, because of limitations in the reporting of these chemical categories. RSEI assigns metal compounds the same toxicity weight as the parent metal, 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 weight.

Because RSEI results reflect changing population size at the local level, a facility's relative contribution to the risk-related score could increase or decrease even without changes in its releases over time. Therefore, population trends should be considered when examining a facility's environmental management practices for the causes of changes in relative risk over time.  RSEI results have greater certainty when examining national or other aggregated levels as compared to disaggregated results at the local or facility level.

Review important caveats of the RSEI model and strengths and limitations associated with RSEI.

More detailed information on the data and methods used in RSEI can be found in the Methodology Document for RSEI Model Version 2.3.2, July 2013(PDF) (89 pp, 381K, About PDF).

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How RSEI Works

The model uses the reported quantities of TRI releases and transfers of chemicals to estimate the impacts associated with each type of air and water release or transfer by every TRI facility. The risk-related impacts potentially posed by a chemical release are a function of chemical toxicity, the fate and transport of the chemical in the environment after it is released, the pathway of human exposure, and the number of people exposed.

The RSEI model starts with TRI releases. For each exposure pathway from each chemical release, the model generates an “Indicator Element”. For instance, a release of the chemical benzene to air via a stack from the “ABC” Facility in 2010 leading to inhalation exposure is an “Indicator Element.” Each Indicator Element is associated with a set of results, including risk-related results or scores, hazard-based, and pounds-based results.

Risk-related results Surrogate Dose x Toxicity Weight x Population
Hazard-based results Pounds x Toxicity Weight
Pounds-based results TRI Pounds released

Risk-related Results

The toxicity, surrogate dose, and population components are multiplied to obtain a risk score for the “Indicator Element”. The surrogate dose is determined through pathway-specific modeling of the fate and transport of the chemical through the environment, combined with subpopulation-specific exposure factors. The score is a unitless measure that is not independently meaningful, but is a risk-related estimate that can be compared to other estimates calculated using the same methods. If the Indicator Element cannot be modeled, because of the lack of data needed for modeling or because the exposure pathway is not currently modeled, then the risk-related score is zero.

The model calculates risk-related results for the entire population and also for the following subpopulations: children under 10, children aged 10 to 17, males aged 18 to 44, females aged 18 to 44, and adults aged 65 and over. In addition the model also calculates “'Modeled Pounds,”' which is simply the number of pounds that can be modeled, before fate and transport modeling and exposure assumptions have been applied.

Hazard-based Results

Each “Indicator Element” also is associated with a hazard-based result, calculated by multiplying the pounds released by the chemical-specific toxicity weight for the exposure route (oral or inhalation) associated with the release. For these results, no exposure modeling or population estimates are involved. If there is no toxicity weight available for the chemical, then the hazard score is zero.

The model also calculates “Modeled Hazard”,' which is the chemical-specific toxicity weights multiplied by the Modeled Pounds (as described above), and “Modeled Hazard x Pop,” which multiplies modeled hazard by the exposed population, but without the fate and transport modeling that would be found in risk-related results.

Pounds-based results

These results include only the pounds of releases reported to TRI, and are available for all Indicator Elements. The model also provides TRI pounds with toxicity weights, which just sums the pounds for the chemicals that have toxicity weights in the RSEI model.

Combining Indicator Elements

Once results are calculated for each Indicator Element, they can be combined in many different ways. All of the results are additive, so a result for a specific set of variables is calculated by summing all the relevant individual Indicator Elements.

This method is very flexible, allowing for countless variation in the creation of results. For example, results can be calculated for various subsets of variables (e.g., chemical, facility, exposure pathway) and compared to each other to assess the relative contribution of each subset to the total potential impact. Or, results for the same subset of variables for different years can be calculated to assess the general trend in pounds-based, hazard-based, or risk-related impacts over time.

It must be reiterated that while changes in results over the years would imply that there have been changes in hazard- or risk-related environmental impacts, the actual magnitude of any specific change or the reason may not be obvious. Although the value itself may be useful in identifying facilities or chemicals with the highest potential for hazard or risk, the weight does not represent a quantitative estimate or provide an exact indication of the magnitude of individual hazard or risk associated with that facility or chemical.

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RSEI Tutorial

RSEI tutorials are designed to give you a quick introduction to how the model works, and how to quickly find basic information that most users are interested in.  There are three tutorials currently available to help acquaint you with various aspects of the model:

These tutorials may also be found in Chapter 4 of the User's Manual for RSEI Model Version 2.3.2, July 2013 (PDF) (172 pp, 1.8 MB, About PDF).

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