Understanding RSEI Results
Watch a short video that will help you understand the results EPA's Risk-Screening Environmental Indicators (RSEI) model provides and how those results should be interpreted. Note that this video describes Version 2.3.5 and the downloadable version of EasyRSEI. EasyRSEI is now a web-based dashboard with no downloading required.
On this page:
- What is a RSEI Score?
- What does a high RSEI Score mean?
- What is RSEI Hazard?
- Why are RSEI Score and RSEI Modeled Hazard different?
- What are RSEI Modeled Pounds?
- Why are cancer and noncancer effects combined?
- What are toxicity-weighted concentrations?
On other pages:
- More information on getting RSEI results from EasyRSEI and RSEI Geographic Microdata.
- Other sources for environmental information
- RSEI data and modeling
- Tips on how RSEI should be used and some helpful examples
RSEI produces different kinds of metrics (RSEI Score, RSEI Hazard, toxicity-weighted concentration) at different levels of aggregation (facility, chemical, industry sector, state, etc.).
RSEI Score is the most commonly used metric and can be found in applications like EasyRSEI and Envirofacts, along with RSEI Modeled Hazard. Toxicity-weighted concentrations, along with RSEI Scores, are provided in the RSEI Geographic Microdata for air and water.
What is a RSEI Score?
A RSEI Score is a unitless value that accounts for the size of the chemical release, the fate and transport of the chemical through the environment, the size and location of the exposed population, and the chemical's toxicity. The graphic below summarizes how RSEI Scores are constructed. A RSEI Score is calculated as toxicity weight multiplied by the exposed population multiplied by the estimated dose.
RSEI Scores are only meaningful in comparison to other RSEI Scores. Ranking facilities, chemicals, industries, or other aggregations by RSEI Score can highlight situations that might warrant concern and need additional investigation. Creating trends for various groupings using RSEI Scores illustrates the change in potential risk for that grouping over time.
RSEI Scores do not describe a level of risk (such as the number of excess cancer cases), and cannot be used solely to draw conclusions about risk. RSEI does not perform a risk assessment, but is rather a screening-level tool to help identify situations of potential concern. Additional investigation should always be performed before any conclusions regarding risk are made (see additional sources of environmental information to help assess situations of potential concern).
What does a high RSEI Score mean?
RSEI Scores are designed to be compared to each other. A RSEI Score 10 times higher than another RSEI Score suggests that the potential for risk is 10 times higher. Relatively small chemical releases may lead to high RSEI Scores if the toxicity weight is particularly high or if the modeled exposed population is large. Conversely, large chemical releases may lead to low RSEI Scores if the toxicity weight is low or if the modeled exposed population is small. High RSEI Scores identify areas that may warrant further investigation, and do not conclusively demonstrate sources of risk. A facility with a high RSEI Score may not be the primary source of chronic human health risk in an area. RSEI Scores point to chemical releases that may warrant further investigation.
Because RSEI Scores reflect changing population size at the local level, a facility's RSEI Score could increase or decrease even without changes in the facility's 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.
A low RSEI Score indicates low potential concern from reported TRI releases, but other kinds of environmental risk may also be present, including pollution from mobile sources like cars and trucks, hazardous waste, and unreported releases from facilities.
What is RSEI Hazard?
RSEI Hazard, also called toxicity-weighted pounds, is a result that accounts for the size of the release and the chemical’s toxicity. Unlike RSEI Score, RSEI Hazard does not include environmental fate and transport modeling or adjustments for population exposure. RSEI Hazard should be interpreted carefully; in some cases, high RSEI Hazard may not be associated with high potential risk for human exposure.
RSEI Hazard can be calculated for any TRI release or transfer. When RSEI Hazard is calculated over the set of modeled releases (on-site releases to air, water, and off-site transfers to POTWs or incineration), it is labeled “RSEI Modeled Hazard” to emphasize that not every possible release or transfer is included.
Why are RSEI Score and RSEI Modeled Hazard different?
Generally, cases where RSEI Score is relatively low and RSEI Modeled Hazard is relatively high indicate that something in the environmental modeling is mitigating the impact of the release on human health—for instance, a small exposed population around a facility in a remote area, a low calculated concentration due to a fast rate of chemical decay, or treatment in an off-site incinerator or water treatment plant. Transfers to off-site incineration of highly toxic organic chemicals receive very high RSEI Hazard values due to the size of the transfers and the toxicity of the chemicals; however, incineration destroys almost 100 percent of the chemicals, so these kinds of transfers are unlikely to result in adverse human health effects.
Note that RSEI Hazard is often higher than RSEI Modeled Hazard for the same facility because more releases and transfers are included.
What are RSEI Modeled Pounds?
Only certain releases and transfers are modeled in RSEI (direct air and water releases, and transfers to publicly owned treatment works (POTWs) or to off-site incineration). Pounds for those releases and transfers are labeled “RSEI Modeled Pounds” in charts and tables to emphasize that the value may not match the total pounds listed in TRI for a facility or a geographic area.
Why are cancer and noncancer effects combined?
RSEI Score and RSEI Hazard results combine cancer and noncancer effects so users only need to compare one set of numbers. However, you can also look at cancer and noncancer effects separately, in EasyRSEI and in other RSEI data products.
What are toxicity-weighted concentrations?
Toxicity-weighted concentrations are provided in the RSEI Geographic Microdata (for air and water). This is an appropriate metric for comparing levels of potential impact between geographic areas and for smaller-scale environmental justice analyses, because, unlike RSEI Score, it does not include the number of people exposed. With population excluded from the calculation, the toxicity-weighted concentration allows thinly populated, but highly burdened areas to be identified, but it may not be as useful as RSEI Score for other screening activities, such as prioritizing facilities for pollution prevention activities.
Users should note that toxicity-weighted concentrations and RSEI Scores are aggregated differently. RSEI Scores are designed to be fully additive- you can add RSEI Scores for groups of chemicals, facilities, geographic areas, etc., and, because the impact is expressed as dose rather than concentration, you can add RSEI Scores for air and water together. Toxicity-weighted concentrations can be added across chemicals for the same geographic area, but must be averaged when combining geographic areas; air and water toxicity-weighted concentrations cannot be added.