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Risk-Screening Environmental Indicators (RSEI) Model

RSEI and NATA

The Risk-Screening Environmental Indicators (RSEI) model and the National Air Toxics Assessment (NATA) are both EPA tools that can provide information about potential impacts from toxic chemicals. Main differences between the two models include:

  • NATA models air toxics only, and includes more sources and processes than RSEI.
  • RSEI models water releases in addition to air releases.
  • RSEI uses Toxics Release Inventory (TRI) data, while NATA uses the National Emissions Inventory (NEI) and includes point, nonpoint, mobile, fire and biogenic sources. NATA also includes estimates for background and secondarily formed air toxics.
  • NATA releases single-year snapshots every few years, while RSEI releases a full time series every year.

The following table provides a summary comparison of the two tools. More information can be found in the RSEI methodology document and on the NATA 2014 website.

Model Component

Comparison of RSEI and NATA

Purpose

NATA is a national-scale, screening-level assessment of outdoor sources of air toxics emissions. NATA estimates cancer risks and noncancer health effects from about 180 Clean Air Act air toxics plus diesel particulate matter (DPM). These estimates help state, local and tribal air agencies identify which pollutants, emission sources and places they may wish to study further to better understand any possible risks to public health from air toxics.

RSEI is more narrowly focused on releases reported to the Toxics Release Inventory (TRI) and so provides a risk-related perspective on toxic emissions from industrial facilities, allowing screening and prioritization within those kinds of releases only.

Media

RSEI models water releases in addition to air releases.

NATA predicts risks and health effects associated with outdoor air emissions only. 

Coverage

RSEI uses TRI data on toxic releases from industrial facilities, which is a type of source included in NATA (although not all TRI facilities are included in NATA). NATA also considers background pollution, onroad and nonroad mobile sources, and smaller sources not reportable to TRI like dry cleaners.

NATA includes emissions, ambient concentrations, and exposure estimates for about 180 air toxics, and estimates cancer risks and noncancer health effects for about 140 of these. This is roughly one-third of the chemicals reportable to TRI. However, many of the reportable TRI chemicals not covered by NATA have relatively low release volumes and lower toxicity

Emissions RSEI uses TRI, while NATA uses the National Emissions Inventory (NEI).
Modeling

RSEI and NATA both use AERMOD (a standard EPA dispersion model) to estimate ambient concentrations of air toxics. NATA also uses a photochemical grid model (CMAQ) for about 50 air toxics. NATA then uses a special hybrid method to estimate ambient concentrations for these pollutants by combining both models’ outputs. NATA also uses an exposure model, HAPEM 7, to include the impacts of a typical person’s activity patterns (such as commuting or going outdoors); RSEI assumes continuous exposure at a person’s place of residence.

Frequency

RSEI is updated annually, and all years of TRI data (from 1988 to present) are included in each annual update to create a comparable time series data set. 

NATA is typically updated every 3 years in conjunction with updates to the NEI. Because of changes in methods between assessments, different NATA years cannot be compared.

Metrics

NATA’s results are expressed in terms of excess risks (for cancer effects) and hazard indexes (for noncancer effects). 

RSEI facility-level results are usually expressed in terms of RSEI Hazard (pounds*toxicity) or RSEI Score (toxicity*dose*population). RSEI also produces geographically based Microdata, with results expressed as toxicity-weighted concentration or RSEI Score.

Outputs

NATA results can be examined by chemical and source type at various levels of aggregation from census tract to the national level. NATA is not designed to identify specific sources as contributing to risks.

RSEI results are designed to be filtered by one or more dimensions like industry, facility, chemical, year or state. Metrics are additive and comparable across any aggregations. Using the Microdata, users can link cumulative potential burden in any specific geography with the facility releases potentially causing the impact.

Scale

NATA combines sources and modeling from different levels of resolution (that is, the locations of some types of sources are not as precise as the point sources), and so the lowest level of resolution at which results are expressed is the census tract level.

RSEI sources are all point sources. Results are calculated at the level of the 810 meter grid cell and can be summed to the census block level (which is more granular than tract).

Toxicity RSEI and NATA use similar toxicity data, although NATA performs some toxicity adjustments that RSEI does not. RSEI combines cancer and noncancer effects into RSEI Score and RSEI Hazard and also reports them separately. NATA maintains separation of cancer and noncancer results.