AERA | Risk screening (RASS and Q/CHI)

In the Air Emissions Risk Analysis (AERA) process, “quantitative analysis” specifically refers to the estimation of additional lifetime cancer risks and hazard indices using the Risk Assessment Screening Spreadsheet (RASS) or the Q/CHI spreadsheet. The RASS typically generates a more conservative risk estimate and the Q/CHI spreadsheet refines the risk estimate using different dispersion techniques. For additional information on air dispersion modeling used in the RASS and Q/CHI spreadsheet see the AERA Dispersion Modeling Web page. These methods can either be used separately or iteratively. Using the spreadsheets in an iterative manner can eliminate or reduce time and effort spent completing more refined modeling. For example, using the RASS may eliminate chemicals or exposure times to be assessed in a more refined analysis if risks are below thresholds.

Risk Assessment Screening Spreadsheet (RASS)

The primary steps necessary in preparing a quantitative analysis of facility emissions for the RASS include:

  • generating the list of chemicals emitted or potentially emitted at a facility,
  • estimating emissions for the chemicals emitted that have at least one inhalation health benchmark value (IHB) in the RASS,
  • selecting dispersion factors for estimating air concentrations,
  • using the IHBs and multimedia factors in the RASS to estimate potential risks, and
  • documenting the results.

The RASS can be used as a screening tool, initially evaluating a broad selection of chemicals with screening dispersion modeling. The screening process identifies and eliminates chemicals or their emission source(s) if additional lifetime cancer risks and hazard quotients do not present a level of risk that warrants further study. Eliminating chemicals or sources from further study requires documentation. The project-proposer may use relatively more conservative assumptions (which would result in higher screening level risk estimates) or may use more accurate (refined) information, which should result in more accurate and lower risk estimates.

RASS for air dispersion modeling

RASS separates the dispersion modeling from the rest of the assessment, allowing project proposers to input dispersion factors using various levels of conservatism. The following methods may be used with the RASS.

  1. Default Look-up Table. The RASS contains a generally conservative look-up table developed using the EPA preferred model AERMOD. The look-up table provides default dispersion factors that are used to estimate air concentrations. With the exception of stack height and receptor distance, worst case conditions for other parameters were used to develop the look-up table dispersion factors.
  2. Other Refined Modeling. Project proposers may also use dispersion factors from other site-specific modeling (e.g., PSD modeling or Title V modeling) if available. If using other site-specific modeling, users must follow standard EPA guidance and practices, except they must set up the model to report the maximum (high-first-high, H1H) ambient value, and not any other lesser values (e.g., H2H for each year, or H6H over 5 years) as is done in criteria pollutant assessments for comparison with NAAQS or PSD increments. Additionally, the model must be set up to report the highest monthly average so the AERA analysis can account for risks from pollutants that have "subchronic" health effects.

While only locked RASS versions are now available on the Web site, unlocked versions are available by request and approval from the MPCA. The SIC code is now required in all RASS submittals.

For ethanol facilities with the SIC code 2869, additional rows of chemicals have been added after the original set of chemicals. These additional thirteen chemicals are the ethanol sector specific chemicals for which MDH developed interim exposure values. These rows also appear in the RiskCalcs, Concs and ToxValues worksheets.

MPCA updates the RASS annually. Please contact MPCA risk assessment staff for a current copy before beginning work on an AERA.

Q/CHI Spreadsheet

The Q/CHI spreadsheet can further refine the analysis. A project proposer enters emission rates that are divided by the inhalation health benchmark into AERMOD. The result from AERMOD is either a cancer risk or a hazard index at every receptor location, pairing the result in space and time. Those chemicals, sources and exposure scenarios exceeding “risk-driver” identification thresholds (a hazard quotient of 0.1 for non carcinogens and/or cancer risk of 1E-6) in the RASS should be moved forward into the Q/CHI spreadsheet, providing that appropriate documentation is provided to the MPCA. The primary steps necessary for preparing a quantitative analysis using the Q/CHI spreadsheet can be found in the Readme tab of the Q/CHI spreadsheet.

Q/CHI spreadsheet for air dispersion modeling

The Q/CHI spreadsheet uses a different methodology than the RASS. In Q/CHI spreadsheet, the look up table and the DISPERSE program are not used to generate dispersion factors. The Q/CHI spreadsheet does, however, utilize AERMOD as its dispersion tool. In the Q/CHI spreadsheet, emission rates (Q) are divided by inhalation health benchmarks (CHI). The Q/CHI values, in units of g/sec per mg/m3, are summed through pollutants for each respective stack. Each stack-specific Q/CHI sum is then input into AERMOD as an “emission rate.” The Q/CHI sums are then “dispersed” to produce cancer risks or hazard indices at every receptor location. Similar calculations and modeling are completed for PBTs, however in this case a Multi-pathway Screening Factor is multiplied by each Q/CHI value before summation and dispersion modeling. These PBT analyses produce dispersed multi-pathway risks (farmer and resident ingestion). The Q/CHI method eliminates the conservative assumption that the maximum risk is calculated regardless of it occurring at the same time and the same place. If facility risks are still above risk guidelines after the refinement of Q/CHI modeling, additional refinement or mitigation may be necessary. Discussion of refinement options, including dispersion refinements, can be found in the AERA Guide.

Risk Drivers in Q/CHI: Once the Q/CHI dispersion modeling is completed, pollutant-specific risks are calculated in the Q/CHI spreadsheet. The stack-specific risks should be input into each risk driver worksheet (e.g., "Q_CHI Acute drivers”, etc.), and the results will be summarized on the “Risk Driver Summary” worksheet.

The Q/CHI spreadsheet can be used in an iterative manner with the RASS. The RASS may be used first, to eliminate pollutants that are not risk drivers for any pathway. The RASS may also be used to eliminate exposure pathways if the summed risks for that exposure pathway are below risk guidelines. This type of screening should be described in the appropriate forms and provided to the MPCA.

To maximize the use of the Q/CHI methodology the user can use the results from AERMOD to generate plot files for use with Geographic Information System (GIS), SURFER, or other graphical tools.

MPCA updates the Q/CHI annually. Please contact MPCA risk assessment staff for a current copy before beginning work on an AERA.