Air Emissions Risk Analysis (AERA) — Risk Assessment Screening Spreadsheet (RASS) and Q/CHI Spreadsheet

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.

The 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.

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. An additional RASS accommodating up to 25 stacks is also available.

• Protected RASS for 25 Stacks (zipped file size = 983KB) (March 2012) (aq9-22)

The 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.

• Protected Q/CHI for 25 Stacks (aq9-23) (July 2012) (zipped file size = 1.5 MB)

Who to Contact

Environmental Analysis and Outcomes Division, Risk Evaluation/Air Modeling Unit

• Mary Dymond, 651-757-2327
• Kristie Ellickson, 651-757-2336
• Heather Magee-Hill, 651-757-2545