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Step 5: Risk Assessment

For all risk assessment levels, the submitted risk assessment report should include a brief summary detailing all relevant aspects of the risk assessment, summary tables indicating the total risk by TEU and exposure scenario, total facility risk compared to Risk Action Levels (RALs), and an uncertainty analysis. Also, include all modeling files with this submittal.

DEQ will verify that all risk calculations were done appropriately. This is expedited by providing calculation workbooks and/or code. By providing these files, DEQ can quickly identify discrepancies. It is also critical to provide a crosswalk of receptors to exposure locations (typically submitted at the Modeling Protocol step) so that DEQ can confirm the appropriateness of the exposure location assignments.

​The risk assessment report should include a summary of the information provided in the Modeling Protocol and Risk Assessment Work Plan (if applicable), as well as information to further support the risk calculations. The level of detail required in the modeling results varies by the risk assessment level selected. You should provide sufficient information to allow DEQ to duplicate the results of the modeling and risk assessment during DEQ's review process. Section 3.7 of DEQ's Recommended Procedures for Toxic Air Contaminant Health Risk Assessments presents recommended report content by risk assessment level.

Level 1 and Level 2 risk assessments (using AERSCREEN)

 Report the following information:

  • Summary report detailing all required elements of the risk assessment (See Section 3.7 of the Recommended Procedures)
  • Provide a map depicting the source location and maximum risk by exposure location
  • For each Toxic Air Contaminant (TAC), provide the Risk Based Concentrations (RBCs) from OAR 340-245-8040 Table 4, dispersion factors from OAR 340-245-8010 Table 2​, maximum exposure concentrations, and total excess cancer risk and hazard quotients across all exposure scenarios reported by individual Toxics Emissions Unit (TEU) and for the facility as a whole.
  • Demonstrate how the total risk across the entire facility was calculated and compared to the risk action levels.


Level 2, Level 3, and Level 4 risk assessments

Report the following information:

  • Summary report detailing all required aspects of the risk assessment (See Section 3.7 of the Recommended Procedures)
  • For each TAC, provide the RBCs from OAR 340-245-8010 Table 2​, location of maximum exposure concentration, maximum exposure concentration, total excess cancer risk and hazard quotients across all exposure scenarios, reported for both TEUs and for the facility as a whole.
  • Demonstrate how the total risk across the entire facility was calculated and compared to the risk action levels.
  • Provide all modeling input and output files to DEQ. Specifically, DEQ requests the following files in order to verify both the modeling accuracy and the reported risk values:
    • AERMOD input file
    • AERMOD source and receptor files
    • Terrain data files
    • Building Profile Input Program (BPIP) files
    • Meteorology data (.sfc and .pfl files)
    • Any other input files needed to run the model
    • Table listing any referenced receptor IDs, geocoordinates (UTM, lat/long), and assigned exposure location.

In addition to the information above, DEQ requests the following information. If a facility does not provide this data, DEQ may create it to better understand the risk near a facility.

  • Figures showing the concentration/risk plots and gradients around the facility for each exposure scenario.
  • When modeling risk using the Risk Estimate Emission Rate (REER) approach, present results in units of risk. Isopleths should represent total risk for each exposure scenario.

Risk should be reported for each Toxics Emissions Unit (TEU) and then totaled across all TEUs for reporting total facility risk. The risk should be identified for each exposure scenario. For most risk assessments, this will result in 7 risk values (residential cancer, child cancer, worker cancer, residential noncancer, child noncancer, worker noncancer, and acute). If a facility has 3 TEUs, it will result in up to 21 risk values – i.e., 3 for each TEU, each reporting 7 total risk values by exposure scenario. Facilities may also elect to report risk by target organ, which will increase the number of non-cancer risk results by the multiple of applicable target organs.

​Noncancer chronic and acute risk are evaluated based on Hazard Index (HI) calculations that are then compared against the non-cancer Risk Action Levels (RALs).

Changes to the non-cancer Risk Action Levels for existing sources were adopted in May 2020. Prior to this rule update, at existing facilities all 182 Toxic Air Contaminants (TACs) with noncancer health effects were regulated at a TBACT RAL of HI 5. If the facility's noncancer Hazard Index was above 5, the facility was required to reduce risk to below HI 5 or install TBACT on all significant TEUs.

The new rules reduced the existing facility TBACT RAL to HI 3 for 158 of the 182 TACs that are expected to have developmental or other severe human health effects. The remaining 24 of the 182 TACs are still regulated at HI 5. Facilities with both HI 3 and HI 5 chemicals will require an additional calculation step to determine compliance with RALs.

Please refer to OAR 340-245-8010 Table 2​ to confirm which TACs are regulated as HI 3 or HI 5, and the 'HI Quick Reference Guide' and Recommended Procedures for Toxic Air Contaminant Health Risk Assessments for further details.

​Noncancer chronic and acute risk are evaluated based on Hazard Index (HI) calculations that are then compared against the non-cancer Risk Action Levels (RALs). For new facilities the noncancer Risk Action level is 1.

​​A Risk Determination Ratio, is used for existing facility that emit a mix of HI3 and HI5 toxic air contaminants, and it is calculated by the following equation:



T = Type of risk (acute or chronic noncancer risk)

E = Exposure Location (residential, non-residential child, worker, or acute)

x = specific TAC emitted at facility

​When comparing calculated risk to the Aggregate Toxics Emissions Unit (TEU) Level and the Source Permit Level [OAR 340-245-0200(4)(a)(A)], round the calculated risk to one decimal place. When risk is compared to all other risk action levels, round the calculated risk to the nearest whole number, unless a Risk Determination Ratio (RDR) is being calculated for existing facilities emitting a combination of Hazard Index (HI) 3 and HI 5 TACs. If calculating an RDR, risk should be rounded to one decimal place. [OAR 340-245-0200(4)(a)(B)].

​CAO rules require that a quantitative or qualitative uncertainty evaluation be included in a Level 3 and Level 4 risk assessment. Specifying the assumptions and uncertainties inherent in the risk assessment helps place the risk estimates in proper perspective. Another use of uncertainty characterization can be to identify areas where a moderate amount of additional effort (such as better emission estimate methods, or characterization of chemical species) might significantly improve the evaluation.

Often it is difficult to quantitatively evaluate uncertainty. Generally, though, you can make a determination that an uncertainty will likely result in an underestimate or overestimate of risk. In some cases it will be unknown whether risks will be under- or over-estimated. There are various types of uncertainties associated with a risk assessment, including the following four major categories:

  • Selection of Toxic Air Contaminants (TACs) for evaluation
  • Emission rate calculations
  • Exposure assessment assumptions
  • Derivation of toxicity values

Selection of TACs for Evaluation
If there are no Risk Based Concentrations (RBCs) for some of the reportable emitted chemicals at a facility, state that risks from these chemicals cannot be quantitatively evaluated, which will likely result in an underestimation of risk. Also, some processes may emit chemicals that are not anticipated from currently available emissions data.

Emission Rate

Often emission rates are calculated using published emission factors developed by EPA or other entities. In general, these factors are designed to be protective, and should not underestimate emissions and risk. If these factors are applied to similar processes for which the factors were not developed, emission estimates may be under or overestimated. You can partially quantify uncertainty in emission rates estimates by evaluating the range of emission factors that could be used. The use of source test data will reduce uncertainty related to emissions and associated risk.

Modeled and Measured Exposure Concentrations

Uncertainty associated with calculating actual exposures and therefore estimating risk can include potential inaccuracies in the emission inventory, variability in estimates of emission rates, uncertainties in air dispersion models, and protectiveness inherent in the exposure assumptions incorporated into the derivation of RBCs. TACs that are missed in the emission inventory or have underestimated emission rates will result in an underestimation of risk. Protective assumptions used in models will likely overestimate risk.

There is often high uncertainty associated with ambient monitoring air concentration results. To reduce uncertainty, a sufficient number of monitors need to be deployed, and they need to be appropriately placed to obtain representative data. Detection limits need to be adequate to detect TACs above RBCs. Uncertainty associated with evaluating non-detected concentrations needs to be discussed.


Sources of uncertainty for toxicity values can be discussed. For carcinogens, EPA has weight of evidence categories that can be presented and evaluated. For noncarcinogens, TRVs have associated uncertainty factors that can also be presented and discussed. Most risk assessments will include a summation of risk from multiple TACs. The assumption of dose additivity (inherent in the rule requirements for summing risk) does not consider possible synergistic or antagonistic effects. The potential for under- or over-estimating summed risks can be discussed. Evaluating noncancer risk by target organ (Appendix F) is one way to reduce uncertainty in the noncancer risk evaluation.

​​DEQ and OHA considered the following authoritative sources in developing toxicity reference values used to calculate risk-based concentrations.
  • United States Agency for Toxic Substances and Disease Registry: The ATSDR is a federal public health agency within the United States Department of Health and Human Services. The agency focuses on minimizing human health risks associated with exposure to hazardous substances.
  • EPA Integrated Risk Information System (IRIS): EPA's IRIS Program supports this mission by identifying and characterizing the health hazards of chemicals found in the environment. Each IRIS assessment can cover a chemical, a group of related chemicals, or a complex mixture.
  • EPA Provisional Peer-Reviewed Toxicity Values: EPA's PPRTV Program supports the Agency's mission to protect human health and the environment by identifying and characterizing the health hazards of, and providing an important source of toxicity information and toxicity values for, chemicals of concern to the Superfund Program.
  • California Office of Environmental Health Hazard Assessment: OEHHA is the lead state agency for the assessment of health risks posed by environmental contaminants.  

Cleaner Air Oregon frequently asked questions for facilities 

The frequently asked questions (FAQs) on this page are provided to answer common questions about completing air toxics risk assessments for the Cleaner Air Oregon program, and ensure consistency across assessments. As the Cleaner Air Oregon process evolves, DEQ will update this page with common questions or unique scenarios. If there is a question you would like to see answered or have feedback on a response below, please contact us at

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