This section addresses frequently asked questions and issues related to an AERA. It also contains helpful information for consultants and facilities conducting an AERA. For further information see the
PDF icon AERA Guide (aq9-18). For assistance, contact Kristie Ellickson, 651-757-2336.

General questions

The Air Emissions Risk Analysis (AERA) is a standardized screening process used by the Minnesota Pollution Control Agency to assess risk to human health from air emissions from a facility. The MPCA intends for the AERA to streamline the process of assessing risk from on-site air emissions and to reduce the use of resources for both businesses and the State. The potential human health effects from air emissions are examined both quantitatively and qualitatively in an AERA. The quantitative analysis consists of estimating emission rates, toxicity data, and air toxics concentrations from air dispersion modeling to calculate a number known as a risk estimate. These calculations are completed in the Risk Assessment Screening Spreadsheet (RASS) or the Q/CHI spreadsheet. The qualitative analysis focuses on issues such as the facility’s proximity to local residences, safety factors used in emission calculations, hours of operation, and actions the company will take to minimize risk (e.g., fencing a property boundary to restrict public access or changing out equipment to less polluting units). The quantitative and qualitative components of a project are considered together in describing a facility’s operating scenario and its potential to pose risk to human health. An AERA generates a risk estimate based on exposure to chemicals, toxicity of chemicals, the amount and types of chemicals emitted, and the location of the facility in relation to nearby residences. 

The AERA is intended to answer questions regarding potential human health risks from air emissions within a facility’s controlled boundary. In general, the AERA is designed to inform the environmental review and permitting processes where potential health risks may exist. The AERA may accompany the facility’s air permit application, provide information to an Environmental Assessment Worksheet, or assist in scoping an Environmental Impact Statement. The AERA results are detailed in a MPCA staff conclusion that states whether or not the analysis is adequate and provides a recommendation as to whether permitting should include any actions to assure protection of the public health. MPCA risk managers then use the information summarized in the AERA to consider the potential for mitigation or risk/emissions reduction activities to be addressed in an EAW or a permit. The AERA also provides useful information for determining if an Environmental Impact Statement is necessary.

A facility or their consultant submits an AERA for review and a team at the MPCA reviews the AERA, including a permit engineer, an air dispersion modeler, a risk assessor, and possibly an environmental review staff person. A permit engineer reviews the facility’s processes and emission factors. An air dispersion modeler then uses meteorological, spatial, topographical and other data to determine air toxic’s concentrations surrounding the facility boundary. The risk assessor for the project assesses data provided by the engineer and air modeler and looks at chemical and exposure scenarios to determine what the potential risks could be to public health. If environmental review is involved in the project, environmental review staff summarizes the information from the AERA in the environmental review documents.

Comments can be made on an AERA that has been completed in the context of the public comment period for the Environmental Review documents or the air permit. Projects that are placed on public notice are announced through local media and can be accessed through the MPCA Public Notice Web page. During a comment period members of the public may comment on the proposed project including the AERA. While the AERA is completed prior to the comment period, the MPCA will address any questions or concerns raised with regard to the methodology or results of the AERA.

The AERA can be used as either a stand-alone analysis to be presented to the public or as an iterative screening process indicating if more refined analyses are appropriate. Information from an AERA is used to consider one of the following four possible courses of action:

  1. The need for further refined and focused analysis of the issues identified through the screening process.
  2. If no further analysis is needed and the project proposer can complete the environmental review and/or permitting process.
  3. Implementation of additional pollution prevention and/or mitigating measures to reduce or better disperse emissions.
  4. Recommendation that risk be evaluated within an Environmental Impact Statement.

"Cumulative" can mean many things, including multiple pollutants, multiple endpoints, multiple sources, on-site and off-site, and multiple exposure routes.

The sources of emissions in AERAs are limited to on-site emissions, including on-site point, area and mobile sources. AERAs do however assess various exposure routes including inhalation and ingestion of home-grown vegetables, beef, chicken, eggs and the ingestion of local fish. An AERA is also cumulative in that it addresses exposures to multiple pollutants for multiple health endpoints.

A facility may be required to complete a cumulative AERA. Cumulative AERAs include both on-site and off-site sources as described in the two cumulative air emissions risk analysis guidance documents. The framework behind these documents and their development, accords with EPA guidance and the requirements within Minnesota State EQB rules concerning Environmental Assessment Worksheets. EQB rules concerning EAW completion require information concerning the assessment of cumulative potential impacts. The cumulative potential impacts inquiry requires a Responsible Governmental Unit “to inquire whether a proposed project, which may not individually have the potential to cause significant environmental effects, could have a significant effect when considered along with other projects that:

  1. are already in existence or planned for the future;
  2. are located in the surrounding area; and
  3. might reasonably be expected to affect the same natural resources.” This requirement was defined in the Minnesota Supreme Court decision between Citizens Advocating Responsible Development vs. Kandiyohi County Board of Commissioners. This court appeal is also known as the CARD decision.

The AERA does not generally evaluate the following:

  • Chemicals lacking approved inhalation health benchmarks
  • Chemicals lacking emission estimates or emission factors
  • Ecological risks from air emissions
  • Dioxin/furan/polychlorinated biphenyls ingestion of breast milk by infants
  • Traffic emissions not addressed by environmental review methods
  • Emergency generator emissions
  • Multimedia chemicals with oral toxicity values but no inhalation health benchmarks
  • Visibility impacts from a facility
  • Carbon dioxide emissions and Climate Change
  • Odors
  • Emergency operations
  • Airborne microorganisms

The quantitative threshold of 1.0 for acute and chronic risk and 1/100,000 for cancer risks is not a regulatory “bright line.” Potential calculated risks under these guidelines suggest a very low probability of the health endpoint assessed, however estimated risks above these guidelines do not guarantee health effects. This is a guideline that allows risk assessors and risk managers to determine whether or not further analysis is necessary or if mitigation should be considered. If a facility’s initial AERA assessment shows that a risk that is greater than a quantitative, or Risk Assessment Screening Spreadsheet, guideline the facility may consider refinements in the toxicity assessment, modeling or emission estimates. This may include fully refined dispersion modeling, limited hours of operation, or separation by health endpoint. If an AERA is submitted with refinements and risk estimates are still significantly above risk guidelines, the MPCA will meet with the project proposer to discuss appropriate courses of action. Possible actions may include re-evaluating emission factors used, focusing on qualitative elements of the analysis (e.g., location of nearest residences), prescribing engineering interventions such as increased stack heights for greater dispersion or installation of additional emission controls (e.g., low NOx burners), requiring pollution prevention efforts such as product substitutions or process improvements or risk reduction activities such as off-sets. If engineering interventions are adopted, they may become enforceable permit conditions. The AERA is an iterative process and will likely require supplemental information on the part of the proposer or revisions to the original submittals at the MPCA’s request. The MPCA also will consult with the Minnesota Department of Health for information to further evaluate exposures and potential health risks.

Criteria pollutants are pollutants for which the EPA has established PDF icon National Ambient Air Quality Standards or NAAQS. The criteria pollutants are:

Criteria pollutants are regulated under the Clean Air Act and must meet the NAAQS established for each pollutant. Similarly, the state has PDF icon Minnesota Ambient Air Standards or MAAQS that must also be met. Lead and NO2 are assessed in the MPCA risk assessment spreadsheet as air toxics.

Respiratory sensitizers are specific chemicals that can cause severe adverse reactions, sometimes at minute concentrations, for people that have been sensitized by previous exposure to the chemical. Initial exposure to a sensitizer may not elicit a response; however, subsequent exposures may result in more severe reactions. Ambient air concentrations that trigger such a response may be well below levels that would affect non-sensitized individuals and may be below inhalation health benchmarks as well. MPCA’s list of respiratory sensitizers can be found in the Risk Assessment Screening Spreadsheet. This information was provided by the Minnesota Department of Health.

Developmental toxicants are specific chemicals that can cause adverse health effects to a developing fetus including birth defects, low birth weight, biological dysfunctions, and neurological deficits. Maternal exposure during pregnancy can harm fetal development or cause death to the fetus. Chemicals that are developmental toxicants are designated as such in the RASS. These chemicals have “ceiling values” which are thresholds that should not be exceeded.

Persistent, bioaccumulative, and toxic pollutants (PBTs) are highly toxic, long-lasting substances that can build up in the food chain to levels that are harmful to human and ecosystem health. They are associated with a range of adverse human health effects, including effects on the nervous system, reproductive and developmental problems, cancer, and genetic impacts. In the AERA process, ingestion risks are assessed from PBTs due to concerns that chemicals emitted into air will be deposited on the land, taken up into produce and livestock, and then consumed by humans. These chemicals are associated with multi-pathway concerns. Multi-pathway concerns refer to exposure from contact with soil, plants, or water bodies on which an emitted chemical has been deposited as well as the ingestion of fruits, vegetables, beef, milk, pork, chicken, eggs, fish, and soil. The Multi-pathway Screening Factors are derived from the list of PBTs in the RASS. PBTs include chemicals such as dioxins, polycyclic aromatic hydrocarbons, and metals such as, arsenic and mercury.

The AERA does not assess all air emissions from a facility. Often, there is limited information to develop emission estimates or health benchmarks for all of the pollutants that could be emitted from a facility. Much of the information regarding toxicity or how a pollutant may move through the environment is unknown. Often, there are no emission estimates or health benchmarks for all of the pollutants that could be emitted from a facility.

The MPCA conducts AERAs using peer-reviewed scientific information and toxicological values reviewed and approved by regulatory agencies in consultation with the Minnesota Department of Health. The MDH also promulgates Health Risk Values (HRVs) and Health Based Values (HBVs), which are toxicity values used in the AERA process. When insufficient information is available to develop a toxicity value, the MPCA may use a surrogate inhalation health benchmark for another chemical to assess potential health risks from a related chemical or chemical mixture. This process is designed to be protective of public health, and is part of the screening process to identify chemicals of potential interest for further evaluation. An example of this in the RASS is the general chemical group of aldehydes which has a surrogate benchmark value originally belonging specifically to formaldehyde.

A water body may be considered “fishable” if it typically contains water year-round in a year that receives at least 75 percent of the normal annual precipitation for that area. For facilities that emit PBTs or chemicals with multi-pathway concerns with stack heights less than 100 meters, the facility should evaluate the fish consumption pathway for water bodies that meet this criterion within a 3 km radius (approx. 2 miles). For facilities with stack heights greater than 100 meters, lakes, rivers and streams for the area within a 10 km radius (6 miles) should be considered. The facility should also show water bodies outside the specified area that may be fed by rivers and streams lying within the radius of interest and determine whether or not these water bodies should be evaluated. For information about how the MPCA assesses risks from mercury in fish, see the Minnesota Mercury Estimation Method.

The AERA is an iterative process, sometimes requiring more refinement in emissions estimations, toxicity values or air dispersion modeling. Potential risk estimates are calculated in the RASS by summing cancer risks or hazard quotients across pollutants at their maximum modeled concentrations. These modeled air concentrations of the specific pollutants are not necessarily paired in time and space. The Q/CHI spreadsheet allows calculations or Q/CHI sums which are then modeled in AERMOD to provide cancer risk quotients or hazard indices where pollutant maxima paired in time and space. Once AERMOD modeling is complete, hazard indices and cancer risk quotients can be input into the Q/CHI spreadsheet where pollutant-specific risk values are calculated and risk drivers are summarized.

A facility proposer is required to conduct a Cumulative Air Emissions Risk Analysis if completing an EAW, a scoping EAW for preparing of an Environmental Impact Statement or in some circumstances outside of the environmental review process. The MN EQB rules concerning EAW completion require information concerning the assessment of cumulative potential impacts. The cumulative potential impacts inquiry requires a Responsible Governmental Unit “to inquire whether a proposed project, which may not individually have the potential to cause significant environmental effects, could have a significant effect when considered along with other projects that:

  1. are already in existence or planned for the future;
  2. are located in the surrounding area; and
  3. might reasonably be expected to affect the same natural resources.” This requirement was defined in the Minnesota Supreme Court decision between Citizens Advocating Responsible Development vs. Kandiyohi County Board of Commissioners. This court appeal is also known as the CARD decision.

There are two documents describing both:

Project proposer questions

The AERA is generally done as part of an Environmental Assessment Worksheet and/or an air emission permit application or permit amendment application and is submitted in conjunction with those documents. The MPCA may also request that an AERA be completed in the event that there are concerns or complaints about health and/or environmental risk by the public who may be in the vicinity of the emission sources. Our suggestion for initially approaching an AERA is to first learn about the process. The next step is to complete the Risk Assessment Screening Spreadsheet (RASS) and supporting forms or to contact one of the staff listed on this webpage. More specific guidance can be found in thePDF icon Air Emission Risk Analysis (AERA) Guide.

An AERA must first be completed by the project proposer and preparation time will vary depending on the certainty of emission factors, chemicals that will be emitted, complexity of emission sources and how much plans change from the initial construction plans. Once the AERA has been submitted to the MPCA, staff review the document, request supporting information, and prepare summary documents that can be used to inform an EAW or permit. The timeline for an AERA will coincide with the schedules established for the EAW or permit. Previous AERAs have typically taken from one month to six months to complete. The length of time required for a project depends on several factors including Agency work load, the quality of the submittals received by the Agency and the responsiveness of the project proposer to MPCA information requests.

The most expeditious approach generally means thorough planning and active engagement on the part of the project proposer and facility representatives. The AERA’s primary function is to provide complete and correct information to the public and to decision-makers. To facilitate that, there are several things that a project proposer can do to keep a project on track:

  • Allow a reasonable amount of time between application and initial construction dates.
  • Talk with MPCA staff to determine what information is needed prior to submittals.
  • Provide complete information in the application and submittals; follow AERA guidance.
  • Meet with MPCA staff regularly and respond to requests in a timely manner.
  • Provide MPCA staff with the information they request and need.

Proposers may have the option of requesting that their air permit be formally expedited which, in turn, may expedite the review of the AERA. The company is charged for this service based on the amount of staff time required. This is highly dependent on availability of staff and resources and should not be relied on as the best option.

The AERA provides guidance and is not a rule; therefore a facility does not have to use the tools provided by the MPCA. However, the MPCA staff believes that using the AERA guidance, forms, and RASS and MPCA tools will expedite the review necessary for the Environmental Review and air permitting processes by providing reviewers assurance that toxicity values and modeling data are applied correctly. The MPCA staff strongly recommends using the RASS and MPCA guidance whenever feasible.

No, inhalation health benchmarks are not “bright lines” or regulatory standards. The AERA should not be viewed as a pass/fail exercise. There are several factors that influence risk management recommendations. Some factors are quantitative (safety factors in emission rates, certainty in toxicity values, refinement of dispersion modeling, etc.) while other equally important considerations are qualitative (proximity to nearby residences and businesses, how frequently the facility will operate, compliance history, etc.). All of these issues may affect the conclusions from an EAW or whether a permit is ultimately issued.

A team of program managers from the MPCA makes recommendations regarding the adequacy of a facility’s AERA results and its air permit based on staff recommendations. The managers may decide to approve permit issuance, require MPCA staff to evaluate certain aspects of a project further, or gather more information on a topic of public interest.

MPCA managers make risk management decisions based on staff recommendations that provide a total picture of the facility. They review and consider the staff summary and recommendation along with permit or environmental review issues. This includes, but is not limited to:

  • the type of permit,
  • the amount of emissions,
  • the types of control equipment to be installed,
  • the proposed monitoring or testing,
  • the previous experience with the industry,
  • the safety factors in emission rates,
  • the certainty in toxicity values,
  • the refinement of dispersion modeling,
  • the proximity to nearby residences and businesses,
  • how frequently the facility will operate,
  • the compliance history at an existing facility,
  • local community concerns, and
  • other issues that may be related to the proposed project.

The MPCA strives to balance all of these concerns to the best of its ability.

The AERA summary form includes the following:

  • Risk estimates
  • Facility location information (including the nearest resident)
  • Nearby land use and zoning information
  • Conformity with the RASS and approved toxicity values
  • Unique chemical or toxicity information
  • Verification of emission rates
  • Verification of modeling results
  • Maps and information identifying the locations of maximum impact
  • Fish consumption pathway analysis (if applicable)
  • A description of how conservative or refined the analysis is
  • Any important circumstances or considerations specific to the project
  • Comments from the Minnesota Department of Health (if applicable)
  • Analysis of mercury emissions
  • If applicable, a summary of the cumulative air emissions risk analysis

This list will vary depending on applicability to each specific project. The impact analysis also contains a recommendation as to whether or not any mitigation should be required by the permit.

While MPCA staff must make a final recommendation on an AERA for a project, the project proposer may disagree with a certain position or approach the Agency takes. We would encourage the consultant and facility representative to bring these issues to our attention early in the process, and to contact the assigned section manager if they are unable to resolve the issue at the staff level. At that time, managers will talk with MPCA staff regarding the dispute and make a decision as to whether or not any changes are necessary. This decision will be based on the specific needs of the project as well as deference to standard MPCA practices.

The MPCA has a hierarchy of chemical and toxicological databases that it uses complete risk assessment calculations. As toxicity values are updated in the separate databases based on new scientific information, the MPCA updates its own list of chemical information. This is done primarily with input from the Minnesota Department of Health. The chemical values used are known as Inhalation Health Benchmarks (IHBs). The IHBs can be found in the Risk Assessment Screening Spreadsheet (RASS).

Multi-pathway Screening Factors were developed to analyze potential risk from persistent, bioaccumulative and toxic (PBT) chemicals and are defined as the ratio of the risk from the indirect exposure pathway (e.g., oral or non-inhalation) to the risk from the inhalation exposure pathway. The MPCA has applied Multi-pathway Screening Factors to the maximum estimated inhalation risk in the RASS. The MPCA staff developed the factors in the RASS using the Industrial Risk Assessment Program (IRAP-h View) software, a computer program developed by Lakes Environmental Software that incorporates the algorithms contained in US EPA’s Human Health Risk Assessment Protocol for Hazardous Waste Combustors (HHRAP).

Multi-pathway Screening Factors were developed for both cancer and chronic non-cancer endpoints for adult farmer and adult resident scenarios but not for the fish consumption pathway due to the great number of variables inherent in estimating fish concentrations. For most volatile compounds, the inhalation risks are higher than the non-inhalation risks, so the Multi-pathway Screening Factor will be less than one. On the other hand, chemicals that accumulate in the food chain may have relatively high non-inhalation risks and a multi-pathway screening factor greater than one. Multi-pathway screening factors were only developed for pollutants with ratios greater than one. For a list of multi-pathway screening factors, see the Risk Assessment Spreadsheet (RASS).

The completion of an AERA has both indirect and direct benefits. Facilities that complete an AERA may initially determine that the potential human health risk posed by emissions from their facility is unacceptable. Based on these preliminary results, the project proposer may choose to improve emission controls or efficiencies within the plant prior to submitting their AERA results and thereby reduce emissions from their facility (e.g., pollution prevention measures). Emission reductions (risk reductions) may also occur as a direct result of the AERA process and recommendations on how to mitigate potential risk. Some examples of mitigation include process improvements, product substitutions, diesel idling prevention measures, voluntary mercury limits, and stack testing to better characterize emissions. Recommendations such as these are discussed with the project proposer to ensure that they are feasible and will be effective. In some cases, facilities have worked together with the MPCA and the Minnesota Technical Assistance Program (MnTAP) to reduce air emissions and implement pollution prevention measures. These measures, in some cases, have helped to reduce operating costs and to address community concerns regarding odors or potential health risks. For more information about potential projects and Pollution Prevention practices, see Industry Best P2 Practices or MnTAP.

Yes. Certain types of emission sources do not need to be included in the quantitative assessment, as described below.
Exclude an “insignificant activity” as defined in Minn. R. 7007.1300 (and its associated emissions) from the quantitative risk analysis if:

  • the activity emits a pollutant that does not have an inhalation health benchmark listed in the RASS, or
  • the activity emits a pollutant that is also emitted by sources/units already included in the emission inventory, and the contribution of the individual activity is less than 1% of the total emission inventory for a pollutant (hourly for acute and annual for chronic). 

If an emissions source does not meet one of these two tests, then it should be included in the RASS. Some types of insignificant activities can emit substantial amounts of air toxics or small amounts of highly toxic pollutants; these should not automatically be excluded from the AERA process.

Internal combustion engines associated with emergency generators and fire pumps are not quantitatively assessed. Further discussion of these sources is required in the qualitative portion of the AERA. See section 3.3.2 of the PDF icon AERA Guidance for the definition of “emergency generator” and additional guidance related to describing these emission units.

Emergency generators are also generally excluded from the AERA process, however, a facility may choose to include these sources in the RASS (to the extent possible) or address them qualitatively in an AERA. These sources would not be excluded from a Cumulative Air Emissions Risk Analysis that might be completed in addition to the AERA.

The AERA is a “screening” level analysis and therefore, should be based on assumptions that are protective of public health, particularly with respect to emission rates and inhalation health benchmarks. But a project proposer may choose how much specificity or refinement they want to add, particularly in the areas of emission estimates and modeling, which may be more representative of actual operating scenarios. In almost any project, there are pros and cons to presenting information that is likely an overestimate of risk, not the least of which is ease of communicating the likely actual scenario to the public. However, MPCA staff and project proposers ultimately have to describe how close to reality (i.e., expected conditions) the analysis is. If a project proposer submits information that is likely to underestimate risk, MPCA staff may ask them to re-evaluate the information or substitute more health-protective inputs themselves. It may expedite the review process if proposers or their consultants provide information regarding uncertainties in the assessment, including assumptions that may overestimate and underestimate risks.

An important element of a human health risk assessment is the transparent communication of uncertainty and variability. A portion of the uncertainty in a final risk estimate stems directly from the assumptions used to characterize potential human exposures. The USEPA and MPCA recommend estimating risks based on a set of default exposure assumptions, called the “Reasonable Maximum Exposure” (RME). One approach to communicate the uncertainty associated with the default exposure assumptions is to provide risk estimates using multiple human exposure assumptions. Risk results using central tendency human exposure factors should not replace risk estimates based on the RME and should not be considered a refinement to screening level risk assessments that follow MPCA’s Air Emissions Risk Analysis (AERA) guidance. Including central tendency exposure estimates would generally be appropriate in larger multi-pathway risk assessments, where more discussion of uncertainty is warranted. Information to estimate potential human health risks using central tendency values for human exposure factors can be found on the Multi-pathway Risk Analysis webpage.

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