AERA guide | Section 5: Cumulative Air Emission Risk Analysis

This section of the AERA guidance contains a summary of MPCA’s methods for conducting a cumulative AERA and discusses the regulatory framework for this requirement. The cumulative information recommended for a Cumulative Air Emissions Risk Analysis is documented by the project proposer in the AERA-19 form.

The three main objectives of this section are:

  • To describe how to complete a cumulative AERA, and the roles of MPCA technical staff and project proposers
  • To describe the EQB regulatory requirement to consider the cumulative potential effects
  • To describe who is required to, or may be requested to, complete a cumulative AERA

The following concepts of cumulative risk assessment methodologies are shared by AERAs and cumulative AERAs:

  • Non-cancer health endpoints are summed at the screening level
  • The probability of disease of all cancer types are summed as one total cancer risk estimate
  • Qualitative contextual information is included
  • Risk estimates are reported as a summation of all pollutants
  • Risk Estimates are summed from multiple media (air toxic deposition and subsequent uptake into soil, plants and animals)
  • Risk estimates are summed from multiple pathways (food ingestion, incidental soil ingestion and inhalation)
  • Risk estimates are for a total facility and include multiple emission units

A cumulative AERA builds on the facility-specific AERA methodology to include air emissions from all appropriate offsite sources. A cumulative AERA incorporates offsite air emissions sources utilizing modeling, monitoring, or qualitative data and information. The choice of the type of information depends on the specifics of the project and the available data.

Approach development and resources

The MPCA developed an approach to meet the intent of the environmental review rules and to be consistent with the Minnesota Supreme Court’s CARD decision.

In developing an approach, the MPCA examined USEPA guidance on cumulative risk assessment, relevant literature, and past MPCA practices USEPA has published documents describing the framework for and tools for use in cumulative risk assessments:  USEPA Cumulative Risk Assessment Framework Document and USEPA’s Cumulative Risk Resource Document

The MPCA also considered data quality and availability, tools, method, time and resource availability among other technical factors.

The MPCA always needs to be consulted before conducting a cumulative AERA. In special circumstances, project-proposers may suggest deviations from this guidance. The cumulative AERA form, and a cumulative AERA form example, are available on the AERA Forms and Deliverables website

Potential air emissions sources to be considered

On-site sources

For projects requiring a cumulative AERA, risks based on emissions from stationary sources at the existing, modified and/or proposed facility will already have been estimated in the facility-based AERA. These AERA results need to be included in a cumulative AERA. Tailpipe emissions and dust suspension from mobile sources are included in AERAs if they remain on-site; idling mobile sources are included in an AERA on a case by case basis. If facility-based mobile source emissions are not evaluated in the AERA, their inclusion in the cumulative AERA will be considered.

Offsite sources

Off-site emissions sources considered for inclusion in a cumulative AERA include:

  • Vehicle tailpipe emissions and dust suspension for vehicles that leave the facility
  • Offsite point sources (within 10km)[1]: Examples are facilities with Title V and registration air permits. MPCA staff will provide project proposers with a list of existing and future off-site point-sources from submitted permit applications upon request
  • Offsite area sources. Examples include smaller stationary emission sources such as dry cleaners, gasoline service stations, or residential wood combustion appliances
  • Ambient background air concentrations related to sources not associated with sources listed above. The inclusion or exclusion of a quantitative analysis of a specific off-site source depends on the following factors: population density, proximity of existing facility and receptors to nearby point and area sources, local traffic counts and the potential increase in traffic counts associated with proposed project, and availability and quality of data. The inclusion or exclusion of off-site point sources in a cumulative AERA is described in general in the EAW language in terms of “geography” and “timing”. The MPCA’s interpretation of these terms is discussed below, and is the basis for the inclusion or exclusion of off-site point sources.


Based on the EAW language, the project proposers are asked to include any point sources that are existing at the time of the submission of permit or environmental review materials. A historical facility that has closed down operation and is no longer emitting air toxics need not be included as a point source in the cumulative AERA. Potential future projects need to be included as point sources for consideration in the cumulative AERA if the facility/project has submitted plans, a permit application or environmental review materials to the State of Minnesota or a local governmental unit.


Geography is the other area of consideration for the inclusion of projects/facilities in a cumulative AERA. The major factors contributing to the inclusion or exclusion of an off-site source on the basis of geography are: distance to receptors of interest, emission rates, dispersion characteristics, and the pollutants emitted.

Cumulative AERA sources of information: Modeling, monitoring, and qualitative

Quantitative representation of off-site air toxics sources can be derived from either modeled air concentrations and/or ambient monitoring data; and then supplemented with qualitative information. Off-site point sources of air toxics need to be considered in the cumulative AERA within 10 km of the proposed project’s receptors of maximum impact.

Including modeled air concentrations in a cumulative AERA

The MPCA prefers that project proposers use modeled air concentrations for facilities when possible. In general, there is less uncertainty as to source apportionment when source-specific modeled air concentrations are used rather than ambient monitoring data. Source-specific modeled air concentrations also provide information on a broader range of pollutants than does ambient monitoring data. Since source-specific modeled air concentrations are generally not available, relevant air monitoring data sets may be used.

Off-site point sources within 10 km

The origin, or centroid, of a cumulative AERA is the maximum modeled location where people could live. Generally, off-site point sources that are farther than 10 km from the maximum modeled location where people could live do not need to be modeled individually. Instead, they need to be considered as ambient background sources. Point sources within 10 km from the maximum modeled location where people could live need to be identified on a map and considered quantitatively or qualitatively in a cumulative AERA.

Whether to assess off-site point sources quantitatively or qualitatively depends on the availability and quality of air toxics data or existing air dispersion modeling results. Modeling information is sometimes available for off-site point sources that have undergone AERA review or air permitting.. If an AERA was completed for the off-site source in question, a RASS is generally available. This RASS needs to be used to calculate dispersion factors for the distance between the off-site point source and the maximum modeled location where people could live. Off-site point sources with a dispersion factor of greater than or equal to 1µg/m3/g/s (the lowest numerical value for a dispersion factor in the RASS look-up table) needs to be assessed quantitatively when possible. A dispersion factor (greater than or equal to 1) at the location of the receptor of interest is an indication that the off-site point source has the potential to impact air concentrations at that receptor location. If refined modeling has been completed for the off-site point source, this modeling data may be used to estimate a refined dispersion factor.

If there are no modeled air toxics data from offsite point sources within 10 km of the maximum modeled location where people could live, further analysis is conducted by gathering the following information: the off-site facility’s likely emissions profile, processes and fuel type, historical regulatory compliance, public complaints, and dispersion characteristics of the source (stack height, prevailing wind direction). This analysis is completed by the project proposer with support and assistance from MPCA staff. Should any of this information lead MPCA to consider this off-site source to have the potential to increase cumulative air emissions risks by more than pollutant risk driver levels (greater than 0.1 for non-cancer, greater than 1E-06 for cancer), the project proposer may be asked to submit emission rates for risk drivers and to develop modeling files.

If further analysis is not possible, yet the MPCA cannot eliminate potential concern for that facility, then ambient monitoring data that best reflects the off-site point sources needs to be selected and the off-site source can be discussed qualitatively.

Off-site mobile, area and ambient background sources

Air concentrations from off-site mobile, area, and other ambient background sources need to be considered quantitatively to the greatest extent possible, and supported by qualitative information. Air concentrations associated with these sources may be included in a cumulative AERA with modeled concentrations from MPCA’s Minnesota Risk Screening (MNRiskS) tool. MNRiskS includes air concentrations and risk estimates from all MN air pollutant sources including mobile and area sources. These mobile and area source emissions are from the MN emissions inventory and include general flowing traffic. MNRiskS does not contain idling vehicle emissions.

Including monitored air concentrations

The most recent and relevant ambient monitoring data may be used to assess offsite sources when modeled air concentrations are not available. Ambient monitoring data may also be used in combination with modeled air concentrations from an off-site point source. In some cases this may result in over-counting measured pollutants from the ambient monitoring data set that is also modeled. A solution to this issue is to add in only the modeled pollutants that are not also measured within the ambient monitoring dataset. The limitations of using monitoring data include:

Some potential risk-drivers may not be measured (acrolein, dioxins/furans, PAHs, PCBs, acidic aerosols)

  • Detection limit constraints
  • Relatively few monitors
  • No direct source apportionment
  • Potential for over-counting existing components of a proposed project
  • date of monitoring data

Included in Table 13 below are averaged risk results from active Minnesota Statewide Ambient Monitoring Network air sites.  If there are no off-site sources to be evaluated quantitatively, the project proposer needs to choose risk results from ambient data according to zip code-based population density. In this case, the ambient monitoring data are used to represent off-site area, mobile, point, and ambient background sources. These data are presented as risks that can be summed with the on-site modeled risks.

Table 13:  Risk results from ambient monitoring data to be used in cumulative air emission risk analyses

Averaged risk values from statewide MN air monitoring network
Mostly rural sites (Zip code population density of less than 500 people per square mile): Intermediate sites (Zip code population density between 500 and 2,999 people per square mile):
Acute Hazard Index Values: 0.5
Acute Hazard Index Values: 0.6
Chronic Non-Cancer Hazard Index Values: 0.6
Chronic Non-Cancer Hazard Index Values: 0.8
Cancer risk values in 100,000: 2
Cancer risk values in 100,000: 4

Note:  Zip code population density can be found at The population density needs to be found from the maximum modeled location where people could live.

A more refined analysis of population density using census data is available upon request from MPCA staff.

Project proposers with conditions described in Table 15 need to request specific ambient monitoring data.

Conditions under which MPCA will customize site-specific Ambient Monitoring Data for use in Cumulative Air Emission Risk Analyses

  • Unique site location (e.g. Iron Range)
  • Unique population consideration (e.g. potential environmental justice area)
  • Unique modeled off-site sources (e.g. coal or biomass energy producer with recently modeled emissions)
  • Urban sites (Zip code population density of over 3,000 people per square mile)
  • Recently monitored ambient data within 10km of the facility
  • Sites described in specific legislation

Explanation of risk calculations from ambient monitoring data

Risk estimates from all MPCA ambient monitoring data are available upon request. These risk estimates are updated annually as appropriate for a given monitor or group of monitors. Measured air toxics concentrations are included in the risk estimates if over 10% of the pollutant measurements are above the detection limit within a year. The following steps are carried out to produce risk estimates from ambient monitoring data:

  1. The detected data and flagged non-detects are used to calculate a 95% upper confidence limits of the arithmetic mean (95UCL-AM). This calculation is completed for pollutant and available year using Kaplan Meier statistics.
  2. Chronic risk estimates (cancer and non-cancer): The 95% UCL-AM is then divided by the appropriate inhalation health benchmark (IHB) to calculate a risk estimate (cancer risk, chronic non-cancer) for each pollutant with an IHB in the RASS.
  3. Acute risk estimates (non-cancer): Air toxics are measured for 24 hours on a frequency of 1 in 6 days to ensure measurements are made on every day of the week. Hourly maximum concentrations are estimated from these 24hr measurements by multiplying the second high 24hr air measurement by 5. For pollutants that are measured hourly, the second highest measured value is used directly. Each 2nd high value or 24-hour value multiplied by 5 is divided by the appropriate inhalation health benchmark to calculate an acute non-cancer hazard quotient.
  4. The pollutant specific risks are summed within the following categories: chronic cancer risk, chronic non-cancer hazard indices, and acute non-cancer hazard indices.
  5. All air toxics are not measured at every site, and in these cases risks are summed by pollutant category and then averaged by population density, region, or other monitoring location category.

Population density based risk estimates

Many proposed facilities do not have nearby monitoring locations. In these cases monitoring data are averaged by population density (rural, intermediate, and urban) or region (Iron Range).

Including qualitative information in a cumulative AERA

Quantitative estimates of risks from monitored or modeled data need to be supplemented by qualitative information. Sources of qualitative information include but are not limited to: distance to the proposed project, processes and fuel type, historical regulatory compliance, public complaints, source dispersion characteristics, land use or zoning, population density, distance to receptors of interest, likely emissions profiles, prevailing wind direction, current mobile source activity, and a projection of how mobile source activity may change.

The risk results reported in Table 8 include inhalation-only risks. Past AERAs have shown that potential on-site risk drivers are often from ingestion pathways. In some instances, MPCA has included a qualitative discussion of background risk from food basket surveys (e.g. population averaged baseline risks from dioxins in food products) in the cumulative AERA. Until methods for the quantitative estimation of non-inhalation risks from off-site emissions are developed, project proposers may discuss non-inhalation risks from off-site emissions qualitatively.

All mandatory AERAs require a cumulative AERA, because the AERA threshold is tied to the trigger requiring environmental review. The MPCA, however, retains the discretion to request a cumulative AERA for facilities completing a non-mandatory AERA. These decisions are influenced by the following factors:

  • Proposed project emissions (typically greater than 100 tons per year)
  • Data availability and quality
  • Proximity of receptors
  • Presence and location of nearby sources
  • Emissions of nearby sources
  • Potential environmental justice areas
  • Ambient air concentrations based on monitoring data.

This list is not exhaustive and additional information may be considered.

Minn. R. 4410.1700, subp. 7, item B, states that the Responsible Governmental Unit (RGU) must consider specific factors identified in the rule to decide whether a project has the potential for significant environmental effects when determining the need for an EIS. One of these factors is the “cumulative potential effects of related or anticipated future projects.” The RGU is to identify any past, present or reasonably foreseeable future projects that may interact with the project described in the EAW in such a way as to cause cumulative potential effects. Furthermore, the RGU is asked to describe the nature of the cumulative potential effects and summarize any other available information relevant to determining whether there is potential for significant cumulative potential effects.

In the Minnesota Supreme Court CARD Decision, the court described how an RGU should apply the rule criterion on cumulative potential effects for determining when to order an EIS.

The type of final risk management decisions made at the MPCA include determination of adequacy of an environmental review document, a negative or positive declaration for the continuation to an EIS, or the issuance of a permit. These final risk management decisions are made by the MPCA commissioner, a delegate of the commissioner, or the MPCA Board. An AERA and a cumulative AERA are only a portion of the information used in these larger agency-wide decisions. Decisions that include consideration of an AERA and/or a cumulative AERA incorporate both the qualitative and the quantitative information.

Cumulative risk goals and facility risk guidelines

The U.S.EPA guidance for conducting air toxics analyses and making risk management decisions at the facility and community-scale level considers a range of “acceptable” cancer risks from one in a million to one in ten thousand. For non-carcinogenic pollutants, USEPA considers a reference level, or hazard index, of 1.  Exposures above the reference level may have some potential for causing adverse effects.

Cumulative risk guidelines are currently unavailable, but are under discussion at the state and national level. Several conceptual starting points include: the acceptance of U.S. EPA’s cancer risk range of 1 in a million to 1 in ten thousand; using a hazard index of 1 for similar non-cancerous human health systemic effects; or developing “acceptable” risk increments for each source contributing to risk at a selected receptor  (similar to the PSD process).

Ingestion-based risks from off-site sources are not quantitatively assessed in the cumulative AERA. There are methodological challenges associated with cumulative ingestion-based risk analyses. The uncertainty associated with these challenges could be propagated with the summation of multiple pathways, routes and endpoints. Other issues not addressed by the cumulative AERA are similar to those not included in facility AERAs and include: indoor air quality, pollutants without health benchmarks, chemical transformation, pollutant interaction effects that are greater than or less than additive (e.g. synergistic toxicity, antagonistic absorption), occupational exposures, and personal micro-environmental exposure.