Cumulative Air Emissions Risk Analysis – AERA
A cumulative air emissions risk analysis estimates potential risks from air toxics emissions from a proposed project in addition to existing sources and planned future sources. The cumulative air emissions risk analysis is intended to provide information for assessing “cumulative potential impacts” in an Environmental Assessment Worksheet, but may also be prepared for air permitting outside of Environmental Review.
In considering whether an Environmental Impact Statement should be prepared (Minn. R. 4410.1700, subp. 7, Item B), an Environmental Assessment Worksheet must consider cumulative potential impacts to determine whether a proposed project has the potential for significant environmental effects. Cumulative potential impacts include environmental or human health effects that could result from a proposed project in conjunction with other related or anticipated future sources in a given area. The potential effects from any one project may be small; however, the aggregated effects from all the sources together may be significant.
In the Citizens Advocating Responsible Development (CARD) vs. Kandiyohi County, the Minnesota Supreme Court described how a project proposer or responsible government unit (RGU) should apply the rule criterion on cumulative potential effects. This court case is commonly referred to as the CARD decision.
Who Completes a Cumulative Air Emissions Risk Analysis?
Cumulative air emissions risk analyses are conducted in conjunction with the AERA as part of the Environmental Review Process by project proposers. Types of projects can include those that meet the mandatory categories described in Minn. R. 4410.4300, subp 15, or Minn. R. 4410.4400, ethanol facilities increasing production by five million gallons per year, or proposed electric production facilities greater than or equal to 25 MW. A cumulative air emissions risk analysis can also be conducted in conjunction with the AERA outside of the environmental review process, when an AERA is requested at the Agency’s discretion.
For facilities not completing an EAW, but that are completing an AERA, MPCA staff determine on a case-by-case basis which facilities will complete a cumulative air emissions risk analysis. 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,
- Air toxics concentrations from ambient air monitoring data.
This list is not exhaustive and additional information may be considered.
What is Assessed in a Cumulative Air Emission Risk Analysis?
A cumulative air emissions risk analysis evaluates risks from multiple sources, on-site and off-site. On-site sources include point, area and mobile sources associated with the existing facility and the proposed project. Off-site sources include nearby point, area, mobile sources and regional background. The air concentrations from these emissions are either generated from air dispersion modeling and/or ambient monitoring data. The degree to which air emissions are assessed from each source is dependent on the availability of modeling and monitoring data and tools.
How to Complete a Cumulative Air Emissions Risk Analysis
The methodology for assessing cumulative risks from air pollutants differs depending on whether air concentrations were modeled or measured within ambient monitoring networks. Generally, the Agency prefers that a project proposer use modeled air concentrations based on source-specific emissions data when the necessary data are available. 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 provide information on a broader range of pollutants than does ambient monitoring data. Whether to include an off-site point, area or mobile source is based, generally, on the magnitude of the emissions from the nearby source and on the distance between the source and the receptors of interest for the risk analysis being conducted. However, emissions data or modeling results for specific chemicals are often not available.
When modeled emissions data are not available, ambient monitoring data can be used to represent full or partial human health risks from related project air emissions for measured pollutants. Data from statewide ambient monitoring stations are separated into low, intermediate and high population density categories. The potential risks from each population density category were calculated from the appropriate ambient monitoring air concentrations and then summarized for acute, chronic non-cancer and chronic cancer endpoints. The risks from on-site and off-site modeled data are summed with the appropriate monitored risks.
How are Risk Management Decisions Made on Cumulative Risk?
Risk management is a distinctly different process from risk assessment. In the risk management process, the results from a risk assessment are integrated with other considerations, such as the potential benefits of the project, the practicality of implementing various risk reduction activities and policy concerns to evaluate the significance and reasonableness of the potential risk associated with a proposed project.
MPCA has historically considered an estimated excess lifetime cancer risk associated with a facility’s total air emissions of one additional cancer case in a population of 100,000 people as a threshold criterion above which further analysis, refinement or mitigation should be pursued. MPCA has also considered EPA’s guidance when considering incremental risks from a facility. For non-cancer endpoints, it has been MPCA’s practice to consider total facility noncancer hazard ratios that are less than one (from an individual facility) to be insignificant. These threshold determinations have been made regardless of background concentrations or contributions from other sources, barring unique circumstances regarding location or other qualitative concerns. This is consistent with the general proposed approach offered in EPA’s Risk and Technology Review (RTR) Assessment Plan. The EPA National Contingency Plan suggests considering an upper bound cancer risk of 1 additional case of cancer in a population of 10,000 people.
Cumulative Air Emissions Risk Analysis at the Minnesota Pollution Control Agency - Background Document (March 2009)
To Learn More
Risk analysis and risk assessment are complex and this summary provides only a basic overview. To learn more about conducting a cumulative air emissions risk analysis, please see the following Web pages:
- Environmental Review
- Environmental Review - Cumulative Risk
- Minnesota Administrative Rules
- The CARD decision
- Environmental Quality Board
Who to Contact
Risk assessment
- Mary Dymond, 651-757-2327
- Kristie Ellickson, 651-757-2336
Environmental Review Supervisor
- Craig Affeldt, 651-296-6062
