Industry not Breathing Easier Following Issuance of New Air Quality Standards for Fine Particulate Matter


On March 6, 2024, the United States Environmental Protection Agency (EPA) published in the Federal Register its long-awaited reconsideration of the National Ambient Air Quality Standard (NAAQS) for particulate matter (PM). In this rulemaking, EPA left untouched most of the PM NAAQS standards, but the Agency reduced the primary annual NAAQS standard for PM2.5 by 25% from 12 µg/m3 to 9 µg/m3. This decision is likely to plunge many counties in the United States into nonattainment and will make air permitting for large emission sources throughout the country more challenging. While under the Clean Air Act, nonattainment designations will not be made for a few years, permitting impacts associated with implementation of the new standard will be felt almost immediately.

What is PM2.5?

 PM2.5, which is colloquially known as “soot,” is a fine particulate that can lodge deep in the lungs. It is composed of both primary particles, which are directly emitted as fine PM, and secondary particles, which originate from precursor gases/compounds in the atmosphere that become new particles or condense on to existing particles. 

Given the composition of PM2.5, regulations implemented to address compliance with the PM2.5 NAAQS target not only sources that emit direct PM2.5 emissions but also sources that emit PM2.5 precursors like sulfur dioxide, nitrogen oxides, volatile organic compounds, and ammonia. These precursors are emitted from numerous sources, including power plants, industrial boilers, stationary internal combustion engines, industrial processes, agricultural activities, and motor vehicles. Essentially, the PM2.5 NAAQS standards touch almost all aspects of industry and American life.

Adding to the complexity, several natural sources of PM2.5 can impact air quality and the ability of an area to demonstrate compliance with the NAAQS. In particular, wildland fires, which include both wildfires like those that impacted the United States in the summer of 2023 and prescribed fires that are set to reduce fire risk are a significant source of PM2.5. EPA stated in the preamble to the final rule, emissions from wildland fires account for 44% of primary PM2.5 emissions and emissions from wildfire comprise 29% of primary PM2.5 emissions.

The NAAQS for PM2.5

EPA issued the first NAAQS for PM in 1971. At that time, the NAAQS standard pertained to larger particles known as total suspended particulate (TSP). In EPA’s subsequent reviews of the PM NAAQS, however, the Agency changed the standards to address health effects associated with exposure to smaller particulate sizes. In 1987, EPA changed the indicator for particles covered by the NAAQS from TSP to PM10, and in 1997, EPA retained NAAQS for PM10, but for the first time issued separate NAAQS for PM2.5, which applied on an annual and 24-hour basis, respectively.

Historically, the NAAQS standard for PM2.5 has been controversial, and many areas of the country have struggled to meet the annual primary PM2.5 NAAQS standards as they have been ratcheted down. Prior to this rulemaking, the last time the annual primary PM2.5 standard was revised was in 2012 when the standard was revised from 15 µg/m3 to 12 µg/m3. In 2014, EPA began another review of the PM NAAQS, and, in 2020, the then EPA administrator determined to retain the existing PM2.5 standards, which was not consistent with staff recommendations. In 2021, after President Biden took office, EPA reconsidered the 2020 determinations regarding the PM NAAQs. This reconsideration resulted in the new annual PM2.5 NAAQS standard of 9 µg/m3

What Happens Next?

The revised NAAQS standards will undoubtedly be challenged. Whether the standards will be stayed while the challenge is pending, however, is not known. In any event, it will be several years before the revised annual PM2.5 NAAQS standard is fully implemented. Absent a stay of the new standard, however, its impact will be felt in the air permitting context immediately after it becomes effective on May 6, 2024.

Attainment/Nonattainment Designations. Revision of a NAAQS standard kicks off a process for states to evaluate air quality within their borders and propose to EPA designation of areas that either attain the standard (attainment areas) or do not (nonattainment areas). Under the Clean Air Act, EPA is required to make NAAQS attainment determinations within 2 years of issuance of the standard. For areas designated as nonattainment, states are required to submit state implementation plans (SIPs) to meet the revised NAAQS standards within 3 years of NAAQS standard issuance, and for areas designated as “moderate” nonattainment, attainment of the NAAQS must be demonstrated within 6 years of standard issuance, or by spring 2030.      

In this rulemaking, EPA tries to minimize the potential impact of the new standards, by noting that the majority of counties with monitors are already meeting the new standard based on 2020-2022 data. EPA stated that there were 119 counties, the majority of which are in the western United States, with design values above the 9 µg/m3 standard and 50% of these counties are in existing PM2.5 non-attainment areas. EPA further projects that by 2032, all but 52 counties in the 48 contiguous states would meet the revised PM2.5 standard when one considers implementation of various final or proposed federal rulemakings. These projections are likely overly rosy. 

Initial attainment/non-attainment designations will be based on data likely collected between 2022 and 2024. This data will be impacted by the 2022 and 2023 wildfires, at a minimum. There is an exceptional events process through which air quality data impacted by events like wildfires can be excluded from consideration when evaluating compliance with NAAQS requirements. This process, however, is complex and time consuming. In this rulemaking, EPA states that it is committed to ensuring that the exceptional events process is clear, and the Agency plans to issue new tools that can be used by states to support an efficient process. How this process will play out in practice given the likely number of exceptional event demonstrations remains to be seen.

Permitting impacts. While nonattainment designations are a few years away, companies that are planning to construct a new facility or modify an existing one will need to consider the revised PM2.5 NAAQS standard immediately. 

As of May 6, 2024, large emission sources of PM2.5 seeking Prevention of Significant Deterioration (PSD) permits to construct or expand their plants will need to use the revised PM2.5 standard when demonstrating compliance with the PSD program. For emission sources with pending PSD applications, they will need to work with their state permitting agency to ensure the pending project meets the new requirements. Consideration of the revised NAAQS standard may be challenging for many facilities. At 9 µg/m3, the revised annual PM2.5 NAAQS standard may be very close to background PM2.5 emissions. With the background PM2.5 levels so close to the standard, it may be difficult for projects to show compliance with the standard. This could particularly impact projects at natural gas fueled power plants and projects with large combustion sources. 

For sources located in current PM2.5 non-attainment areas, the requirements will not change. After completion of the attainment designation process – likely in or after 2026, however, additional locations may be subject to more stringent non-attainment New Source Review requirements, including the requirement to obtain emission offsets, which may be scarce and pricey. 

Quarles continues to monitor and share developments in the environmental and energy regulations. If you have any questions concerning the revised PM2.5 NAAQS and how it may impact you, please contact your Quarles & Brady attorney or:

Cynthia A. Faur: (312) 715-2609 /

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