On October 2, 2026, EPA Region 8 notified Montana of its intended approach for finalizing area designations under the revised annual particulate matter with an aerodynamic diameter of 2.5 microns or less (PM2.5) National Ambient Air Quality Standard (NAAQS) of 9.0 micrograms per cubic meter (µg/m³), despite ongoing litigation and appeals surrounding the 2024 standard. EPA is currently operating under a court-ordered deadline requiring final area designations by February 6, 2027.
EPA’s preliminary review of Montana’s 2023 through 2025 monitoring data indicates that Lincoln County, specifically the Township of Libby, may not meet the revised annual PM2.5 standard and may be designated nonattainment. This assessment is consistent with Montana’s initial recommendation and is unlikely to surprise many Montana stakeholders. Libby sits within a narrow mountain valley where winter temperature inversions can trap wood smoke and other particulate emissions near the ground, contributing to elevated PM2.5 concentrations during the winter heating season. The area has a long history of particulate matter challenges and was previously designated nonattainment for the 1997 annual PM2.5 standard before being redesignated to attainment in 2023 after demonstrating compliance with the standard and implementing a long-term maintenance plan. The potential return to nonattainment reflects the much more stringent 2024 annual standard of 9.0 µg/m3, compared with the 15.0 µg/m3 standard under which Libby was redesignated in 2023.
Outside of Libby, EPA currently intends to designate the remainder of Montana as attainment/unclassifiable. However, EPA also identified Hill County and Teton County as areas where available monitoring data are insufficient to calculate the required three-year average and invited Montana to provide additional information before final designations are issued.
EPA Acknowledges Uncertainty
Notably, EPA’s designation letter emphasizes that final designations remain subject to change. The agency specifically highlighted several issues that could influence final outcomes, including:
- Potential exceptional-event demonstrations associated with wildfires and prescribed burns;
- Questions raised by states and industry groups regarding potential high bias associated with certain PM2.5 monitoring technologies; and
- Ongoing litigation challenging the 2024 PM2.5 standard itself, including pending Supreme Court review.
Montana based its initial recommendations on the three-year average of monitoring data collected from 2021 through 2023, while EPA intends to use the more recent three-year period of 2023 through 2025. The addition of 2024 and 2025 data could change the calculated air quality levels for individual monitors. However, Montana recommended that EPA designate the Libby area as nonattainment in their February 4, 2025 letter, so the state’s position is unlikely to change.
EPA requested that Montana submit any additional information for consideration by November 1, 2026, to help ensure EPA has sufficient time to evaluate it before finalizing designations. The letter acknowledges that the agency is operating under an expedited, court-ordered schedule and may lack some of the information or technical demonstrations that could support a different final designation.
Final area designations are currently expected by February 6, 2027, unless further judicial action alters the schedule.
Why This Matters
The 2024 annual PM2.5 standard lowered the allowable concentration from 12.0 µg/m³ to 9.0 µg/m³, creating a more stringent threshold for determining whether areas attain the standard. While Montana appears largely unaffected compared to some regions of the country, the proposed designation for Libby would continue the community’s long history of PM-related air quality challenges.
Although nonattainment designations are often associated with industrial permitting concerns, Libby’s PM2.5 challenges have historically been driven by a combination of residential wood heating, winter inversions, and other localized factors. Consequently, the proposed designation stems largely from longstanding community air quality challenges rather than emissions from major industrial sources.
Even so, a nonattainment designation can carry important regulatory implications. New and modified major sources locating within a designated nonattainment area may become subject to Nonattainment New Source Review (NNSR) requirements, including emissions offset obligations, Lowest Achievable Emission Rate (LAER) requirements, and additional permitting complexity. Future industrial development projects may therefore face greater regulatory hurdles and longer permitting timelines than similar projects located in attainment/unclassifiable areas.
What Montana Facilities Should Watch
Although most of Montana is currently expected to receive attainment/unclassifiable designations, project developers and facility owners should continue monitoring EPA’s designation process, particularly for projects in or near Libby. Companies planning major emission increases, new source construction, or large permitting actions over the next several years should evaluate whether changing PM2.5 attainment status could affect permitting strategy, project schedules, or regulatory requirements. If you would like to discuss how this proposed designation affects your Montana projects, reach out via email to Ed Warner or call 720.638.7647.
