On October 6, 2026, the United States Environmental Protection Agency (U.S. EPA) published a notice in the Federal Register that they had issued the required “120-day letters” to each of the 50 U.S. states related to the revised 2024 annual fine particulate matter (PM2.5) National Ambient Air Quality Standard (NAAQS). 120-day letters are the formal notices U.S. EPA sends to governors (and tribal leaders) near the end of the initial area designations process for a new or revised NAAQS to outline their proposed attainment designations related to that NAAQS. They come from Clean Air Act (CAA) section 107(d)(1)(B)(ii). The process around the 2024 annual PM2.5 NAAQS designations, however, has been anything but typical. This blog discusses the timelines, the letters and their implications, and the next steps.
What is the Purpose of a 120-Day Letter?
A typical 120-day letter explains U.S. EPA’s intended designation for each area (nonattainment, attainment/unclassifiable, or unclassifiable). Where U.S. EPA intends to designate a nonattainment area, the letter includes a technical support document covering the boundary analysis: the violating monitor, emissions data, traffic and commuting patterns, meteorology, geography, and jurisdictional boundaries. It often redraws a state’s proposed boundary, for example by adding a county with large sources that U.S. EPA concludes contributes to the violation. In the case of PM2.5, the extent of these boundaries can be tricky to determine and often larger than for other criteria pollutants, because some portion of the PM2.5 recorded at a monitor may be in the form of sulfates and nitrates that were emitted in gaseous form by sources well upwind of the monitor and later solidified into fine particulate and those sources may need to be considered when determinizing the size of the nonattainment area. Areas designated nonattainment for PM2.5 also automatically include sulfur dioxide (SO2) and nitrogen dioxide (NOX) as nonattainment precursors pollutants to PM2.5. In addition, as part of establishing nonattainment areas, states with U.S. EPA approval are required to determine if volatile organic compounds (VOC) and ammonia (NH3) significantly contribute to the nonattainment status determination and therefore should also be established as nonattainment precursor pollutants to PM2.5.
The Long and Confused PM2.5 NAAQS Designation Process
After U.S. EPA promulgates a NAAQS, states are required under CAA section 107(d) to submit designation recommendations to the U.S. EPA by a date the U.S. EPA specifies, which in the case of the 2024 annual PM2.5 NAAQS revision was February 7, 2025. These initial recommendations were to be based on the 2021-2023 design values at ambient air monitors within their states, with the expectation that U.S. EPA would modify those recommendations based on 2022-2024 design values.
In the event, only 45 of the 50 states submitted their required letters, with 5 states: Arizona, Arkansas, Illinois, Indiana, and Missouri not submitting at all. Of the 45 states that did submit their proposed designations, 39 recommended no nonattainment areas in their states. This is well documented in a white paper published by Earthjustice. To summarize:
- Of the 45 states that submitted, only six: Alaska, California, Michigan, Montana, Ohio, and Pennsylvania, recommended areas within their states for nonattainment.
- Of the 39 that recommended that their entire states be designated attainment/unclassifiable, 16 had monitors that showed nonattainment.
- Of those 16, nine (Alabama, Georgia, Idaho, Louisiana, Mississippi, North Carolina, Oregon, Tennessee, and Washington) submitted exceptional event analyses, suggesting events such as wildfire smoke or fireworks, as their reason for failing to meet the standard and that removing those events from the data showed that the monitors would have been in attainment if not for those events.
- One, Kansas, argued that its Teledyne T640 monitors have problems that result in concentrations that are biased high, and that they read high even after U.S. EPA, who is aware of the problem, adjusted to concentrations to correct the bias. North Dakota and Oklahoma made the same argument, though neither had a violating county.
- Texas was a special case in that the state agency, the Texas Commission on Environmental Quality (TCEQ) recommended 4 counties be designated nonattainment, but Texas’ Governor Greg Abbot overrode his own state agency and recommended the entire state be designated attainment/unclassifiable).
- The remaining 5 were the 5 that did not submit their recommendations at all.
U.S EPA does not have to accept the recommendations of the states. If it plans to modify a state’s recommended designation or boundary, the statute requires it to tell the state no later than 120 days prior to finalizing the designations. In practice, U.S. EPA sends them to every state, including states where it agrees with the recommendation, though it is not at all uncommon for the letters to add additional nonattainment areas, or adjust the boundaries of the nonattainment areas recommended by the state.
The Designation Timeline – How Did We Get Here?
According to the original schedule, U.S. EPA should have issued the 120-day letters by October 9, 2025, to give states 120 days to respond in time for the final designation date, which would have been February 6, 2026, based on the 2022-2024 design values. However, no letters were issued in 2025, and no designations were made in 2026. Complicating the matter was the legal effort by U.S. EPA and industry to reverse the 2024 NAAQS revision on the grounds that the previous administration’s off-cycle review was illegal. In March 2025, U.S. EPA announced that it would reconsider the 2024 revision to the annual PM2.5 NAAQS, and work on the designations apparently stopped.
After missing the deadline to get the 120-day letters out in November of 2025, U.S. EPA asked the U.S. Court of Appeals for the D.C. Circuit to vacate the 2024 standard. February 7, 2026, rolled around with no action by U.S. EPA, including a failure to request a one-year extension as allowed under CAA Section 107(d)(1)(B)(i). Outside groups immediately filed lawsuits. Then, to the surprise of many, on June 26, 2026, the D.C. Circuit unanimously upheld the 2024 standard, thereby removing U.S. EPA’s primary reason for delaying designations. The U.S. Court of Appeals for the 9th Circuit then set a designation date of February 6, 2027, thus requiring the 120-day letters to go out in October 2026, getting us to where we are now.
What is in the Letters?
Unlike typical 120-day letters from NAAQS revisions of the past, the recent letters contain little or no technical data, and no technical support document. Each of the letters discusses the litigation history and says that U.S. EPA is submitting the letters under protest. It also notes that U.S. EPA is appealing the D.C. Circuit Court decision to the Supreme Court of the United States, which is somewhat ironic in the sense that they are appealing a court case that they effectively won (although it was the previous administration’s version of U.S. EPA that set the 2024 standard). Every letter then says that absent relevant action from the judiciary, the U.S. EPA intends to finalize designations for the 2024 annual PM2.5 NAAQS based on the most recent monitoring data (2023-2025 data). There are two versions of the letters:
- States with no violating monitors: These letters are firm. The state recommendation letters say there are no violating monitors, and there are no violations based on the 2023-2025 design value, so the U.S. EPA intends to designate the entire state as “attainment/unclassifiable”.
- States with potentially violating monitors: These letters are deliberately noncommittal. In each case, U.S. EPA says preliminary 2023–2025 data shows one or more monitors that may not meet the standard. It adds that absent further processing, such as an exceptional events determination, the area, including nearby areas that contribute to the area not meeting the NAAQS, may be designated as nonattainment. They do not identify which areas might be nonattainment, let alone provide any indication of what the boundaries of those areas might be. The letters then give three possible outcomes instead of one intended designation:
- Nonattainment, if the data stands, but again no details.
- Attainment/unclassifiable, which U.S. EPA may designate if sufficient and appropriate information is received or becomes available prior to February 6, 2027.
- Unclassifiable, because of the difficulties associated with gathering and processing the information necessary for the designation process within such an abbreviated timeline. This is critical because a footnote almost hints that if a state should claim insufficient time to review, it can gain unclassifiable status rather than nonattainment.
U.S. EPA also indicates that some states have invalid design values for some of their monitors, but does not explain why they are invalid, or what U.S. EPA intends to do in terms of designations in those areas.
In the case of the states that identified nonattainment areas based on the 2021-2023 data, they get the same form letter as the states with 2023-2025 monitors that may be out of attainment, with the enclosure being the areas that the states recommended for nonattainment. They are given the same opportunity to submit supplemental data based on the 2023-2025 design values and to potentially declare their states as unclassifiable based on insufficient time to review.
What Does This All Mean?
U.S. EPA was clearly caught unprepared for the D.C. Circuit Court decision, and as a result many things remain up in the air in the PM2.5 designation process. The important outstanding questions are:
- Will U.S. EPA designate any areas in the United States nonattainment for the 2024 annual PM2.5 NAAQS or will they just issue blanket unclassifiable designations citing insufficient time?
- Will those states that were prepared to designate nonattainment areas continue to do so if the 2023-2025 data warrants such a designation (and will U.S. EPA approve them)?
- What will the Supreme Court decide, if they even agree to hear the case, and on what timeframe?
- If the 2024 PM2.5 NAAQS is vacated by the Supreme Court and the standard returns to the previous value, do the CAA anti-backsliding provisions prevent a return to an attainment classification for those areas that were designated nonattainment?
- If areas where the monitors show a violation of the NAAQS are designated unclassifiable rather than nonattainment, how would anyone permit a large project in those areas when cumulative air dispersion modeling would not be possible because the monitors by themselves are already over the NAAQS, and nonattainment permitting mechanisms would not be available?
On top of all this, the CAA dictates that U.S. EPA must review each NAAQS every five years to determine if the NAAQS is still adequate and correct, and 2025 was technically the year where that review should have been completed for the PM2.5 NAAQS. However, U.S. EPA has done very little to initiate that review.
It would appear that U.S. EPA, like they did prior to the D.C. Circuit court ruling, is banking that the Supreme Court will not only agree to hear their case, but also will rule in their favor, and therefore are doing the bare minimum required with respect to designations. In fact, there is some suggestion that what they’ve done in issuing these 120-day letters may not satisfy CAA requirements because of the lack of information presented in them. U.S. EPA is also appealing to the 9th Circuit with respect to the February 2027 designation deadline.
What’s Next?
In a nutshell, nobody really knows for sure. This leaves continued uncertainty for industry as to what the attainment status of various areas of the U.S. that are over the 2023-2025 design value might ultimately be, and what the permitting implications are.
ALL4 will continue to track regulatory and technical developments related to the 2024 annual PM2.5 NAAQS revisions as the designation date closes in. ALL4 can help you review your area’s ambient background and assess whether you are likely to be able to permit a project in your area. ALL4 has been heavily involved in advocacy work around this issue and is well connected with the industry associations and law firms working to achieve clarity. If you’d like to discuss the rapidly changing situation around the PM2.5 NAAQS or need help in assessing its potential impact on your facility, please contact Rich Hamel or your ALL4 project manager.
