The Battle for the PM2.5 Annual NAAQS is Back On

Back on February 7, 2024 I wrote a blog on how the “hour of doom” was upon us as the PM_2.5 Annual NAAQS had been lowered from 12.0 µg/m³ to 9.0 µg/m³.  I likened the lowered NAAQS to Osgiliath falling in J.R.R. Tolkien’s “The Return of the King,” despite valiant efforts from the Men of Gondor.

Well, to continue my “Lord of the Rings” analogies, after Osgiliath fell Aragorn summoned the Dead Men of Dunharrow to fight for the men of Middle Earth.  These “dead men” were zombie-like warriors from long ago who had been cursed to never die until they could fulfill their (previously ignored) oath to fight for Isildur and his heirs.  Because Aragorn was a descendant of Isildur, they took the opportunity, helped defeat Sauron (the picture above), and were released from their curse.  To put it all in layman’s terms, these un-dead warriors helped the good guys win.

So with that context…could the PM_2.5 Annual NAAQS of 9.0 µg/m³ be returning to save the day like the Dead Men of Dunharrow?

On November 24, 2025 (the day before Thanksgiving), the U.S. Environmental Protection Agency (EPA) filed a motion for vacatur in the D.C. Circuit, asking the court to strike down its March 2024 tightening of the primary annual National Ambient Air Quality Standards (NAAQS) for fine particulate matter (PM_2.5), which lowered the threshold from 12.0 µg/m³ to 9.0 µg/m³.  The agency argued the revised standard was finalized “without the rigorous, stepwise process” required under the Clean Air Act.  This filing is legally notable because agencies rarely ask courts to discard their own rules.

Beyond public-health and regulatory headlines, this legal move carries significant implications for air dispersion modeling, which plays a vital role in permitting for emissions sources under programs like New Source Review, Prevention of Significant Deterioration, and State Implementation Plans.  Since the PM_2.5 Annual NAAQS was lowered to 9.0 µg/m³ the regulated community has been living with a lot less “head room,” when it comes to NAAQS compliance, with background concentrations in some instances taking up nearly all the NAAQS—leaving very little “head room” for modeled impacts.  So this (anticipated for some time) revisiting of the PM_2.5 Annual NAAQS could have huge ramifications for the modeling community.  If the court grants vacatur and the standard reverts to 12.0 µg/m³, many ongoing modeling efforts may get instant relief by no longer requiring onerous operational restrictions, controls, etc.

With regards to timing of how this plays out, last week's motion asks the court to act with an eye on the February 7, 2026 nonattainment designation deadline.  That means that we could get a short, expedited decision over the coming weeks to a few months, with more complex briefings or appeals stretching out over a year.  Possible outcomes include immediate vacatur (reverting back to the 12.0 µg/m³), remand without vacatur (keeping it at 9.0 µg/m³ while EPA redoes its process) or a partial remedy tailored to procedural defects.

In short: for modelers and permitters, the legal fight isn’t academic. A favorable ruling for vacatur could restore greater regulatory “breathing room,” but until the court acts (and potentially until EPA issues updated guidance), many projects will remain in limbo.