The NO2 Standard Isn't Changing in 2026: What EPA's NAAQS Review Draft ISA Actually Means for Air Permitting
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The NO2 Standard Isn't Changing in 2026: What EPA's NAAQS Review Draft ISA Actually Means for Air Permitting

EPA's draft ISA opens the NO2 NAAQS review — but the standard isn't changing in 2026. What the science step means for air permitting and modeling headroom.

by iSi Environmental

If you saw a headline this week about EPA reviewing the nitrogen dioxide standard and wondered whether your air permitting strategy just changed — it didn’t. On July 17, 2026, EPA published the Integrated Science Assessment for Oxides of Nitrogen — Health Criteria, External Review Draft (91 FR 44841), the first scientific product of its active review of the NO2 NAAQS. There is no proposed rule, no new number, and no compliance obligation. The notice says so itself: “The draft ISA does not represent, and should not be construed to represent, any final EPA policy, viewpoint, or determination.”

That’s the whole story a plant manager needs today. But there are three reasons to read the rest of this post. First, the ISA is the document that — once finalized — becomes the scientific foundation for whether the current 100 ppb 1-hour and 53 ppb annual standards survive, and NO2 already bites in permitting: the 1-hour standard is a recurring dispersion-modeling constraint for facilities with boilers, engines, and turbines. Second, the last time EPA ran this review, in 2018, it retained both standards unchanged — a precedent that matters, because NAAQS reviews do not automatically produce tighter numbers, and the current administration’s deregulatory posture in air programs makes a tightening proposal plausible but far from the base case. Third, the one genuinely time-bound item here is voluntary: a public comment window on the draft science, open through September 15, 2026.

Below are the twelve questions we’re hearing from facilities with combustion sources — answered directly, with the primary sources linked.

Is the NO2 air quality standard changing in 2026?

No. EPA released a draft Integrated Science Assessment on July 17, 2026 (91 FR 44841) — the first science step in a multi-year review. No proposed rule exists, and the notice states the draft ISA represents no final EPA policy or determination. Comments are due September 15, 2026.

Read that carefully: a draft science document is not a standard change, and it is not a signal that one is imminent. The review process is governed by Clean Air Act sections 108 and 109 — section 109(d)(1) requires EPA to periodically review each NAAQS and revise it only “as may be appropriate.” Retention is a legitimate and historically common outcome. The 2018 NO2 review ended with both standards kept exactly where they were (EPA NO2 NAAQS timeline). Anyone telling you to re-plan capital projects around a tighter NO2 standard this year is ahead of the record.

What is the current NO2 NAAQS?

Two primary standards: a 1-hour standard of 100 ppb, met when the 3-year average of the 98th percentile of daily maximum 1-hour concentrations stays at or below that level, and an annual standard of 53 ppb (EPA primary NO2 NAAQS). Both were retained unchanged in EPA’s April 2018 review.

NO2 is the indicator species for the broader oxides-of-nitrogen family, which is why permit reviewers talk about “NOx emissions” but model ambient impacts as NO2. The 1-hour standard is the one that does the work in practice. The statistical form matters too: because compliance is measured at the 98th percentile of daily 1-hour maximums averaged over three years, short-duration peak impacts from intermittent sources — emergency engines are the classic case — drive modeling results in ways an annual average never would.

What is an Integrated Science Assessment (ISA)?

The ISA is EPA’s synthesis of the latest health science for a criteria pollutant, required as “air quality criteria” under CAA section 108(a)(2) (91 FR 44841). Once finalized — after public comment and CASAC review — it becomes the scientific foundation for EPA’s decision to retain or revise the NAAQS.

The draft now out for comment will be reviewed by the Clean Air Scientific Advisory Committee (CASAC) at a public meeting to be announced in a separate Federal Register notice. After the ISA is finalized, EPA staff prepare a policy assessment that translates the science into standard-setting options, and only then does a proposed rule follow. An ISA answers “what does the health evidence show?” — not “what should the number be?” Conflating the two is how a science document gets misread as a regulatory change.

What did EPA’s last NO2 review conclude?

The review completed in April 2018 retained both primary standards without revision — 100 ppb 1-hour and 53 ppb annual — concluding the existing standards protected public health with an adequate margin of safety (EPA NO2 NAAQS timeline). That precedent matters: NAAQS reviews do not automatically produce tighter numbers.

The 2018 review examined the same core categories of health evidence this new ISA updates — respiratory effects, asthma exacerbation, short-term exposure studies — and found the standards adequate. Combine that with two more facts: ambient NO2 concentrations have kept declining since 2018, and no area of the country violates the current standards. Commenters on the draft ISA will press both points hard, and they cut directly against any assumption that this review ends in a tighter standard.

When could a revised NO2 standard actually take effect?

Realistically, years out. This review’s call for information issued December 9, 2022 (87 FR 75625); the draft ISA arrived July 2026 — three and a half years for one science document. Still ahead: final ISA, policy assessment, CASAC review, proposed rule, public comment, and final rule — then designations and SIP revisions.

EPA’s own Integrated Review Plan (Vol. 2, March 2024) lays out the remaining sequence. Even in the scenario where EPA ultimately proposes and finalizes a tighter standard, the machinery that converts a revised NAAQS into facility-level consequences — area designations, state implementation plan revisions, updated permit modeling thresholds — runs on its own multi-year clock. Permitting impacts before the late 2020s are unlikely. That is not a reason to ignore the review; it is the reason to treat it as a planning signal instead of a fire drill.

How do I comment on the draft NO2 ISA?

Submit to Docket ID EPA-HQ-OAR-2023-0317 at regulations.gov — EPA’s preferred method — by September 15, 2026 (91 FR 44841). Email to a-and-r-Docket@epa.gov with the docket ID in the subject line is also accepted. Comments are public once posted; don’t include confidential business information.

To be precise about what this date is: a voluntary comment window on a draft science document, nothing more. Missing it carries zero compliance consequence. What it does offer is the earliest opportunity in this review to put data on the record — and the administrative record built now shapes everything downstream. If your trade association tracks NAAQS reviews, this is the window where industry monitoring data, exposure studies, and attainment-record arguments get filed. The upcoming CASAC public meeting (dates to be announced in a separate Federal Register notice) is the other early signal worth watching: CASAC deliberations telegraph where the science review is heading.

Which facilities would a tighter NO2 standard affect most?

Facilities with significant NOx combustion sources — industrial boilers, stationary engines, combustion turbines, process heaters — especially those planning PSD-permitted expansions (91 FR 44841). NO2 is the indicator for the NOx family, and permit applicants must model ambient NO2 impacts against the NAAQS before construction.

That watch-list spans manufacturing, food processing, power generation, oil and gas, and any campus running a large backup-generator fleet. The common thread is the permitting pathway: Prevention of Significant Deterioration (PSD) applicants — and nonattainment NSR applicants where applicable — must demonstrate through dispersion modeling that a project won’t cause or contribute to a NAAQS violation. Facilities planning expansions with new combustion equipment in the next 3–7 years have the most practical stake in where this review lands. Small facilities without PSD-scale NOx emissions can file this one under monitoring-only and move on.

How does the NO2 NAAQS affect air permitting today?

PSD applicants must demonstrate through dispersion modeling that a new or modified source won’t cause or contribute to a violation of the 100 ppb 1-hour NO2 standard (EPA primary NO2 NAAQS) — a frequent constraint for engine and turbine projects. Title V and SIP-based NOx limits also trace back to these standards.

This is the part of the story that exists regardless of what the review concludes. The 1-hour NO2 standard is already one of the tighter modeling demonstrations in a PSD application: near-field impacts from stacks, engine exhausts, and building downwash frequently consume most of the available increment between background concentrations and 100 ppb. Every future permit application at your site will be modeled against whatever standard exists at that time — which is why the practical planning question isn’t “is the standard changing?” but “how much modeling headroom does my site have under the current one?”

Are any areas in nonattainment for NO2?

No area of the country currently violates the NO2 NAAQS; ambient concentrations have declined for decades, and areas are designated attainment/unclassifiable (EPA NO2 NAAQS timeline). That clean national record is a central reason EPA retained the standards in 2018 — and a data point commenters will press in this review.

A nationwide attainment record is rare among criteria pollutants — compare ozone, where dozens of areas cycle through nonattainment classifications. For NO2, decades of mobile-source controls and combustion improvements have pushed monitored concentrations well below the standards in most locations. This is the strongest counter-signal against the tightening narrative: the argument “the current standards are demonstrably working” is sitting right there in EPA’s own monitoring data, and it will be filed into Docket EPA-HQ-OAR-2023-0317 by every industry commenter paying attention.

Would a tighter NO2 standard change my existing Title V permit?

Not directly or immediately. A revised NAAQS works through designations, SIP revisions, and new-permit modeling first (EPA Integrated Review Plan, Vol. 2). Existing permit limits stay in force; the squeeze appears when you next modify equipment or expand — tighter modeling thresholds and potentially stricter SIP-derived NOx limits over a multi-year runway.

Your current Title V permit does not reopen because EPA publishes a science document, and it would not reopen automatically even if EPA finalized a revised standard. The exposure is prospective: the next boiler replacement, engine addition, or capacity expansion that triggers permit review gets modeled against the standard in force at that time. That’s the mechanism by which a NAAQS revision — if one ever comes out of this review — would reach your facility, and it’s why the honest framing here is planning signal, not enforcement risk.

What are the penalties for violating CAA permit limits?

Clean Air Act §113(b) civil penalties run up to $124,426 per day, per violation at 2025-adjusted levels (40 CFR 19.4), which carry into 2026 because the annual inflation adjustment was canceled. NAAQS reviews don’t change penalty exposure — permit compliance does.

Two clarifications worth making. First, nothing about this ISA or the broader NAAQS review creates enforceable requirements or new penalty exposure — a draft science assessment carries no enforcement history because there is nothing in it to enforce. Second, the number itself: $124,426/day is the 2025-adjusted maximum carried forward into 2026, not a 2026 increase. Where CAA penalties actually land on facilities is the unchanged, unglamorous stuff — NOx limit exceedances found through stack testing or CEMS data, missed monitoring, reporting lapses. Meeting your existing permit conditions is what manages that exposure, this year and every year.

How do Kansas, Missouri, Oklahoma, Texas, and Nebraska handle NO2 requirements?

All five states are attainment for NO2 and implement the federal standard through EPA-approved SIPs (EPA primary NO2 NAAQS) — no state in the region imposes an NO2 ambient standard more stringent than federal. State agencies (KDHE, MoDNR, ODEQ, TCEQ, NDEE) run the minor NSR and modeling programs where the 1-hour standard shows up in practice.

For a facility in iSi’s primary region, that means the standard you model against is the same 100 ppb / 53 ppb federal pair regardless of which side of a state line you sit on. Where the states differ is procedure: each agency sets its own minor NSR applicability thresholds, modeling protocols, and Title V program mechanics, so the same engine project can face different demonstration requirements in Wichita than in Tulsa. If the federal standard ever tightens, these are the agencies that would carry it into practice through SIP revisions — on the same multi-year runway described above.

What to Do With This — and What Not to Do

Nothing on this list is mandatory. That’s the point of reading the record instead of the headlines. The practical moves:

  1. File this as a planning signal, not a compliance item. No requirement exists, no proposed rule exists, and the 2018 retention precedent plus a nationwide attainment record mean a tighter standard is a possibility to track, not an outcome to assume.
  2. If the review matters to your sector, get on the record. The voluntary comment window on the draft ISA runs through September 15, 2026, at Docket EPA-HQ-OAR-2023-0317 — directly or through your trade association. Watch for the CASAC meeting notice.
  3. Quantify your 1-hour NO2 modeling headroom now. This is the one action with value regardless of how the review ends. If you’re planning combustion-source projects — new engines, boiler replacements, an expansion with added NOx — document how much margin your site has between modeled impacts and the 100 ppb standard under current conditions. That number tells you what a future tightening would actually cost you, and it strengthens your next permit application either way.
  4. Keep meeting existing NOx limits and monitoring obligations. Unchanged by this notice, and the only place real penalty exposure lives.

The third item is where iSi’s environmental compliance team spends its time: potential-to-emit calculations, air permit determinations, and dispersion-modeling support that tell you — before you commit capital to an expansion — exactly how much NO2 headroom your site has and which equipment configurations preserve it. If you have a combustion-source project on the 2026–2028 horizon, call (316) 264-7050 and ask for an air permitting review. Knowing your margin while the standard is stable beats discovering it during permit review.

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