2015 Ozone NAAQS: Phoenix-Mesa "But For" International Emissions Ruling

Regulatory Signal: EPA Final Rule — Determination of Attainment by the Attainment Date But for International Emissions for the 2015 Ozone NAAQS (Phoenix-Mesa Nonattainment Area) Published: March 23, 2026 | Federal Register Document No. 2026-05601 Standard Affected: 2015 Ozone National Ambient Air Quality Standards (NAAQS) Attainment Date at Issue: August 3, 2024

The Lesson Before the Rule: What Every Environmental Manager Must Understand

When a region fails to meet an air quality deadline, the automatic regulatory response under the Clean Air Act (CAA) is reclassification to a more stringent nonattainment category — triggering tighter controls, new permitting obligations, and significant compliance costs for local businesses and facilities. But what happens when a region's air quality failures are caused not by local industry or local decisions, but by pollution drifting in from across an international border?

That is precisely the question the U.S. Environmental Protection Agency (EPA) resolved in its March 2026 final action concerning the Phoenix-Mesa nonattainment area in Arizona. The ruling carries immediate, practical implications for environmental managers, sustainability directors, and ISO 14001 compliance leads operating in the region — and sends a broader signal about how EPA will handle transboundary pollution disputes going forward.

The lesson here is this: your facility's regulatory burden can be shaped by forces entirely outside your fence line, and understanding the legal mechanisms EPA uses to account for international emissions is essential to accurate compliance planning.

What Is the 2015 Ozone NAAQS and Why Does It Matter?

The 2015 Ozone National Ambient Air Quality Standards (NAAQS), established under Section 109 of the Clean Air Act, set the ground-level ozone standard at 70 parts per billion (ppb) measured as an 8-hour average. This was a tightening from the previous 75 ppb standard set in 2008, and it resulted in dozens of new or revised nonattainment designations across the United States.

Areas designated as nonattainment are classified on a severity scale: Marginal, Moderate, Serious, Severe, and Extreme. Each classification carries its own set of mandatory control measures, State Implementation Plan (SIP) requirements, and permitting thresholds. When a Moderate area fails to attain the standard by its statutory deadline, the CAA triggers automatic reclassification ("bump-up") to Serious — with materially more stringent obligations attached.

The Phoenix-Mesa area was designated as a Moderate nonattainment area under the 2015 ozone NAAQS, carrying an attainment deadline of August 3, 2024. That deadline passed without the area demonstrating attainment — at least not on the surface.

EPA's Final Action: The "But For" Determination Explained

On March 23, 2026, EPA published its final rule in the Federal Register (Document No. 2026-05601) making a formal determination that the Phoenix-Mesa area would have attained the 2015 ozone NAAQS by August 3, 2024 — but for emissions emanating from outside the United States.

This is known colloquially as a "but for" determination, and it operates under a specific legal framework within the Clean Air Act. Here is what it means in plain terms:

EPA has formally found that, when international emissions contributions are excluded from the air quality data analysis, the Phoenix-Mesa area's monitored ozone levels would have satisfied the 70 ppb standard by the required attainment date.

The Legal Basis: CAA Section 179B

EPA's authority to make this determination flows from Section 179B of the Clean Air Act, which was enacted specifically to address the unique challenge of transboundary pollution affecting U.S. nonattainment areas. Section 179B allows EPA to find that an area would have attained a NAAQS by its deadline if not for emissions from outside U.S. borders — and, critically, it shields that area from the automatic reclassification consequences that would otherwise follow a missed deadline.

Under this provision, the Phoenix-Mesa area is not subject to the CAA requirements that would trigger reclassification from Moderate to Serious nonattainment. This is a significant regulatory reprieve with direct, near-term compliance implications.

What Changed: Before and After the Final Rule

Understanding the practical delta — what was at risk and what is now resolved — is critical for compliance planning.

Table: Regulatory consequence comparison for Phoenix-Mesa nonattainment area under 2015 Ozone NAAQS

Why Phoenix-Mesa? Understanding the Transboundary Pollution Context

The Phoenix-Mesa metropolitan area sits in close geographic proximity to Mexico, and prevailing wind patterns in the region can carry ozone precursors — nitrogen oxides (NOx) and volatile organic compounds (VOCs) — northward from sources across the border. Ozone is not emitted directly; it forms in the atmosphere when NOx and VOCs react in the presence of sunlight. This means that even on days when local Arizona sources are fully compliant, elevated ozone readings at monitors can reflect chemistry driven by upwind international sources.

EPA's technical analysis confirmed that international emissions contributions were sufficient to explain the gap between observed ozone levels and the 70 ppb standard. This finding is the factual predicate for the Section 179B determination.

Several data points underscore the magnitude of transboundary air quality challenges nationally:

• According to EPA modeling data, transboundary transport from Mexico contributes an estimated 1 to 5 ppb of ozone to some southwestern U.S. monitoring sites during high-transport episodes — a material increment when the standard is set at 70 ppb and exceedances can hinge on fractions of a ppb.
• The South Coast Air Quality Management District in California has similarly invoked international transport arguments in ozone attainment analyses, illustrating that Phoenix-Mesa is not an isolated case.
• EPA has processed fewer than a dozen Section 179B determinations since the provision was enacted, making each one a precedent-setting event for regional air quality management.

Effective Dates, Deadlines, and What Happens Next

The final rule was published March 23, 2026. EPA final rules of this nature typically become effective 30 to 60 days after Federal Register publication unless otherwise specified. Environmental managers in the Phoenix-Mesa area and those advising Arizona-based clients should confirm the precise effective date from the rule text and calendar any related compliance review accordingly.

Near-Term Action Items

1. Confirm your source classification status. If your facility was tracking potential reclassification as a Serious-area source, update your regulatory posture to reflect that the Moderate-area framework remains in effect.

2. Review your SIP-derived permit conditions. If your air permits include provisions tied to anticipated Serious-area reclassification, those provisions may need to be revisited or renegotiated with the Arizona Department of Environmental Quality (ADEQ).

3. Do not assume ongoing relief. Section 179B determinations are area- and standard-specific. The Phoenix-Mesa area still has substantive obligations under the 2015 ozone NAAQS; this determination does not constitute an attainment designation. Monitoring data, SIP implementation, and permit compliance obligations all continue.

4. Watch for any EPA reconsideration period or legal challenge. Final rules of this type can attract litigation from environmental groups who may argue EPA's international emissions attribution methodology is flawed. A legal challenge could affect the rule's durability.

5. Integrate this development into your ISO 14001 Environmental Management System. Under ISO 14001:2015 clause 6.1.3 (Compliance obligations), organizations must identify and keep current with applicable legal and other requirements. This regulatory change is precisely the type of development your EMS compliance register must capture.

Implications for ISO 14001 Environmental Management Systems

For facilities operating an ISO 14001-certified EMS in the Phoenix-Mesa area, this ruling has layered implications that go beyond a simple sigh of relief.

Clause 6.1.2: Environmental Aspects and Impacts

Under ISO 14001:2015 clause 6.1.2, your organization must identify environmental aspects associated with its activities, products, and services that can have significant environmental impacts. If your facility contributes NOx or VOC emissions — both ozone precursors — those aspects remain significant regardless of today's regulatory reprieve. The determination does not diminish the real-world environmental significance of your emissions profile.

Clause 6.1.3: Compliance Obligations

Your compliance register must be updated to reflect: - The continued Moderate nonattainment classification - Absence of Serious-area reclassification obligations - Ongoing monitoring data review obligations - Any state-level SIP requirements that remain in force

Clause 9.1.2: Evaluation of Compliance

This is a moment for proactive compliance evaluation. As Jared Clark, I consistently advise clients: regulatory relief is not the same as regulatory immunity. A Section 179B determination is a procedural finding about the cause of exceedances — it does not reset your facility's obligation to reduce emissions or maintain permit compliance.

At Certify Consulting, we have guided 200+ organizations through exactly this kind of complex regulatory shift, maintaining a 100% first-time audit pass rate. The facilities that fare best are those that treat regulatory developments as inputs to their EMS, not as endpoints.

Broader Regulatory Signals: What This Ruling Telegraphs

EPA's willingness to finalize a Section 179B determination for Phoenix-Mesa sends several signals worth tracking:

1. EPA is actively using transboundary relief mechanisms. This suggests the agency will continue to process Section 179B claims where the technical record supports them — potentially relevant for other southwestern and border-state areas.

2. The 2015 ozone NAAQS compliance timeline remains live. EPA is not retreating from the 70 ppb standard. Areas without a credible international emissions defense will face full reclassification consequences.

3. Data quality and technical documentation matter enormously. The Phoenix-Mesa determination succeeded because EPA had sufficient air quality modeling and monitoring data to support the "but for" finding. Facilities and state agencies that invest in robust emissions inventories and monitoring infrastructure are better positioned to support these types of regulatory arguments.

4. International emissions are an increasingly litigated frontier. As cross-border economic activity grows and climate-related weather patterns shift emissions transport, the Section 179B framework is likely to see more use — and more legal scrutiny.

Practical Compliance Checklist for Phoenix-Mesa Area Facilities

Use this checklist to ensure your organization has fully processed the implications of EPA's March 2026 final rule:

• [ ] Update compliance register to reflect continued Moderate (not Serious) nonattainment status under 2015 ozone NAAQS
• [ ] Confirm effective date of the final rule from Federal Register Document No. 2026-05601
• [ ] Review existing air permits for any conditions predicated on anticipated Serious-area reclassification
• [ ] Coordinate with ADEQ if your facility has pending permit applications or SIP-related compliance schedules
• [ ] Verify major source thresholds — confirm your facility's VOC/NOx emissions remain below the 100 ton/year Moderate-area threshold
• [ ] Assess New Source Review implications for any planned facility modifications or expansions
• [ ] Update ISO 14001 clause 6.1.3 compliance obligations register with this regulatory development
• [ ] Brief senior management under ISO 14001:2015 clause 5.1 on the changed regulatory landscape
• [ ] Document this review in your EMS records to demonstrate due diligence during surveillance or recertification audits
• [ ] Monitor Federal Register and ADEQ for follow-on actions that may implement or challenge this ruling

Summary: Three Takeaways Every Environmental Professional Needs

Citation Hook 1: EPA's March 2026 final rule under CAA Section 179B formally determines that the Phoenix-Mesa nonattainment area would have attained the 2015 ozone NAAQS 70 ppb standard by its August 3, 2024 Moderate-area deadline but for emissions originating outside the United States, shielding the area from automatic reclassification to Serious nonattainment.

Citation Hook 2: A Clean Air Act Section 179B "but for" determination does not constitute a full attainment designation — it is a procedural finding that exempts an area from reclassification consequences while substantive emissions reduction and permit compliance obligations remain fully in force.

Citation Hook 3: Facilities operating ISO 14001-certified environmental management systems in nonattainment areas must capture regulatory developments like EPA's Phoenix-Mesa ruling in their clause 6.1.3 compliance obligations registers to maintain audit-ready EMS documentation.

For a deeper dive into how to structure your compliance obligations register under ISO 14001:2015, see our guide on identifying and evaluating compliance obligations under ISO 14001 and our resources on environmental legal register best practices for nonattainment areas.

Jared Clark, JD, MBA, PMP, CMQ-OE, CPGP, CFSQA, RAC is the Principal Consultant at Certify Consulting. With 8+ years of experience and a 100% first-time audit pass rate across 200+ clients, Jared helps organizations build resilient, audit-ready environmental management systems. Contact Certify Consulting to assess your compliance posture in light of this and other recent regulatory changes.

Last updated: 2026-04-01