Ongoing Compliance at the Moment of Redesignation:
Sierra Club v. EPA and the Meaning of “Has Met All Requirements Applicable to the Area”

I. Introduction

In Sierra Club v. U.S. Environmental Protection Agency, Nos. 23‑3581/3583 (6th Cir. Dec. 5, 2025), the United States Court of Appeals for the Sixth Circuit addressed two intertwined questions under the Clean Air Act (CAA):

Whether EPA acted arbitrarily and capriciously in approving Michigan’s request to treat ozone exceedances as “exceptional events” caused by Canadian wildfires; and
Whether EPA lawfully redesignated the Detroit area from “nonattainment” to “attainment” for the 2015 ozone National Ambient Air Quality Standard (NAAQS) even though Michigan had not implemented Reasonably Available Control Technology (RACT) measures that became due after the State submitted its redesignation request.

Judge Helene White, writing for a unanimous panel (Judges Cole, White, and Davis), upheld the EPA’s scientific and technical judgment on the wildfire “exceptional event,” but vacated the EPA’s redesignation of Detroit to attainment. The decision is recommended for publication and thus is precedential in the Sixth Circuit.

The opinion is especially significant because it:

Applies the Supreme Court’s post‑Loper Bright framework, rejecting deference to EPA’s legal interpretation of the Clean Air Act; and
Adopts a strong textualist reading of 42 U.S.C. § 7407(d)(3)(E)(v), holding that a state must have complied with all applicable requirements as of the date of redesignation, not merely as of the date the redesignation application was filed.

The result is a powerful new precedent on the timing and completeness of state obligations when seeking to convert nonattainment areas to attainment under the Clean Air Act, with direct consequences for EPA, states, industry, and environmental groups in the Sixth Circuit and potentially beyond.

II. Overview of the Case

A. Parties and Procedural Posture

The petitioner, Sierra Club, challenged two separate but related final EPA actions concerning the Detroit, Michigan ozone nonattainment area:

EPA’s approval of Michigan’s “exceptional event” demonstration for June 24–25, 2022 ozone exceedances allegedly caused by Canadian wildfire smoke; and
EPA’s final rule redesignating the Detroit area from Marginal/Moderate nonattainment to attainment for the 2015 ozone NAAQS and approving a maintenance plan.

The petition for review arose under the Clean Air Act and Administrative Procedure Act (APA), and the court reviewed EPA’s actions under the familiar “arbitrary and capricious” standard of 5 U.S.C. § 706(2)(A) and the statutory‑authority limitation in § 706(2)(C).

B. Regulatory Context

The dispute sits within the broader CAA framework:

National Ambient Air Quality Standards (NAAQS): Under 42 U.S.C. §§ 7408–7409, EPA sets NAAQS for pollutants such as ozone. Areas are designated “attainment,” “nonattainment,” or “unclassifiable” for each standard. For ozone, attainment is measured using a “design value”—the three‑year average of the fourth‑highest daily maximum eight‑hour ozone concentration. See 40 C.F.R. pt. 50, app. U.
State Implementation Plans (SIPs): States must adopt SIPs, through state rulemaking, showing how they will attain and maintain NAAQS. Once approved, SIPs are enforceable as federal law. 42 U.S.C. §§ 7410, 7413, 7604.
Nonattainment Classifications: For ozone, 42 U.S.C. § 7511a establishes classifications (Marginal, Moderate, Serious, Severe, Extreme), each carrying escalating requirements. Moderate and above require RACT for certain sources, including volatile organic compound (VOC) sources. § 7511a(b)(2).
“Bump‑ups” for Failure to Attain: If an area fails to meet a NAAQS by its statutory deadline, it is automatically reclassified (“bumped up”) to a more stringent level, triggering additional obligations. 42 U.S.C. § 7511(b)(2), § 7511a(i).
Redesignation to Attainment: A governor may request redesignation once an area is meeting a NAAQS. EPA may only redesignate if five criteria in 42 U.S.C. § 7407(d)(3)(E) are satisfied, including that the state “has met all requirements applicable to the area under section 7410 and part D.” § 7407(d)(3)(E)(v).
Clean Data Determinations (CDD): Under 40 C.F.R. § 51.1318, EPA can determine an area is meeting a NAAQS even while it remains designated nonattainment; this suspends certain planning obligations until redesignation or a finding of failure to maintain.
Exceptional Events: EPA may exclude air‑quality data influenced by “exceptional events” (natural events or rare human‑caused events) if the state satisfies strict demonstration requirements. 42 U.S.C. § 7619(b); 40 C.F.R. § 50.14.

C. Detroit’s Regulatory History

The key chronology in Detroit is:

2018: EPA designates the Detroit area as Marginal nonattainment for the 2015 ozone NAAQS. 83 Fed. Reg. 25,776, 25,813 (June 4, 2018).
Attainment deadline: August 3, 2021 for Marginal areas. 87 Fed. Reg. 21,842, 21,849 (Apr. 13, 2022).
January 2022: Michigan submits a redesignation request to move Detroit to “attainment” based on 2019–2021 data. 87 Fed. Reg. 14,210 (Mar. 14, 2022).
April 13, 2022: EPA proposes to find Detroit failed to attain by the August 3, 2021 deadline and proposes to reclassify it as Moderate, with a RACT SIP due January 1, 2023. 87 Fed. Reg. 21,842.
June 24–25, 2022: Detroit monitors record ozone exceedances. Michigan attributes them to Canadian wildfires.
January 2023: Michigan submits an “exceptional event” demonstration asking EPA to exclude June 24–25 data.
February 1, 2023: EPA finalizes the failure‑to‑attain determination and bumps Detroit up to Moderate nonattainment, moving the RACT SIP deadline to March 1, 2023. 88 Fed. Reg. 6,633 (Feb. 1, 2023).
February 3, 2023: EPA proposes a Clean Data Determination (CDD) for Detroit, based on 2020–2022 data excluding the wildfire days. 88 Fed. Reg. 7,382 (Feb. 3, 2023).
May 19, 2023: EPA (1) finalizes the CDD, approving Michigan’s exceptional‑events demonstration, and (2) finalizes the redesignation rule, moving Detroit to attainment. 88 Fed. Reg. 32,584; 88 Fed. Reg. 32,594 (May 19, 2023).

Crucially, Michigan never submitted or implemented a RACT SIP to satisfy the new Moderate‑area requirements due by March 1, 2023. EPA nonetheless proceeded with redesignation, reasoning that § 7407(d)(3)(E)(v) required only that the state had met the requirements that were “applicable” when Michigan submitted a complete redesignation request in January 2022—not those that later became due.

III. Summary of the Sixth Circuit’s Decision

The court split Sierra Club’s challenge into two principal issues:

Exceptional Event Approval. Applying deferential APA review, the court held that EPA’s approval of Michigan’s exceptional‑event demonstration was not arbitrary or capricious. EPA had reasonably concluded that Canadian wildfire smoke caused the June 24–25 ozone exceedances, relying on Brown Carbon (BrC) measurements, particulate data, meteorology, satellite imagery, and a “matching‑day” analysis.
Redesignation to Attainment and RACT. On the legal question whether § 7407(d)(3)(E)(v) allowed redesignation despite Michigan’s failure to implement RACT for the Detroit Moderate nonattainment area, the court rejected EPA’s interpretation. Drawing on Loper Bright and textual analysis, the court held:
- The phrase “has met all requirements applicable to the area” requires that those requirements be met as of the date of redesignation, not just as of the date the state filed a complete redesignation request.
- Because Michigan had not implemented the Moderate‑area RACT obligations that came due March 1, 2023, EPA lacked authority to redesignate the area to attainment in May 2023.

The court therefore:

Affirmed EPA’s approval of the wildfire “exceptional event” and the associated Clean Data Determination; but
Vacated the Detroit redesignation rule because EPA exceeded its statutory authority under § 7407(d)(3)(E)(v).

As a result, Detroit remains designated as a Moderate nonattainment area that is currently “clean” for ozone (under the CDD) but cannot yet be redesignated to “attainment” until it has implemented all applicable RACT obligations.

IV. Analysis of the Opinion

A. Exceptional Events: Deference to Agency Expertise Under the APA

1. Legal Standard

Under the APA, the court must set aside agency action that is “arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law” or “in excess of statutory jurisdiction.” 5 U.S.C. § 706(2)(A), (C). The opinion canvasses standard authorities:

Motor Vehicle Mfrs. Ass’n v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29, 43 (1983): an agency must articulate a rational connection between the facts found and the choice made, and may be reversed if it ignored important factors, contradicted the record, or offered implausible reasoning.
FCC v. Prometheus Radio Project, 592 U.S. 414, 423 (2021): courts check whether the agency acted within a “zone of reasonableness” and reasonably explained its decision.
St. Marys Cement, Inc. v. EPA, 782 F.3d 280, 286 (6th Cir. 2015): courts are “most deferential” when reviewing an agency’s technical or scientific determinations within its expertise.

Here, the court emphasizes that EPA’s approval of an exceptional‑event demonstration is both highly technical and governed by the detailed criteria in 40 C.F.R. § 50.14. The key contested regulatory element was whether Michigan demonstrated a “clear causal relationship” between the Canadian wildfires and the Detroit ozone exceedances. See 40 C.F.R. § 50.14(c)(3)(iv)(B).

2. EPA’s Weight-of-Evidence Approach

EPA did not rely on a single dataset. It adopted a “weight‑of‑evidence” evaluation of Michigan’s demonstration, including:

Brown Carbon (BrC) measurements: BrC is a by‑product of incomplete combustion and is used as an indicator (“tracer”) of wildfire smoke. Elevated BrC was observed in the Detroit monitoring network.
Particulate matter (PM10 and PM_2.5): Michigan looked at coarse (PM10) and fine (PM_2.5) particles. EPA acknowledged that PM10 is typically less useful for long‑range smoke transport, and Michigan did not rely on PM10 alone.
Meteorology: EPA determined that local meteorological conditions on June 24–25, 2022, were not conducive to high ozone based solely on local emissions, and that smoke had been transported to Detroit from Canadian fires via northerly winds and a cold front.
Satellite imagery and back‑trajectories: Michigan tracked smoke plumes and air mass trajectories from major fires into the region.
Matching‑day analysis: Michigan compared June 24–25 with other days exhibiting similar meteorological conditions, showing that such conditions historically did not yield similar ozone levels absent wildfire smoke.

EPA explicitly acknowledged contradictory evidence, such as Lake Michigan Air Directors Consortium (LADCO) screening analyses showing relatively low standard deviations in certain pollutants, and the absence of a strong PM_2.5 peak. EPA explained why, in its expert judgment, that contrary evidence did not outweigh other indicators (especially BrC and meteorology) demonstrating smoke’s presence at ground level at the relevant monitor.

3. Sierra Club’s Challenges and the Court’s Responses

Sierra Club advanced several technical critiques:

Reliance on BrC and PM10: Sierra Club argued BrC peaked on June 23 (not June 24–25) and that EPA had itself cautioned against relying on PM10 for long‑range smoke. The court noted that EPA:
- Expressly recognized PM10’s limitations and did not rely on it alone; and
- Provided a reasoned meteorological explanation for why BrC peaking on June 23 still indicated the presence and persistence of wildfire emissions affecting surface ozone on June 24–25 (transport by northerly winds, post‑frontal subsidence, and stagnant high‑pressure conditions).
LADCO screening analysis: Sierra Club emphasized that LADCO’s screening did not show strong smoke signatures. EPA, however, treated the LADCO results as one data point within a broader body of evidence, finding stronger corroboration in BrC, trajectory, and meteorological data. The court found this balancing of evidence reasonable under the APA.
Omission of certain days in the matching‑day analysis: Michigan excluded some days from its matching‑day dataset on the ground that they were also affected by smoke, but did not separately document smoke’s influence on those days. The court held that, given the extensive supporting evidence (historical ozone distributions, satellite imagery, BrC data, trajectory analyses, meteorology, and the matching‑day comparison), EPA did not act arbitrarily in accepting Michigan’s treatment of that subset.
Failure to analyze local emissions (new sources): Sierra Club argued that newly operating local sources of ozone precursors near the monitor could account for the exceedances and that EPA failed to disentangle their impact from smoke. The court pointed to EPA’s conclusion that “meteorological conditions on the exceedance days examined in conjunction with local and background emissions do not present the conditions conducive to producing elevated ozone concentrations,” so “the exceedances at issue were due to wildfire smoke, rather than local pollution.” 88 Fed. Reg. 32,586.

In each instance, the court emphasized that EPA:

Confronted the contrary evidence directly, rather than ignoring it;
Explained why other evidence was more probative; and
Reasonably applied its weight‑of‑evidence, case‑specific approach under 40 C.F.R. § 50.14.

Given the inherently technical nature of atmospheric science and exceptional‑event causation, the court deferred to EPA’s expertise and upheld the exceptional‑event approval and the associated Clean Data Determination.

B. Redesignation and the Present-Perfect Tense in § 7407(d)(3)(E)(v)

The far more consequential part of the decision lies in the court’s rejection of EPA’s legal interpretation of 42 U.S.C. § 7407(d)(3)(E)(v), which shapes when and how nonattainment areas may be redesignated as attainment.

1. The Statutory Provision and EPA’s Interpretation

Section 7407(d)(3)(E) authorizes redesignation only if five criteria are met. Subsection (v) provides:

The Administrator may not promulgate a redesignation of a nonattainment area (or portion thereof) to attainment unless— […] (v) the State containing such area has met all requirements applicable to the area under section 7410 of this title and part D.

EPA had long interpreted this clause to mean that:

A state needed to have satisfied only those § 7410 and Part D requirements that were “applicable” at the time the state submitted a complete redesignation request; and
New obligations that arose later—for example, due to a bump‑up from Marginal to Moderate nonattainment while the request was pending—did not need to be met before EPA could finalize redesignation.

Relying on that understanding, EPA concluded that Michigan’s failure to submit and implement a Moderate‑level RACT SIP by March 1, 2023 did not bar redesignation because those Moderate RACT obligations arose after Michigan filed its January 2022 redesignation request.

2. Post-Loper Bright: No Chevron Deference, Independent Statutory Construction

The Sixth Circuit explicitly grounded its interpretive method in Loper Bright Enterprises v. Raimondo, 603 U.S. 369 (2024), which overruled Chevron. The court quoted Loper Bright:

Courts must exercise their independent judgment in deciding whether an agency has acted within its statutory authority, as the APA requires… But courts need not and under the APA may not defer to an agency interpretation of the law simply because a statute is ambiguous.

Thus, while giving “due respect” to the Executive Branch’s views, the court undertook its own textual and contextual reading of § 7407(d)(3)(E)(v), unmediated by Chevron’s two‑step framework.

3. Textual Focus on the Present Perfect: “Has Met”

The central interpretive move concerns the verb tense in § 7407(d)(3)(E)(v): “has met.” The court drew on several authorities:

United States v. Wilson, 503 U.S. 329, 333 (1992): Congress’s use of verb tense is “significant” in statutory interpretation.
Commonwealth of Ky. v. EPA, 165 F.3d 26 (6th Cir. 1998) (unpublished): In construing “has attained” in § 7407(d)(3)(E)(i), the court had already observed that the present perfect “denotes past action with an abiding effect or continuing relevance.” That case held “has attained” requires attainment to continue until the date of redesignation.
Hewitt v. United States, 606 U.S. 419 (2025): Interpreting the First Step Act’s phrase “a sentence… has been imposed,” the Supreme Court emphasized that the present perfect indicates an event that remains true with “continued legal validity” at the time of application.

Applying this logic, the court reasoned that “has met” in § 7407(d)(3)(E)(v) likewise refers to:

past action with an abiding effect or continuing relevance, and therefore requires that all applicable requirements must be met until the date of redesignation.

If Congress had meant to refer only to requirements that were met as of some past point (such as the date of the application), it could have used a different construction (for example, “had met”) or explicitly tied “applicable” to the time of submittal. The use of “has met” suggests an ongoing state of compliance as of the time EPA exercises its redesignation authority.

The court also observed that EPA’s reading would effectively render the auxiliary “has” meaningless: the statute would have the same operative effect if it simply said the state “met all requirements applicable…,” which offends the rule against superfluity (Kungys v. United States, 485 U.S. 759, 778 (1988)).

4. Statutory Context: Escalating Requirements, Bump-Ups, and SIP Enforceability

The court’s reading of “has met” is reinforced by the broader statutory scheme:

Escalating Requirements via Bump-Ups: Under § 7511(b)(2) and § 7511a, nonattainment areas are subject to stricter requirements (like RACT) when they fail to attain NAAQS by statutory deadlines. The trigger for a bump‑up is the failure to meet the deadline, not a worsening of air quality.
Continuing Applicability of SIPs Despite Redesignation Requests: Section 7407(d)(3)(D) expressly provides that submitting a redesignation request “shall not affect the effectiveness or enforceability of the applicable implementation plan.” In other words, a pending redesignation request does not freeze the regulatory landscape or excuse states from complying with new SIP obligations arising from a bump‑up.
Nature of the “Applicable Implementation Plan”: Once a plan (including any revisions like a RACT SIP) is approved, it becomes the “applicable implementation plan” and is enforceable as federal law. States cannot sidestep that by failing to submit required SIP revisions.

Taken together, the scheme contemplates:

States may file redesignation requests while still subject to evolving and escalating obligations; and
EPA must ensure those obligations have been met as of the date it promulgates the redesignation, not merely as of an earlier application date.

Allowing EPA to disregard obligations that became due while the request was pending, the court warned, would:

“render toothless” the requirement that states continue complying with the Act and their SIPs even after submitting redesignation requests;
Create a perverse incentive to file early redesignation requests to lock in a more favorable set of obligations; and
Effectively let states avoid required RACT and other controls by “simply request[ing] redesignation and ignor[ing] any changes required after the date of the request.”

5. Rejection of EPA’s “State-Focused” Timing Argument

EPA advanced a textual argument based on § 7407(d)(3)(E)(v) being phrased in terms of the state’s obligations (“the State… has met all requirements”), whereas the other subparagraphs (i)–(iv) refer to determinations the Administrator must make. EPA argued:

Because (v) is state‑focused, it should be read as referring to the period of the state’s active role (i.e., up to submission of the request), implying a cut‑off at the application date; and
The Administrator’s conditions in (i)–(iv) govern EPA’s role at the time of redesignation, but (v) does not.

The court rejected this for four reasons:

Section 7407(d)(3)(D) expressly denies any “freeze” effect: The statute forbids using a redesignation request to delay or suspend SIP obligations. States have a continuing duty of compliance, and (v) naturally captures that ongoing responsibility.
States’ primary responsibility is central to the CAA: The CAA places “responsibility for meeting the NAAQS” on states. Referring to the state’s compliance in (v) is entirely consistent with that structure and does not imply any timing carve‑out.
The operative constraint is on the Administrator: Section 7407(d)(3)(E) begins with “The Administrator may not promulgate a redesignation… unless…,” and all five subparagraphs limit EPA’s authority. Subparagraph (v), no less than (i)–(iv), describes a condition the Administrator must ensure is satisfied at the time of redesignation. Prior Sixth Circuit precedent, such as Greenbaum v. EPA, 370 F.3d 527, 531 (6th Cir. 2004), had already treated (v) as a constraint on EPA (“The EPA may not redesignate an area… unless… the EPA has determined that the State… has met all applicable SIP requirements…”).
Contextual reading is required: Following Southwest Airlines Co. v. Saxon, 596 U.S. 450, 455 (2022), the court read the phrase “has met all requirements applicable” in context, as part of a single list of conditions limiting EPA’s redesignation authority, rather than isolating the state‑focused wording in (v).

6. EPA’s Practical Concerns and the Court’s Workability Response

EPA warned that if “applicable” requirements are measured as of the date of redesignation, not the date of application:

New obligations (like RACT after a bump‑up) could arise while a request is pending; and
The statutory 18‑month window for EPA to act on a redesignation request might be difficult to meet if additional SIP submissions are required mid‑stream.

The court was unpersuaded, suggesting practical solutions:

EPA can deny the redesignation request within the 18‑month period if newly applicable requirements remain unmet and invite the state to submit a new request once compliance is achieved.
EPA itself has flexibility in setting RACT compliance dates; for Detroit, EPA acknowledged it could have given up to two years for the RACT SIP, but chose a shorter period (until March 1, 2023) for reasons of consistency and the ozone season. EPA can manage deadlines and redesignation timing to avoid conflicts.
If EPA grants redesignation before stricter requirements come due, those later obligations simply never become “applicable” to the area as a nonattainment area, avoiding the problem entirely.

What EPA cannot do, the court held, is:

require RACT implementation by a certain date, and then ignore a state's failure to comply in assessing a redesignation request.

7. Application to Detroit and the Holding

Applying its construction, the court concluded:

Detroit was reclassified to Moderate nonattainment effective February 1, 2023.
The Moderate‑area RACT SIP and implementation were due March 1, 2023.
Michigan had not submitted or implemented a RACT SIP by that date.
When EPA promulgated the redesignation rule on May 19, 2023, Michigan therefore had not “met all requirements applicable to the area under section 7410 and part D.”

EPA’s redesignation action thus exceeded its statutory authority under § 7407(d)(3)(E)(v), and the court vacated the redesignation rule while leaving the exceptional‑event approval and Clean Data Determination intact.

V. Precedents and Authorities: How They Shaped the Decision

A. Administrative Law and Arbitrary-and-Capricious Review

Motor Vehicle Mfrs. Ass’n v. State Farm: Provided the baseline standard for reviewing EPA’s technical decisions on exceptional events—requiring rational explanation and consideration of relevant factors.
FCC v. Prometheus Radio Project: Reinforced the theme of a “zone of reasonableness” and highlighted the limited nature of arbitrary‑and‑capricious review, especially for predictive or technical judgments.
St. Marys Cement, Inc. v. EPA: A Sixth Circuit case that underscored deference to EPA’s scientific expertise in environmental matters, which the panel followed in upholding the exceptional‑event determination.

B. Clean Air Act Structure and SIP Obligations

Greenbaum v. EPA, 370 F.3d 527 (6th Cir. 2004): The panel relied on Greenbaum for the proposition that § 7407(d)(3)(E) collectively imposes conditions on EPA’s authority to redesignate and that subsection (v) is part of those constraints.
Wall v. EPA, 265 F.3d 426 (6th Cir. 2001): Emphasized states’ primary responsibility for meeting NAAQS, supporting the idea that § 7407(d)(3)(E)(v)’s focus on state compliance is consistent with the broader scheme and not evidence of a timing exception.
Commonwealth of Ky. v. EPA, 165 F.3d 26 (6th Cir. 1998) (unpublished): Provided the direct antecedent for reading “has attained” in § 7407(d)(3)(E)(i) as requiring continued attainment up to the redesignation date; the panel applied the same logic to “has met” in subsection (v).

C. Statutory Interpretation and Verb Tense

United States v. Wilson, 503 U.S. 329 (1992): Recognized that verb tense matters in statutory construction.
Hewitt v. United States, 606 U.S. 419 (2025): Supplied an up‑to‑date Supreme Court analysis of the present perfect (“has been imposed”) to support the proposition that Congress uses that tense to refer to conditions that remain true and legally valid at the time of application.
Southwest Airlines Co. v. Saxon, 596 U.S. 450 (2022): Reinforced that statutory terms must be read in context and with an eye to the statute’s structure, not in isolation.
Kungys v. United States, 485 U.S. 759 (1988): Warned against interpretations that render statutory language redundant or superfluous, which the court invoked in rejecting EPA’s reading that would effectively erase “has.”

D. Post-Chevron Administrative Law

Loper Bright Enterprises v. Raimondo, 603 U.S. 369 (2024): By overruling Chevron, Loper Bright obliged courts to exercise independent judgment in interpreting statutes, without automatic deference to reasonable agency readings of ambiguous provisions. The panel explicitly applied this framework in deciding that EPA’s interpretation of § 7407(d)(3)(E)(v) was not controlling and, ultimately, was inconsistent with the statute.

VI. Impact and Implications

A. For EPA’s Redesignation Practice

The core holding—that a state must have met all requirements applicable at the time of redesignation, including those that arose while the redesignation request was pending—directly challenges EPA’s historical approach to redesignations under § 7407(d)(3)(E)(v).

Consequences include:

Revision of EPA Guidance and Practice: EPA will likely need to revisit redesignation guidance and precedents that pegged “applicable” requirements to the date of complete submittal.
Closer Coordination of RACT and Redesignation Timing: EPA may have to:
- Set RACT compliance dates cognizant of pending or anticipated redesignation requests; and
- Choose between (a) acting quickly on a redesignation before new obligations come due, or (b) requiring states to fully comply with new requirements before redesignation.
Potential for More Denials or Delays: Where bump‑ups or new CAA requirements arise while a request is pending, EPA may be forced to deny or hold redesignations until the new obligations are actually met.

B. For States in the Sixth Circuit

For Michigan, Ohio, Kentucky, and Tennessee, the decision has immediate operational significance:

No “Safe Harbor” from Subsequent Obligations: Filing a redesignation request no longer offers any prospect of “locking in” a snapshot of requirements. States must plan on satisfying all obligations that come due up to the date EPA issues a final redesignation rule.
Greater Incentive to Meet Deadlines: If an area fails to attain by its deadline and is bumped up, the new set of requirements will have to be satisfied before redesignation—regardless of when the redesignation request was filed.
Strategic Use of Clean Data Determinations: States may still seek Clean Data Determinations to secure suspension of certain planning requirements (e.g., for reasonable further progress, some contingency measures) while remaining nonattainment, but this ruling clarifies that CDDs do not suspend RACT obligations or otherwise soften the requirements of § 7407(d)(3)(E)(v).

C. For Environmental and Industry Stakeholders

Environmental Groups: The decision gives environmental petitioners a powerful statutory hook to challenge redesignations whenever:
- A nonattainment area was bumped up to a more stringent classification; and
- The state has not yet fully implemented the more stringent requirements by the time of redesignation.
It elevates the role of timing and compliance in litigation and may lead to more challenges focusing on whether RACT and other Part D requirements are actually in place rather than only promised.
Regulated Industry: Businesses may experience:
- Longer periods under nonattainment designations (which carry stricter New Source Review, offset ratios, and other controls), even when ambient air quality is currently meeting NAAQS; and
- More pressure on states to adopt and enforce RACT and other measures that can impose additional costs.

D. Exceptional Events and Future Wildfire Smoke Disputes

While the redesignation ruling is the more novel and impactful, the court’s treatment of exceptional events also matters in a climate‑driven era of increasing wildfire smoke:

Affirmation of the “Weight-of-Evidence” Model: The court validates EPA’s practice of integrating multiple data types—BrC, PM, meteorology, satellite imagery, trajectories, and historical comparisons—rather than relying on a single diagnostic.
Limited Role of Screening Tools: LADCO‑type screening analyses are useful but not dispositive; EPA may override them where other evidence of smoke influence is strong.
Deference to Technical Judgment: Absent clear methodological errors or disregard of contrary evidence, courts will likely continue to uphold EPA’s exceptional‑event determinations, even in a post‑Loper Bright world, because these are questions of fact and expertise rather than law.

E. The Post-Loper Bright Landscape

Sierra Club v. EPA exemplifies a bifurcated approach in the post‑Loper Bright era:

Law: On legal questions (such as the meaning of “has met all requirements applicable to the area”), the court does not defer to EPA’s “reasonable” interpretation. It instead undertakes a full textual and structural analysis, consulting grammar, tense, context, and related precedents.
Science and Fact: On scientific and technical determinations (such as whether Canadian wildfire smoke caused particular ozone exceedances), the court remains “most deferential,” reviewing only for arbitrariness or failure to explain.

This division likely foreshadows future environmental litigation: more success for challengers on pure questions of statutory interpretation, but continued high barriers for overturning EPA’s technical judgments absent clear record defects.

VII. Simplifying Key Concepts

A. NAAQS, Design Values, and Nonattainment

NAAQS: EPA sets nationwide limits for pollutants like ozone to protect public health and welfare.
Design value for ozone: EPA takes the highest 8‑hour ozone reading each day, picks the fourth‑highest day for each year, and then averages that value over three consecutive years. If that average exceeds the NAAQS, the area is in nonattainment.
Nonattainment Classes: Marginal and Moderate are lower‑level nonattainment classifications; each higher class brings more stringent control requirements and longer deadlines.

B. SIPs and RACT

SIP: A state’s blueprint—approved by EPA—showing how it will meet and maintain air quality standards. Once approved, its provisions become enforceable federal law.
RACT: “Reasonably Available Control Technology” is defined in Sixth Circuit precedent (State of Mich. v. Thomas, 805 F.2d 176, 180 (6th Cir. 1986)) as:

the lowest emission limitation that a particular source is capable of meeting by the application of control technology that is reasonably available considering technological and economic feasibility.
RACT for Moderate Areas: For ozone, Moderate nonattainment areas must apply RACT to specified categories of VOC sources and certain other pollutants, with EPA guidance listing affected source categories.

C. Clean Data Determinations vs. Redesignation

Clean Data Determination (CDD): EPA’s finding that monitoring data show an area is currently meeting a NAAQS, even if the area’s formal legal designation remains nonattainment. A CDD suspends some planning requirements, but not all (like RACT).
Redesignation to Attainment: A formal change in an area’s legal status from nonattainment to attainment. It has greater legal and regulatory consequences—especially relaxed permitting and planning obligations—and is governed by the strict five‑part test in § 7407(d)(3)(E).

D. Exceptional Events

Definition: Events that affect air quality, are not reasonably controllable or preventable, and are either natural or rare human‑caused events. 42 U.S.C. § 7619(b); 40 C.F.R. § 50.14(a)(1).
Why They Matter: If a state proves that a high pollution reading was caused by an exceptional event, EPA must exclude that data from regulatory decisions, such as determinations of attainment, failure to attain, or redesignation.
Key Elements of a Demonstration:
- A conceptual model explaining how the event affected air quality;
- Evidence of a clear causal relationship between the event and the exceedance;
- Comparisons to other days and historical data;
- Proof that the event was not reasonably controllable or preventable; and
- Proof that the event was a natural event or a non‑recurring human‑caused event.

E. The Present Perfect Tense in Law

Present perfect (“has met,” “has attained,” “has been imposed”): Indicates that something happened in the past and remains true or legally effective at present.
Why It Matters Here: The court used grammar to conclude that “has met all requirements applicable to the area” means those requirements must be currently satisfied at the time EPA acts to redesignate, not just satisfied at some earlier moment in time.

VIII. Conclusion

Sierra Club v. EPA is a leading Sixth Circuit decision at the intersection of environmental law and modern administrative law doctrine. On the technical side, it affirms that courts will continue to defer heavily to EPA’s scientific judgments about complex phenomena like wildfire smoke and ozone formation, so long as EPA explains its reasoning and engages with contrary evidence.

But on the legal side, the opinion is transformative. In a post‑Loper Bright landscape, the court independently interprets the Clean Air Act’s redesignation provision, focusing on the present‑perfect tense and statutory context to hold that a state must have complied with all applicable CAA and SIP requirements—as they exist at the time of redesignation—before EPA may lawfully change a nonattainment area to attainment. EPA’s longstanding practice of treating “applicable” requirements as those in place at the time of redesignation‑request submittal cannot survive this textual and structural analysis in the Sixth Circuit.

The practical effect is that Detroit remains a Moderate nonattainment area that is currently meeting the 2015 ozone standard but has not yet earned the more permanent benefits of attainment status because Michigan has not implemented required RACT controls. More broadly, the decision forces EPA and states to treat statutory and SIP obligations as truly ongoing and binding up to the moment of redesignation, rather than as boxes checked at the time of application.

In sum, Sierra Club v. EPA stands for a clear new rule in the Sixth Circuit:

A nonattainment area may not be redesignated to attainment unless, at the time EPA acts, the state has actually complied with all Clean Air Act and Part D requirements that are then applicable to the area—including any that arose while the redesignation request was pending.

This principle will shape future ozone and other NAAQS redesignations, ensuring that legal status changes reflect not only current air quality but full compliance with the statutory control regime Congress designed to secure lasting attainment and maintenance of clean air.

Ongoing Compliance at the Moment of Redesignation: Sierra Club v. EPA and the Meaning of “Has Met All Requirements Applicable to the Area”