Ongoing Compliance as a Prerequisite for Clean Air Act Redesignation: Commentary on Sierra Club v. EPA (6th Cir. 2025)

I. Introduction

This commentary analyzes the Sixth Circuit’s published decision in Sierra Club v. EPA, Nos. 23‑3581/3583 (6th Cir. Dec. 5, 2025), a petition for review challenging two Environmental Protection Agency (EPA) actions under the Clean Air Act (CAA) concerning the Detroit, Michigan ozone nonattainment area:

EPA’s approval of Michigan’s “exceptional events” demonstration excluding wildfire‑influenced ozone data from 2022; and
EPA’s rule redesignating the Detroit area from nonattainment to attainment of the 2015 ozone National Ambient Air Quality Standards (NAAQS).

The case arises at the intersection of complex air‑quality planning rules, wildfire‑driven pollution episodes, and a new post‑Loper Bright landscape in which courts no longer defer to agency interpretations of ambiguous statutes under Chevron.

The key legal issues were:

Whether EPA’s approval of Michigan’s exceptional‑event demonstration for June 24–25, 2022 wildfire smoke was arbitrary or capricious under the Administrative Procedure Act (APA).
Whether EPA could lawfully redesignate the Detroit area to attainment when Michigan had not yet implemented “reasonably available control technology” (RACT) requirements that became applicable after Detroit was “bumped up” from Marginal to Moderate nonattainment.

The Sixth Circuit:

Affirmed EPA’s exceptional‑events approval, deferring to the agency’s technical evaluation of wildfire impacts on ozone; but
Vacated EPA’s redesignation rule, holding that 42 U.S.C. § 7407(d)(3)(E)(v) requires that a state has met all requirements applicable to the area as of the date of redesignation, not merely as of the date it submitted its redesignation request.

The decision therefore establishes an important new precedent in the Sixth Circuit: where an area is reclassified to a more stringent nonattainment category while a redesignation request is pending, the state must satisfy the additional CAA requirements (including RACT) before EPA may promulgate redesignation.

II. Summary of the Opinion

A. Statutory and Regulatory Background

Under the CAA, EPA sets NAAQS for pollutants that endanger public health or welfare and are emitted widely from multiple sources (42 U.S.C. §§ 7408–7409). For each NAAQS, EPA designates geographic areas as:

Attainment (meeting the standard),
Nonattainment (not meeting the standard), or
Unclassifiable.

Ozone nonattainment areas are further classified as Marginal, Moderate, Serious, Severe, or Extreme (42 U.S.C. § 7511a). Each classification carries increasingly stringent planning and control obligations, including:

State Implementation Plans (SIPs) specifying how the state will attain and maintain NAAQS (42 U.S.C. § 7410); and
RACT requirements for certain sources, particularly those emitting volatile organic compounds (VOCs) and oxides of nitrogen (NO_x), which are ozone precursors.

If an area fails to attain by the statutory deadline, EPA must “bump up” the area to the next higher classification and impose the corresponding additional requirements (42 U.S.C. § 7511(b)(2)). Once an area is actually attaining the NAAQS, the Governor may request redesignation to attainment. EPA may only redesignate if five statutory criteria are satisfied, including that:

the State containing such area has met all requirements applicable to the area under section 7410 of this title and part D.
— 42 U.S.C. § 7407(d)(3)(E)(v)

EPA can also make a separate “Clean Data Determination” that an area is currently meeting the NAAQS. That determination suspends certain planning obligations but does not itself redesignate the area (40 C.F.R. § 51.1318).

And, critically, EPA may exclude data affected by an “exceptional event” (such as wildfire smoke) when determining attainment, but only if the state submits a detailed technical demonstration and EPA finds, after notice and comment, that the event clearly caused the exceedance (42 U.S.C. § 7619(b); 40 C.F.R. § 50.14).

B. Factual and Procedural History

In June 2018, EPA designated the Detroit area as Marginal nonattainment for the 2015 ozone NAAQS, with an attainment deadline of August 3, 2021.

In January 2022, Michigan submitted a request to redesignate Detroit to attainment, based on 2019–2021 monitoring data.
In March 2022, EPA proposed to approve the redesignation.

However, on April 13, 2022, EPA separately proposed to find that Detroit had failed to attain by its August 2021 deadline and to reclassify the area from Marginal to Moderate. That reclassification would trigger new RACT obligations, with an initial proposed compliance deadline of January 1, 2023.

Meanwhile, during the 2022 ozone season:

On June 24–25, 2022, monitors in the Detroit area recorded ozone levels above the 2015 NAAQS.
Michigan attributed these exceedances to wildfire smoke from Canada and, in January 2023, submitted an “exceptional events” demonstration to EPA, asking to exclude those days from attainment calculations.

On February 1, 2023, EPA finalized the failure‑to‑attain finding and bumped Detroit up to Moderate nonattainment, moving the RACT SIP deadline to March 1, 2023. Michigan never submitted the required Moderate‑area RACT SIP.

On February 3, 2023, EPA proposed a Clean Data Determination that Detroit was attaining the 2015 ozone standard for 2020–2022, based on design values that excluded the June 24–25 wildfire days. After notice and comment, on May 19, 2023, EPA:

Finalized the Clean Data Determination; and
Finalized the redesignation rule moving Detroit from Moderate nonattainment to attainment.

In the redesignation rule, EPA interpreted § 7407(d)(3)(E)(v) to require that Michigan meet only those SIP requirements that were “due” as of the date the state submitted a complete redesignation request, not those that became due later (such as the Moderate‑area RACT obligations that took effect after the bump‑up).

Sierra Club petitioned for review, challenging:

EPA’s exceptional‑events approval for June 24–25, 2022; and
The redesignation rule, on the grounds that Detroit had not lawfully attained, air‑quality improvements were not shown to be due to permanent and enforceable reductions, and Michigan had not “met all requirements applicable to the area” as required by § 7407(d)(3)(E)(v).

C. Disposition

Applying APA review standards:

The court held that EPA’s exceptional‑events approval was not arbitrary or capricious. EPA reasonably applied a “weight‑of‑evidence” approach and adequately addressed Sierra Club’s criticisms of the data and analyses.
The court upheld EPA’s finding under § 7407(d)(3)(E)(iii) that air‑quality improvements were due to permanent and enforceable emission reductions, not just COVID‑19‑related economic slowdowns.
However, the court rejected EPA’s statutory interpretation of § 7407(d)(3)(E)(v) and held that the phrase “has met all requirements applicable to the area” refers to requirements applicable as of the date of redesignation, not merely as of the date of the state’s application.

Because Michigan had not implemented the Moderate‑area RACT requirements by the time EPA promulgated the redesignation rule in May 2023, EPA lacked authority to redesignate Detroit to attainment. The Sixth Circuit therefore:

Affirmed EPA’s Clean Data Determination and exceptional‑events approval;
Vacated the redesignation rule; and
Remanded to the agency.

III. Analysis

A. Precedents and Authorities Cited

1. APA and Agency Reasoning

The court grounded its APA analysis in well‑established Supreme Court precedents:

Motor Vehicle Mfrs. Ass’n v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29 (1983)
This case supplies the canonical arbitrary‑and‑capricious standard: an agency must articulate a “satisfactory explanation” including a “rational connection between the facts found and the choice made,” and may not ignore important factors or rely on reasoning that is counter to the record.
FCC v. Prometheus Radio Project, 592 U.S. 414 (2021)
The Court reiterated that arbitrary‑and‑capricious review is deferential: courts ask only whether the agency acted within a “zone of reasonableness” and adequately explained its decision.
St. Marys Cement, Inc. v. EPA, 782 F.3d 280 (6th Cir. 2015)
The Sixth Circuit emphasized particular deference to agency scientific and technical determinations within the agency’s expertise – a principle central to its deference to EPA’s handling of complex air‑quality and wildfire data.

2. Post–Loper Bright Statutory Interpretation

Far more consequential for the redesignation issue, the court relied on:

Loper Bright Enterprises v. Raimondo, 603 U.S. 369 (2024)
The Supreme Court overruled Chevron, holding that courts must exercise independent judgment on questions of statutory interpretation and “may not defer to an agency interpretation of the law simply because a statute is ambiguous.” The Sixth Circuit expressly invoked Loper Bright as the governing interpretive framework: the question is whether EPA acted within its statutory authority, not whether its reading was reasonable under Chevron step two.

The court also drew on textual canons and prior interpretations of similar language:

United States v. Wilson, 503 U.S. 329 (1992)
Cited for the proposition that Congress’s choice of verb tense matters when construing statutory language.
Hewitt v. United States, 606 U.S. 419 (2025)
The Supreme Court interpreted the present perfect tense (“has been imposed”) in the First Step Act as signaling an event that remains legally operative. The Sixth Circuit analogized this to “has met” and “has attained” in § 7407(d)(3)(E).
Commonwealth of Ky. v. EPA, 165 F.3d 26 (6th Cir. 1998) (unpublished table)
Earlier, the court read “has attained” in § 7407(d)(3)(E)(i) to require attainment that “must continue until the date of redesignation,” supporting a parallel reading of “has met” in subsection (v).
Southwest Airlines Co. v. Saxon, 596 U.S. 450 (2022)
Emphasized that statutory words must be read “in their context, not in isolation,” which the Sixth Circuit applied to read § 7407(d)(3)(E)(v) in the broader CAA scheme of escalating nonattainment obligations and SIP effectiveness rules.
Kungys v. United States, 485 U.S. 759 (1988)
Cited (via Justice Scalia’s plurality) for the “cardinal rule” disfavoring interpretations that render statutory language superfluous. The court invoked this to reject a reading of § 7407(d)(3)(E)(v) that would make “has” effectively meaningless.
Security Industry Ass’n v. Board of Governors, 468 U.S. 137 (1984)
Quoted in Wall v. EPA for the proposition that courts must reject agency constructions inconsistent with statutory mandates.

3. Prior CAA Redesignation Cases

The court’s understanding of § 7407(d)(3)(E) also rested on its own Clean Air Act precedents:

Greenbaum v. EPA, 370 F.3d 527 (6th Cir. 2004)
Described EPA’s obligation under § 7407(d)(3)(E): EPA may not redesignate unless it determines that the state has met all applicable SIP requirements under § 110 and Part D. The Sixth Circuit used this to emphasize that subsection (v), though phrased in terms of state compliance, is ultimately a constraint on EPA’s authority.
Wall v. EPA, 265 F.3d 426 (6th Cir. 2001)
Clarified that responsibility for meeting the NAAQS rests with the states; and identified RACT as a core requirement for Moderate ozone nonattainment areas, confirming that RACT is a quintessential “requirement[] applicable to the area” under § 7407(d)(3)(E)(v).
State of Michigan v. Thomas, 805 F.2d 176 (6th Cir. 1986)
Provided a working definition of RACT as the “lowest emission limitation” a source can meet, considering technological and economic feasibility, shaping how the court understood the substance of the RACT obligations at issue.

B. The Court’s Legal Reasoning

1. Exceptional Events and the Clean Data Determination

Sierra Club argued that EPA’s approval of Michigan’s exceptional‑event demonstration for June 24–25, 2022 was arbitrary and capricious, focusing on the requirement that the state prove a “clear causal relationship” between the wildfire smoke and the monitored ozone exceedances (40 C.F.R. § 50.14(c)(3)(iv)(B)).

The Sixth Circuit, applying a deferential scientific‑expertise standard, rejected four main lines of attack.

a. Alleged Failure to Consider Contradictory Local Data

Michigan’s demonstration relied, in part, on a screening tool developed by the Lake Michigan Air Directors Consortium (LADCO) that uses local monitoring data to detect smoke influence. Sierra Club argued that standard deviations of ozone, PM_2.5, and CO below 1 on the exceptional‑event days suggested no meaningful smoke impact.

EPA, however, directly confronted this criticism in the rulemaking record:

It acknowledged that the LADCO analysis did not show a high PM_2.5 peak for the relevant days.
It explained that it evaluates exceptional‑event demonstrations case by case using a “weight‑of‑evidence” approach, not any single indicator.
It pointed to other data—particularly Brown Carbon (BrC) measurements—as more probative of smoke presence in this event.

The court found that this was a textbook example of rational agency decision‑making under State Farm: EPA recognized contrary evidence, explained why it was not dispositive, and relied on a broader dataset.

b. Reliance on BrC and PM₁₀ Data

Sierra Club next attacked the BrC and PM₁₀ evidence EPA used to corroborate smoke influence, noting:

EPA’s own guidance warns PM₁₀ is not usually a good indicator of long‑range smoke transport; and
BrC levels peaked on June 23 (one day before the exceedances), with only modest increases on June 24–25.

EPA responded that:

Michigan did not rely “solely” on PM₁₀ and EPA “recognizes that long‑range transport of wildfire smoke would not typically have an impact on nearby PM₁₀.”
BrC data, interpreted alongside meteorology, still supported smoke influence on June 24–25. EPA reconstructed the meteorological sequence: smoke reached southern Ontario by June 23; northerly winds and a cold front transported and then subsided that smoke into the Detroit boundary layer; resulting in “atypical air quality for such a frontal passage.”

On this record, the court held that EPA had provided a sufficiently detailed and scientifically coherent rationale; the presence of peak BrC one day earlier did not undermine EPA’s conclusion that smoke persisted and affected ozone the following days.

c. Matching-Day Analysis and Omitted Dates

Michigan also conducted a “matching‑day” analysis, comparing June 24–25 to historical days with similar meteorology. It omitted certain days it believed were also influenced by smoke. Sierra Club argued that the omission undermined the integrity of the analysis because Michigan did not present specific evidence that smoke actually affected the omitted days.

The court emphasized that the matching‑day analysis was not the sole or even primary proof; rather, it was one component of an extensive package that included:

Historical comparisons of ozone concentrations;
Satellite imagery of smoke plumes;
Trajectory analyses from Canadian fire locations;
BrC and PM₁₀ data;
Local ozone and PM_2.5 data; and
Meteorological assessments.

Taken together, this body of evidence was enough, in the court’s view, to satisfy the “clear causal relationship” requirement. The omission of some days did not, by itself, render EPA’s reliance irrational.

d. Consideration of Local Emissions

Sierra Club contended that EPA failed to adequately distinguish wildfire contributions from local industrial sources that had recently begun operating near the monitor. The court disagreed, pointing to EPA’s reliance on the matching‑day analysis and meteorological evidence to conclude that:

the 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; the exceedances at issue were due to wildfire smoke, rather than local pollution.

That explicit reasoning—recognizing local emissions but finding them insufficient to explain the unusual exceedances—satisfied the court’s requirement that EPA “reasonably considered the relevant issues and reasonably explained the decision.”

Accordingly, the exceptional‑event approval stood.

2. “Permanent and Enforceable” Emission Reductions vs. COVID-19 Effects

Under § 7407(d)(3)(E)(iii), EPA may not redesignate a nonattainment area to attainment unless it determines that the observed improvement in air quality is due to “permanent and enforceable reductions in emissions” rather than temporary factors like economic downturns.

Sierra Club argued that the 2019–2022 period, which included the COVID‑19 pandemic and associated lockdowns, was so dominated by unusual economic conditions that EPA could not reasonably attribute improvements to permanent controls.

The court again sided with EPA, highlighting several key features of the record:

Michigan’s analysis showed a long‑term decline in point‑source emissions (2012–2020), indicating that improvements preceded the pandemic and could not be fully explained by it.
While the pandemic clearly decreased emissions, vehicle‑miles traveled, and employment between 2019 and 2020, ozone concentrations actually increased from 2019 to 2020, undercutting any claim of a simple cause‑and‑effect relationship between economic slowdown and lower ozone.
EPA identified specific, permanent control measures that had been implemented and were enforceable: regional NO_x controls, federal mobile and stationary source regulations, Detroit point‑source NO_x reductions, and a Michigan low‑volatility (low‑RVP) gasoline program.

In light of this analytical work, the court concluded that EPA had a rational—and indeed detailed—basis for its “permanent and enforceable reductions” determination.

3. The Core Holding: Timing of “All Requirements Applicable to the Area” under § 7407(d)(3)(E)(v)

The most significant aspect of the opinion is the court’s interpretation of § 7407(d)(3)(E)(v): when must a state have “met all requirements applicable to the area” for EPA to lawfully redesignate?

EPA had long operated on the view, and applied it here, that:

“requirements applicable to the area” do not include requirements that “came due after the submittal of a complete redesignation request.”

On that reading, Michigan’s failure to submit and implement the Moderate‑area RACT SIP by March 1, 2023 did not bar redesignation in May 2023, because those RACT obligations became due only after Michigan filed its redesignation application in January 2022.

The Sixth Circuit rejected that approach, applying a textual and structural analysis guided by Loper Bright.

a. Text: The Present Perfect “Has Met”

The court began with the statutory text:

The Administrator may not promulgate a redesignation of a nonattainment area (or portion thereof) to attainment unless … the State containing such area has met all requirements applicable to the area under section 7410 of this title and part D.
— 42 U.S.C. § 7407(d)(3)(E)(v) (emphasis added)

Drawing on Wilson and Hewitt, the panel emphasized that the present perfect tense (“has met”) typically denotes:

a completed action; and
an ongoing or current relevance of that action.

It analogized to its earlier reading of “has attained” in § 7407(d)(3)(E)(i), where the court previously held that “attainment must continue until the date of redesignation.” That parallel, along with the Supreme Court’s use of the present perfect to signal continuing legal effect in Hewitt, led the court to conclude:

“has met all requirements applicable to the area” includes requirements that are in force and applicable at the time of redesignation;
It is not satisfied by showing only that the state had met all requirements that existed when the state’s application was filed.

The court also rejected EPA’s attempt to use linguistic commentary on the present perfect to argue for multiple plausible readings. Even if the present perfect sometimes refers to past actions with present “significance,” EPA failed to identify what the relevant present significance would be under its reading that tied “applicable requirements” to the date of application. Moreover, if “has met” were read as if it simply meant “met before filing,” then the word “has” would do no work—contrary to the anti‑superfluity canon.

b. Context: The Nonattainment and SIP Scheme

Context solidified the court’s conclusion. The CAA’s nonattainment program is designed to ratchet up requirements when areas fail to attain by deadlines—regardless of whether air quality has actually worsened:

If a Marginal area fails to attain by its deadline, EPA must reclassify it upward (to Moderate, and potentially higher) based on prescribed criteria (42 U.S.C. § 7511(b)(2)).
Reclassification automatically imposes stricter requirements, including RACT obligations for additional source categories and pollutants (42 U.S.C. § 7511a(b)(2)).

Crucially, § 7407(d)(3)(D), which authorizes redesignation requests, explicitly provides that filing such a request:

shall not affect the effectiveness or enforceability of the applicable implementation plan for the State.

From this structure, the court reasoned:

The CAA anticipates that obligations may change between the filing of a redesignation request and EPA action on that request. Reclassification (a “bump‑up”) is triggered by missed deadlines, not by the timing of any request.
Once the area is reclassified, the more stringent requirements apply; EPA cannot simply ignore them in redesignation decisions.
A state cannot evade newly applicable obligations—like RACT for Moderate areas—by filing an early redesignation application and then not bringing its SIP into compliance with the new classification.

The Clean Data Determination regulation, which temporarily suspends some planning obligations when an area demonstrates current attainment, further underscores this reasoning: it does not suspend RACT implementation. If EPA’s “application‑date” approach to § 7407(d)(3)(E)(v) were correct, it would effectively create a blanket regulatory escape from RACT that Congress did not authorize.

C. Response to EPA’s “State-Focused” Argument

EPA also argued that § 7407(d)(3)(E)(v) is framed in terms of the state’s obligations (“the State … has met all requirements applicable to the area”), unlike subsections (i)–(iv), which are framed in terms of EPA’s findings. EPA suggested that because the state’s “role in the redesignation process” effectively ends with the application, the statute must lock “applicable requirements” to conditions at the time of state submission.

The court rejected this for four reasons:

Section 7407(d)(3)(D) expressly forbids the state from treating the redesignation request as a shield against evolving SIP obligations. The state’s compliance duties are ongoing.
Under the CAA, “responsibility for meeting the NAAQS rests with the states” (Wall), so it is unsurprising that Congress framed this criterion in terms of state compliance. That does not create a timing exception.
All five clauses of § 7407(d)(3)(E), including subsection (v), are introduced by the same operative phrase: “The Administrator may not promulgate a redesignation … unless.” Thus, they all function as constraints on EPA. Whether a clause focuses textually on EPA’s findings or on state performance, the operative legal prohibition applies to EPA.
Prior Sixth Circuit precedent (Greenbaum) had already described subsection (v) as a limitation on what “EPA may” do, confirming that the subsection speaks to EPA’s authority, not only to state conduct.

d. Workability and the 18-Month Deadline

Finally, EPA invoked practical concerns: it is required by the CAA to act on redesignation requests within 18 months. If a bump‑up and new SIP obligations occur during that window, EPA argued, it could be forced to either:

Act on an incomplete record before the state can satisfy new obligations; or
Violate the 18‑month timeline by delaying action until the state finishes new SIP submissions.

The court found this concern overstated. It pointed out that:

EPA can simply deny or decline to approve the redesignation request within the 18‑month window if the state has not met all applicable requirements as of that time, and invite the state to resubmit once it has done so.
EPA itself has flexibility in setting RACT compliance deadlines; it acknowledged that it could have set Detroit’s RACT SIP due date up to two years after the failure‑to‑attain finding, but chose a shorter timeline for policy reasons (consistency and calendar alignment).

Thus, the statute remains entirely workable even if “has met all requirements applicable to the area” is pegged to the redesignation date. The court concluded that such a reading better reflects congressional design and avoids allowing states to circumvent escalating nonattainment obligations through strategic timing.

Accordingly, the Sixth Circuit held that:

§ 7407(d)(3)(E)(v) permits redesignation only when a state “has met all requirements applicable to the area” at the time of redesignation, rather than at the time a state submits its application for redesignation.

Because Michigan had not implemented the required Moderate‑area RACT measures by May 19, 2023, EPA lacked authority to redesignate Detroit to attainment. The redesignation rule was therefore vacated.

IV. Impact and Implications

A. Significance for Clean Air Act Redesignations

The immediate impact of Sierra Club v. EPA is regional but potentially far‑reaching.

Within the Sixth Circuit (Michigan, Ohio, Kentucky, Tennessee):
EPA may no longer rely on an “application‑date” interpretation of § 7407(d)(3)(E)(v). For any nonattainment area in these states, EPA must verify that the state has satisfied all applicable SIP requirements—including those triggered by any reclassification—as of the date of redesignation.
Nationally:
While the decision is not formally binding outside the Sixth Circuit, it provides a well‑reasoned template—especially post‑Loper Bright—for challengers who wish to contest EPA’s traditional redesignation policy in other circuits. If adopted elsewhere, it could force a recalibration of EPA’s longstanding redesignation practice.

Practically, this means:

States cannot immunize themselves from impending, stricter nonattainment obligations (such as Moderate‑area RACT) by submitting redesignation requests just before bump‑up deadlines.
Regulated industries and communities in nonattainment areas should expect that full implementation of new SIP obligations may be a precondition to redesignation, even if air quality has already improved to attainment levels.

B. The Post–Loper Bright Landscape for EPA

This case is a concrete illustration of how Loper Bright changes judicial review of CAA implementation:

Under Chevron, EPA might have argued that its “application‑date” reading of § 7407(d)(3)(E)(v) was at least reasonable, warranting deference even if not the only plausible interpretation.
Post‑Loper Bright, the court instead asked whether EPA’s reading matched the best interpretation of the statute, using ordinary tools of statutory construction. Finding that it did not, the court invalidated EPA’s approach without any deference.

The opinion signals that:

Courts will closely scrutinize longstanding EPA interpretive policies embedded in guidance or practice, particularly where statutory text is specific and structured.
Arguments based on “practical necessity” or administrative convenience will carry less weight if they conflict with the statute’s grammatical form and structural design.

C. Exceptional Events and Wildfire-Driven Ozone

On the exceptional‑events front, the decision sends a different signal:

Courts remain highly deferential to EPA’s technical judgments about atmospheric chemistry and source attribution, particularly where EPA has assembled multi‑factor, multi‑pollutant evidence and has responded directly to public comments.
Challengers face a steep uphill climb in trying to overturn exceptional‑event determinations, especially where EPA has relied on an established “weight‑of‑evidence” methodology and carefully documented its reasoning.

Given the increasing frequency and geographic reach of wildfires, especially in the context of climate change, this affirmation of EPA’s exceptional‑event framework may prove significant. It confirms that wildfire‑affected data can continue to be excluded from attainment decisions when states make robust, scientifically grounded showings.

D. Effects on Detroit and the Region

For the Detroit area specifically:

The vacatur of the redesignation rule means Detroit remains classified as Moderate nonattainment for the 2015 ozone NAAQS unless and until EPA lawfully redesignates it.
Michigan must now complete and implement the Moderate‑area RACT SIP for Detroit in order to satisfy § 7407(d)(3)(E)(v) and support any future redesignation.
Regulated sources in the area should anticipate continued nonattainment requirements and potentially stricter control measures, including RACT for additional VOC and NO_x sources.

V. Complex Concepts Simplified

A. Key Clean Air Act Mechanics

National Ambient Air Quality Standards (NAAQS)
National pollutant limits set by EPA to protect public health and welfare. Ozone has both short‑term (8‑hour) standards; attainment is based on a three‑year “design value” calculated from monitor data.
Nonattainment Classifications: Marginal vs. Moderate
For ozone, areas that do not meet the NAAQS are categorized from least serious (Marginal) to most serious (Extreme). Higher classifications impose stricter control and planning obligations. Marginal areas have fewer requirements; Moderate areas must, among other things, adopt more extensive RACT controls.
State Implementation Plan (SIP)
Each state’s legally enforceable blueprint—approved by EPA—for attaining and maintaining the NAAQS. Once EPA approves a SIP (or SIP revision), it is enforceable as federal law. States must update SIPs when classification changes or new requirements arise.
Reasonably Available Control Technology (RACT)
A level of emissions control defined as the lowest emission limitation a source can meet, considering technological and economic feasibility. For Moderate ozone areas, RACT must be applied to specified VOC and NO_x source categories identified by EPA.
Clean Data Determination
An EPA finding, through rulemaking, that a nonattainment area is actually meeting the NAAQS based on recent monitoring data. This determination can temporarily suspend certain planning requirements, but it does not change the area’s formal designation; a separate redesignation rule is required.
Redesignation to Attainment
A formal EPA rulemaking that changes an area’s classification from nonattainment to attainment once the statutory criteria in § 7407(d)(3)(E) are met. Unlike a Clean Data Determination, redesignation affects long‑term obligations and planning frameworks.

B. Exceptional Events

An “exceptional event” is:

An event that affects air quality;
Is not reasonably controllable or preventable;
Is either a rare human‑caused event or a natural event; and
Is formally determined by EPA to qualify under its regulations.

States seeking to exclude data influenced by such events must submit a technical demonstration showing, among other things, a “clear causal relationship” between the event and the monitored exceedance. Evidence frequently includes:

Meteorology (wind patterns, pressure systems, fronts);
Satellite imagery and smoke plume tracking;
Pollutant measurements (e.g., ozone, PM_2.5, PM₁₀, BrC);
Trajectory and dispersion modeling; and
Comparisons to historical concentrations and similar meteorological days.

C. “Present Perfect” Verb Tense in Statutes

The court’s grammatical analysis may seem technical but has major legal consequences:

The present perfect tense (“has met,” “has attained,” “has been imposed”) is commonly used in English to describe:
- An event that happened in the past; and
- That still matters or remains true in the present.
Applying this to § 7407(d)(3)(E)(v), the phrase “has met all requirements applicable to the area” implies:
- The state must have complied with all relevant requirements in the past; and
- That compliance must remain current at the moment EPA redesignates the area.

By contrast, if Congress had said “had met all requirements,” that might suggest past compliance without necessarily demanding that all obligations are currently fulfilled. The court’s reliance on grammar is not pedantic; it is integral to the statute’s temporal operation.

VI. Conclusion

Sierra Club v. EPA is a pivotal Clean Air Act case at the intersection of environmental law, administrative law, and statutory interpretation.

On the one hand, the decision reaffirms strong judicial deference to EPA’s scientific expertise in technically complex determinations, such as attributing ozone exceedances to wildfire smoke. The court closely reviewed the record but upheld EPA’s exceptional‑events approval and its conclusion that recent air‑quality improvements were due to permanent and enforceable emission reductions, not merely pandemic‑related economic conditions.

On the other hand, the court applied a rigorously textual, post‑Loper Bright approach to EPA’s legal interpretation of § 7407(d)(3)(E)(v). Rejecting EPA’s longstanding view that only requirements due as of the redesignation application date count, the Sixth Circuit held that the Clean Air Act requires ongoing, up‑to‑date compliance with all applicable SIP and Part D obligations as of the date of redesignation. Because Michigan had not yet implemented the Moderate‑area RACT requirements triggered by Detroit’s bump‑up, EPA lacked authority to redesignate Detroit to attainment.

Going forward, this precedent ensures that redesignation cannot be used to sidestep or dilute the escalating control obligations Congress embedded in the nonattainment program. States seeking redesignation in the Sixth Circuit must not only demonstrate current attainment and permanent emission reductions; they must also fully implement any additional requirements that become applicable during the pendency of their requests.

In a broader sense, the case exemplifies the new balance after Loper Bright: agencies retain deference on scientific and technical questions within their expertise, but courts will independently and textually police the bounds of statutory authority. For Clean Air Act practitioners, regulated entities, and environmental advocates, Sierra Club v. EPA is a clear signal that timing strategies cannot substitute for substantive compliance when it comes to the transition from nonattainment to attainment.