In a recent decision by the United States Court of Appeals for the Sixth Circuit, Sierra Club v. EPA, Nos. 23-3581/3583 (6th Cir. Dec. 5, 2025), the Court invoked its statutory interpretation authority to hold that EPA contravened the plain language of the Clean Air Act (“CAA”) by redesignating the Detroit area as in attainment with the 2015 ozone National Ambient Air Quality Standards (“NAAQS”). Conversely, the Court deferred to EPA’s technical expertise in determining that certain air emissions should be excluded from consideration of the Detroit area’s compliance with the ozone NAAQS because of their qualification as “exceptional events” under the CAA, demonstrating that Courts will continue to defer to agencies’ expertise, but will not defer to agency interpretations of their enabling statutes.

In 2018, EPA designated the Detroit area as being in Marginal nonattainment with the 2015 ozone NAAQS, which are standards promulgated by EPA under the CAA for pollutants that EPA determines “may reasonably be anticipated to endanger public health or welfare.” Consistent with the CAA, Michigan had until August 3, 2021 to demonstrate air quality in attainment with the ozone NAAQS.  In April 2022, following Michigan’s failure to demonstrate attainment for the Detroit area by the August 3, 2021 deadline, EPA proposed to make a determination that the Detroit area had failed to attain the 2015 NAAQS.  The determination was finalized on February 1, 2023.  As a result, the Detroit area was redesignated by operation of law to the escalated category of Moderate nonattainment, thereby triggering increased control requirements for sources emitting ozone precursors.  Following the redesignation, Michigan was, as states with areas in Moderate nonattainment with the ozone NAAQS generally are, required to implement reasonably available control technology (“RACT”) for major sources of emissions of ozone precursors.  EPA provided Michigan with a RACT implementation deadline of March 1, 2023, by which time Michigan was required to update its State Implementation Plan (“SIP”) to demonstrate compliance with the new RACT requirements.

In January 2022, prior to February 1, 2023 finalization of the Detroit area’s redesignation as Moderate nonattainment for ozone, Michigan submitted a redesignation request to EPA, asking EPA to redesignate the Detroit area as in attainment based on 2019-2021 ozone air quality data. While this request was pending in June 2022, the Detroit area recorded two ozone exceedances.  Michigan argued that the exceedances were the result of “exceptional events”—namely, wildfires in Canada—and should be excluded from EPA’s consideration of Michigan’s redesignation submittal.  EPA agreed and promulgated two separate determinations under the CAA: (1) a redesignation of Detroit to attainment for ozone (the “Attainment Redesignation Rule”), and (2) a clean data determination stating that the Detroit area had attained the 2015 NAAQS for the 2020–2022 period (“Clean Data Determination”).  As a result, Michigan never revised its SIP to include the heightened RACT requirements that were applicable upon EPA’s finalization of redesignating the Detroit area as Moderate nonattainment.

Petitioner Sierra Club challenged EPA’s actions on two primary bases: (1) that EPA acted arbitrarily in excluding “exceptional event” air-quality data from June 2022 to make its Clean Data Determination, and (2) EPA lacked authority to redesignate the Detroit area to attainment because Michigan did not satisfy the RACT requirements imposed when the area was designated Moderate nonattainment.

Regarding its first argument, Sierra Club alleged that Michigan did not, to EPA’s satisfaction, establish a “clear causal relationship” required under the CAA between the Canadian wildfire emissions and the exceedances on the relevant days in June 2022. The Court disagreed and noted that sufficient evidence in the administrative record demonstrated that “EPA conducted a thorough evaluation of Michigan’s exceptional event demonstration.”  As a result, the Court concluded that EPA reasonably considered the relevant issues and reasonable explained its decision, thereby satisfying the requirements of the Administrative Procedure Act to avoid agency actions that are arbitrary or capricious.

With respect to Sierra Club’s second argument, the Court considered the language of CAA Section 307(d)(3)(E)(v), which provides that EPA “may not promulgate a redesignation of a nonattainment area (or portion thereof) to attainment unless” the state “has met all requirements applicable to the area under” CAA Section 7410.  Sierra Club specifically asserted that Michigan was required to meet the “applicable requirement” of revising its SIP to reflect the RACT requirements applicable to Moderate nonattainment areas as a prerequisite to being designated as in attainment. EPA, on the other hand, primarily argued that CAA Section 7407(d)(3)(E)(v) excludes requirements that “came due after the submittal of a complete redesignation request,” as was the case here, where the RACT requirements came due while the redesignation request was pending. Further, EPA argued that it is sufficient that the Detroit area was in compliance with the requirements applicable to the area at the time of its redesignation application.

The Sixth Circuit noted that the question was one of statutory interpretation.  Citing to Loper Bright Ent. v. Raimondo, 603 U.S. 369 (2024), the Court held that Section 7407(d)(3)(E) “commands the EPA not to promulgate a redesignation unless the state ‘has met’ (in the present-perfect tense) certain statutory requirements, it follows that the statutory requirements must be met at the time of redesignation.”  In this respect, the Court also noted that if Michigan was only required to meet the statutory requirements at the time it submitted its redesignation request, the provision would “render toothless the requirement that a state continue complying with the CAA even if it already submitted a redesignation request and would allow states to simply request redesignation and ignore any changes required after the date of the request.” The Court found that Congress’ use of the present-perfect tense was important, as “has met” “‘denotes past action with an abiding effect of continuing relevance and not ‘noncontinuing noncompliance.’”  The Court found this interpretation to be consistent with statutory context, given that the CAA “clearly contemplates escalating requirements even when a state has submitted a request to redesignate an area to attainment.”  Accordingly, the Court found that Detroit was required to implement the applicable RACT requirements until the date of redesignation to attainment. As a result, the Court vacated EPA’s Attainment Redesignation Rule.

This holding illustrates federal court’s unwillingness to defer to agencies’ interpretations of their own enabling statute, juxtaposed with a willingness to continue to defer to actions based on technical determinations and expertise.

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