Sixth Circuit Upholds EPA's Exceptional Events Determination In Vacating Detroit's Ozone Attainment Redesignation

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 the U.S. Environmental Protection Agency ("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"). However, the Court deferred to EPA's technical expertise in determining that certain air emissions could be excluded from consideration of the Detroit area's compliance with the ozone NAAQS because of their qualification as "exceptional events" under CAA, demonstrating that Courts will continue to defer to agencies' expertise, but will not defer to agency interpretations of their enabling statutes.

Detroit Ozone NAAQS Attainment History

In 2018, EPA classified the Detroit area as being in marginal nonattainment with the 2015 ozone NAAQS, with a deadline to reach attainment status by August 2021. Subsequently, in January 2022, Michigan requested EPA to reclassify the Detroit area to attainment based on 2019–2021 air quality data. While Michigan's request was pending, the Detroit area exceeded the 2015 ozone NAAQS in June 2022. However, because Michigan believed Canadian wildfires caused these ozone NAAQS exceedances, Michigan requested EPA to exclude the June 2022 air quality data.

In February 2023, EPA determined the Detroit area failed to achieve attainment status by the August 2021 deadline and designated the area as moderate nonattainment. States with areas in marginal nonattainment are required to revise their State Implementation Plans ("SIPs") to implement reasonably available control technology ("RACT") requirements for major sources of emissions of ozone precursors. Michigan did not revise its SIP to include the additional RACT requirements associated with Detroit's moderate nonattainment status designation.

In May 2023, EPA reclassified the Detroit area to attainment status by excluding the June 2022 air quality data, as Michigan requested. EPA determined that Michigan did not need to update its SIP because those additional requirements became applicable after Michigan already requested EPA to reclassify the Detroit area as attainment.

Sierra Club's Challenge to Detroit's Attainment Status Designation

Petitioner Sierra Club challenged EPA's actions on two primary bases: EPA (1) acted arbitrarily in excluding "exceptional event" air-quality data from June 2022 and (2) 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.

With respect to its first argument, Sierra Club alleged that Michigan did not, to EPA's satisfaction, establish the "clear causal relationship" required under 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 reasonably explained its decision, thereby satisfying the requirements of the Administrative Procedure Act.

With respect to Sierra Club's second argument, the Court considered the language of CAA Section 107(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." 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, argued that CAA Section 107(d)(3)(E)(v) excludes requirements that "came due after the submittal of a complete redesignation request." As was the case here, the RACT requirements came due while the redesignation request was pending. EPA further 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 court determined the question to be an issue of statutory interpretation and relied upon the Supreme Court's recent decision in Loper Bright Enters. v. Raimondo, 603 U.S. 369 (2024). Loper Bright requires courts to use their independent judgment in deciding whether EPA acted within its statutory authority. Here, the court determined that the moderate attainment status requirements remain in effect while a reclassification request is pending and that Michigan cannot avoid the moderate attainment status requirements by failing to amend its SIP. The Sixth Circuit, therefore, vacated the Detroit area attainment status designation.

Why This Case Matters

This case is an example of the Sixth Circuit applying Loper Bright and not deferring to EPA's interpretation of CAA. Courts now have greater power to interpret federal environmental statutes, even when those statutes concern technical environmental topics where EPA has more expertise than the courts. This holding illustrates the federal court's unwillingness to defer to an agency's interpretation of its enabling statute, juxtaposed with a willingness to continue to defer to actions based on technical determinations and expertise.

Separately, EPA published a proposed rule on November 19, 2025 determining that the Phoenix area would have achieved attainment with the 2015 ozone NAAQS by the August 2024 deadline but for international air pollution. 90 Fed. Reg. 52,019 (Nov. 19, 2025). Importantly, EPA proposed a new CAA interpretation that would not require states to meet all CAA requirements for an area's classification before EPA can approve a state's attainment demonstration. Sierra Club and others submitted comments disagreeing with EPA's proposed rule on December 19, 2025. EPA will review the comments it received and likely publish its final decision in 2026. If finalized, this would directly conflict with the Sixth Circuit's judgment in Sierra Club v. EPA and would likely be challenged in court.

In our new Loper Bright world, the regulated community can no longer rest assured in EPA's interpretation of environmental statutes. If litigated, EPA's decisions are subject to a specific court's, or even a specific judge's, interpretation. Stites & Harbison will continue to monitor the court dockets and provide updates on how the courts around the country are applying Loper Bright to put EPA's decisions under a microscope.

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