The U.S. Court of Appeals for the Tenth Circuit issued a decision on August 3, 2026, which clarified the scope of the diligent prosecution bar to citizen suits under the Clean Air Act (“CAA”).  The decision was a partial victory for the environmental organizations that brought the lawsuit and adds to the body of appellate decisions aiming to elucidate this heavily litigated area of the law.

The case, GreenLatinos et al. v. Suncor Energy (U.S.A.), Inc., No. 25-1243 (10th Cir.), was filed by several environmental groups that alleged petroleum refineries in Commerce City, Colorado, were violating the CAA and its implementing regulations.  The statute provides a cause of action for citizens to sue based on violations of “an emission standard or limitation” under the CAA.  But these “citizen suits” are precluded where “the [EPA] Administrator or [a] State has commenced and is diligently prosecuting a civil action in a court of the United States or a State to require compliance with the standard, limitation, or order.”  42 U.S.C. § 7604(b)(1)(B).

Suncor argued that the environmental groups’ suit was barred by two consent decrees Suncor’s predecessors had entered into with EPA in 2002 and 2005.  The district court agreed and granted Suncor’s motion to dismiss the complaint.

On appeal, the appellate court reversed and remanded, concluding that the district court had “applied the wrong legal standard.”  The court of appeals then clarified the scope of the diligent prosecution bar within the Tenth Circuit, conducting what it described as a “text-based” analysis of the CAA’s citizen suit and diligent prosecution provisions.

First, the court observed that “the diligent prosecution bar is not jurisdictional” and can therefore be waived, rejecting Suncor’s effort to contend otherwise.  Second, the court confirmed that a previously entered judicial consent decree may bar a citizen suit under the CAA.  In doing so, the court rejected the environmental groups’ argument that a consent decree is a past judicial action rather than one that is presently being enforced.  A consent decree, the court emphasized, “possess[es] at least two defining characteristics: the imprimatur of the judicial court, and the power of that court to review the decree on an ongoing basis.”  Surveying case law, the Tenth Circuit “conclude[d] [that] the unbroken practice of federal courts confirms that a civil action can continue to be diligently prosecuted even after final judgment, so long as that judgment contemplates future conduct by the parties and the participation of the courts.  Consent decrees fit this bill.”  But the court also rejected Suncor’s argument that post-consent-decree administrative actions to enforce emissions standards and limitations could, in and of themselves, serve to bar a citizen suit.  Such administrative enforcement actions could be relevant, however, to the extent they evidenced diligent prosecution of the consent decrees.

The court then turned to what it described as “the second step in the diligent prosecution bar analysis,” which requires determining whether the CAA standards or limitations at issue in the judicial action (here, in the consent decrees) are “the same as the one[s] in the citizen suit complaint.”  Focusing on the definite article “the” in the statutory text “diligently prosecuting a civil action in a court of the United States or a State to require compliance with the standard, limitation, or order,” the court held that “the standards and limitations in the citizen suit complaint must be the same as the ones raised in the proffered judicial action in order for the diligent prosecution bar to apply.”  The court of appeals thus concluded that the district court had erred by finding all of the environmental groups’ claims barred, including some claims involving CAA standards and limitations that, while “substantially similar” to the standards and limitations at issue in the consent decrees, were not identical.  And the court rejected Suncor’s purpose-based argument that applying a strict “sameness requirement” would allow citizen suits to undercut agency enforcement discretion.  The court remanded the action to the district court to apply the correct standard, noting it appeared that some but not all of the environmental organizations’ claims would survive the diligent prosecution bar “because they are not the same as anything asserted in the consent decrees.”

Finally, while noting that the district court applied the wrong standard before it reached diligence, the court provided a brief word on the “diligent prosecution” requirement of the bar, which a court reaches only after determining that “the same standard or limitation” is being enforced “in a present judicial action” (which may include a consent decree).  If these first two elements are met, then “diligent prosecution is presumed.”  With that said, the court noted that courts must employ more than a “casual approach” to assessing whether this presumption may be overcome, and can consider, for example, whether agency actions to enforce provisions of the consent decree impose meaningful consequences on the alleged violator.  This requires a “fact-sensitive inquiry of diligence.”

Citizen suits under the CAA and other statutes, such as the Clean Water Act, often relate to circumstances where EPA and state environmental agencies have taken a variety of enforcement and compliance-related actions over a lengthy period of time.  The scope of preclusion of these citizen suits is a frequently contested issue with many variables.  The Tenth Circuit’s decision in GreenLatinos et al. v. Suncor Energy (U.S.A.), Inc. usefully confirms that previously entered consent decrees, which are subject to continued judicial enforcement, may serve to bar citizen suit claims.  But the court’s emphasis on a strict “sameness requirement” could open up avenues for citizen suit claims based on other emissions standards and limitations, even when there may be substantial similarity or factual overlap between the issues.

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