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The citizen suit provision of the Clean Air Act (CAA) gives standing to citizen groups to bring suits against private actors for violations of the Act. Congress and the courts have established limitations on a citizen’s ability to bring a claim. These include notification of intent to file, a bar when the EPA or state has already “commenced and is diligently prosecuting” an action, or where the claim is barred by common law preclusion doctrines. In a divided decision, the Tenth Circuit held that the doctrine of issue preclusion barred the filing of a CAA citizen suit in Sierra Club v. Two Elk Generation Partners. The court found that the Sierra Club, the citizen group, was barred from filing a claim even though it was not a party to the previous administrative action. Through an expansive interpretation of the parens patriae doctrine, the Tenth Circuit circumvented the true purpose of the CAA’s citizen suit provision