Facility owners and operators with air permits will want to pay close attention to a recent Fifth Circuit ruling on a private citizen’s ability to seek penalties for/and defenses against alleged violations of the Clean Air Act.  Following a bench trial, an appeal, and another bench re-trial, this decade-old case has again gone up to the appeals court and had the trial court’s judgment vacated and remanded, this time to decide the Plaintiffs’ standing to bring the case in the first place and to judge the viability of two key affirmative defenses. Environment Texas Citizen Lobby, Inc. v. Exxon Mobil Corp., 66 F.Supp.3d 875 (S.D. Tex. 2014), vacated and remanded, 824 F.3d 507 (5th Cir. 2016), on remand, No. H-10-4969, 2017 WL 2331679 (S.D. Tex. Apr. 26, 2017), vacated and remanded, slip op. No. 17-20545, 2020 WL 4345337 (5th Cir. Jul. 29, 2020), as revised (Aug. 3, 2020).  According to the majority opinion, the panel gave both the lower court and the regulated community guidelines for when each CAA violation is “fairly traceable” to a plaintiff’s alleged injury to support standing under Article III of the U.S. Constitution. According to the concurrence, the Fifth Circuit’s standing precedents are “a mess” that are trending toward the unconstitutional elimination of “but-for” causation; a paradox that should be clarified by the full court en banc to stop a continuing loop of confusion.