In a 2–1 decision, the U.S. Court of Appeals for the District of Columbia Circuit (D.C. Circuit) largely vacated an Environmental Protection Agency (EPA) rule requiring 32 states to amend their Clean Air Act implementation plans (called State Implementation Plans, or SIPs) by removing provisions providing either exemptions or affirmative defenses to excess emissions from unit startups, shutdowns, or malfunctions (SSM) — times when pollution controls often cannot fully function as needed.[1] Sources in states that still have or revert to allowing an SSM defense may now have this important operational flexibility. However, stakeholders should track the actions of their individual states and keep close watch on whether EPA or environmental groups seek rehearing or appeal the ruling.

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