The Environmental Protection Agency filed a brief September 1 in the U.S. Court of Appeals for the District of Columbia Circuit in defense of its tailpipe rule. The agency said that the Clean Air Act required the EPA to regulate mobile source greenhouse gas emissions after it determined that those emissions posed a hazard to the public and the environment. The agency also claims that legal challenges to the rules, issued under Section 202 of the Clean Air Act, do not address the emissions standards themselves, but instead the triggering effect of the stationary source rules.