Following the Supreme Court ruling Massachusetts v. EPA in 2007 stating that EPA has authority to regulate greenhouse gases emissions, the EPA put in place several regulations in order to regulate these emissions. One of them sets emission standards for new motor vehicules. Some companies challenged these measures arguing they were based on inadequate science and a wrong interpretation of the Clean Air Act as this Act does not authorize EPA to require permit for greenhouse gas emissions from motor vehicules.

The Court rules that the Clean Air Act authorizes the EPA to regulate pollution only when the source of pollution can emit more than 250 tons of pollutants per year. The Court says that greenhouse gases emissions can be considered as pollution so the EPA can set regulations addressing it but only for the most important sources of greenhouses gases. Therefore, the EPA overstepped its authority by regulating smaller greenhouse gases emissions like motor vehicules’.