"Air quality has improved substantially in the United States in the 40 years of EPA's Clean Air Act (CAA) regulation. According to the agency's science advisers and others, however, more needs to be done to protect public health and the environment from the effects of air pollution. Thus, the agency continues to promulgate regulations using authority given it by Congress in CAA amendments more than 20 years ago. Members of Congress from both parties have raised questions about the cost-effectiveness of some of these regulations and/or whether the agency has exceeded statutory authority in promulgating them. Others in Congress have supported EPA, noting that the Clean Air Act, often affirmed in court decisions, has authorized or required the agency's actions. EPA's regulatory actions on GHG [Greenhouse Gas] emissions are the main focus of congressional CAA interest. Although the Obama Administration has consistently said it would prefer that Congress pass new legislation to address climate change, such legislation now appears unlikely. Instead, over the last four years, EPA has developed GHG emission standards using its existing CAA authority. Relying on a finding that GHGs endanger public health and welfare, the agency promulgated GHG emission standards for cars and light trucks on May 7, 2010, and again on October 15, 2012, and for larger trucks on September 15, 2011. The agency subsequently turned its attention to major stationary sources of GHGs (power plants, manufacturing facilities, etc.) and a second round of standards for medium- and heavy-duty trucks."
CRS Report for Congress, R42895