Congressional interest in air quality issues has been dominated for the past six years by efforts to prevent the Environ mental Protection Agency (EPA) from promulgating and implementing new emission control requirements. Often under court order, EPA has used authorities Congress gave it in the Clean Air Act amendments of 1970, 1977, and 1990 to address long-standing issues posed by emissions from various sources. EPA's regulations on greenhouse gas emissions from both mobile and stationary sources and on emissions of all kinds from electric power plants have been of particular interest, as have the agency's efforts to revise ambient air quality standards for ozone and particulate matter. Especially in the House, efforts to restrain agency regulatory actions have occupied a prominent place since the 112 th Congress. Until the 114 th Congress, however, bills to restrain EPA faced a roadblock in the Senate, where Democratic control largely prevented their consideration. With Republicans in charge in the 114 th Congress, the Senate posed less of a roadblock. Still, environmental regulations have been a key element of President Obama's legacy, so legislation restraining EPA—if passed by the House and Senate—would generally have faced a presidential veto. The net result—few environmental bills enacted—thus remained as it had the last six years. In addition to bills targeting specific Clean Air Act rules, in recent years the House has also considered broader legislation designed to address regulation in general—bills such as the REINS Act ( H.R. 427 / S. 226 in the 114 th Congress), which would require congressional approval before regulations classified as major rules could take effect, or the Energy Consumers Relief Act ( S. 156 in the 114 th Congress), which would prohibit EPA from promulgating energy-related regulations estimated to cost more than $1 billion if the Secretary of Energy determines that the regulations would cause significant adverse effects to the economy. If enacted, such legislation would affect new rules under the Clean Air Act as well as other statutes. Given the broad nature of the bills' purpose, they are not discussed here. A continuing focus of congressional interest under the Clean Air Act (CAA) has been EPA regulatory actions to limit greenhouse gas (GHG) emissions using existing CAA authority. EPA actions have focused on six gases or groups of gases that multiple scientific studies have linked to climate change. Of the six gases, carbon dioxide (CO 2 ), produced by combustion of fossil fuels, is by far the most prevalent, accounting for more than 80% of annual anthropogenic emissions of the combined group when measured as CO 2 equivalents. Members from both sides of the aisle, including a majority of the House in the 112 th –114 th Congresses, have expressed concerns about EPA proceeding with GHG regulations that could have major economic impacts. Some argue that the case for GHG controls has not been proven. Others maintain that EPA should delay taking such action until Congress more explicitly authorizes it. EPA finalized GHG standards for power plants on August 3, 2015, finalized a second round of GHG standards for trucks in August 2016, and conducted a mid-term evaluation of the GHG standards for light-duty vehicles in 2016, providing Congress important opportunities to review the agency's actions. Unlike its critics, EPA has concluded that the Clean Air Act requires action to control GHG emissions, without the need for additional congressional authorization: a 2007 Supreme Court decision interpreting EPA's existing CAA authority, Massachusetts v. EPA , found that the agency must weigh whether GHG emissions from new motor vehicles endanger public health and welfare (or present valid reasons why it is unable to make that determination) and, if it concludes that there is endangerment, proceed with regulation of such vehicles. The agency made this endangerment finding in December 2009, and proceeded to promulgate GHG emission standards for new 2012-2016 cars and light trucks, May 7, 2010. Until recently, the prospect of GHG standards for new motor vehicles has not been particularly controversial. In May 2009, President Obama reached agreement with major U.S. and foreign auto manufacturers, the state of California (which has separate authority to set motor vehicle emission standards, if EPA grants a waiver), and other stakeholders regarding the substance of GHG emission and related fuel economy standards. Under the agreement, EPA, the U.S. Department of Transportation (which has authority to set fuel economy standards), and California would establish "One National Program" for GHG emissions and fuel economy. The auto industry supported a national agreement, in part, to avoid having to meet standards on a state-by-state basis; thus, it has not generally supported efforts to block EPA's motor vehicle GHG standards. A second round of standards for cars and light trucks, promulgated in October 2012, was also preceded by an agreement with the auto industry and key stakeholders. CRS has two reports that provide additional details. The second round of GHG standards for cars and light trucks covers model years (MY) 2017-2025. As part of the rulemaking, EPA made a commitment to conduct a Mid-term Evaluation (MTE) for the MY2022-2025 standards by April 2018. The agency deemed an MTE appropriate given the long time frame at issue, with the final standards taking effect as long as 12 years after promulgation. Through the MTE, EPA was to determine whether the standards for MY2022-2025 were still appropriate given the latest available data and information. A final determination could result in strengthening, weakening, or retaining the standards as promulgated. Critics of EPA's light-duty GHG standards have taken the MTE as an opportunity to revisit the rulemaking. Among the potential revisions suggested have been efforts to better harmonize the existing EPA/DOT/CARB standards, ease the MY2022-2025 standards, or eliminate them entirely. On November 30, 2016, EPA released a proposed determination under the MTE stating that the MY2022-2025 standards remained appropriate and that a rulemaking to change them was not warranted. EPA based its findings on a Technical Support Document, a previously released Draft Technical Assessment Report (which was issued jointly by EPA, DOT, and the California Air Resources Board (CARB)), and input from the auto industry and other stakeholders. The proposed determination has opened a public comment period through December 30, 2016, after which the EPA Administrator will review the comments and decide whether she has enough information to make a final determination on the MY2022-2025 standards. The proposed action has significantly accelerated the original timeline for the MTE (which called for a final determination by April 2018), and EPA announced it separately from any DOT (fuel economy) or California (GHG standard) process. Critics reacted swiftly, vowing to work with the new Administration to revisit EPA's determination—citing a "rush to judgment" that they argued contradicted the objectives of the One National Program. For additional information on the MTE, see CRS Insight IN10619, EPA's Mid-Term Evaluation of Vehicle Greenhouse Gas Emissions Standards , by [author name scrubbed]. EPA and the Department of Transportation have also promulgated joint GHG emission and fuel economy standards for medium- and heavy-duty trucks, which have been supported by the affected industries. In his 2014 State of the Union message and a subsequent directive to EPA and DOT, the President directed the agencies to develop a second round of these standards, to be proposed in 2015 and finalized a year later. The rule was finalized on August 16, 2016. The new standards cover model years 2018-2027 for certain trailers and model years 2021-2027 for semi-trucks, large pickup trucks, vans, and all types and sizes of buses and work trucks. According to EPA, The Phase 2 standards are expected to lower CO 2 emissions by approximately 1.1 billion metric tons, save vehicle owners fuel costs of about $170 billion, and reduce oil consumption by up to 2 billion barrels over the lifetime of the vehicles sold under the program. EPA projects the total cost of the rule at $29-$31 billion over the lifetime of model year 2018-2029 trucks. The standards will increase the cost of a long haul tractor-trailer by as much as $13,500 in model year 2027, according to the agency, but the buyer would recoup the investment in fuel-efficient technology in less than two years through fuel savings. In EPA's analysis, fuel consumption of 2027 model tractor-trailers will decline by 34% as a result of the rule. In general, these standards have been well-received. The American Trucking Associations described themselves as "cautiously optimistic" that the rule would achieve its targets: "We are pleased that our concerns such as adequate lead-time for technology development, national harmonization of standards, and flexibility for manufacturers have been heard and included in the final rule." For additional information on the motor vehicle standards, see CRS Report R40506, Cars, Trucks, and Climate: EPA Regulation of Greenhouse Gases from Mobile Sources . The decisions to move forward on GHG standards for new motor vehicles were seen by many, including EPA, as precedents for other potential standards. In December 2010, the agency announced that it had reached a settlement agreement with 11 states, the City of New York, the District of Columbia, and three environmental groups under which it would propose GHG emission standards for power plants by July 2011, and for refineries by December 2011, with promulgation by May 2012 and November 2012, respectively. Power plants are the largest anthropogenic source of U.S. GHG emissions, accounting for 30% of the U.S. total. Petroleum refineries are the second-largest industrial source of GHG emissions. EPA did not meet the deadlines of the two consent agreements, but it did finalize emission standards for new, existing, and modified power plants in August 2015.