Since the 1960s, emission standards for motor vehicles, coupled with standards for the fuels they burn, have reduced emissions from new cars and trucks by at least 95%. Nevertheless, because there are more vehicles on the road, because they are being driven more miles, and because the average useful life of a car exceeds the statutorily defined "useful life" over which emission standards must be met, emissions from motor vehicles continue to be a major component of the nation's air pollution problems. For these reasons, Congress, through the Clean Air Act (CAA) has required emission standards for new automobiles since 1965, and has amended these requirements on several occasions. Further, over the last five decades, the scientific understanding of the effects of air pollutants has led to a tightening of air quality standards. The CAA, as amended, requires the U.S. Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for common pollutants from numerous and diverse sources, which may reasonably be anticipated to endanger public health and welfare. EPA has set standards for six principal pollutants: ozone, particulate matter, nitrogen oxides, sulfur dioxide, carbon monoxide, and lead. The agency reports that in 2013 on-road vehicles accounted for about 34% of carbon monoxide emissions in the United States, 38% of nitrogen oxides, 12% of volatile organic compounds, and 3% of particulate matter. Despite nationwide reductions in each of these major air pollutants by over 50% since 1970, air quality still fails to meet ambient standards in areas where about one-third of the nation's population lives. Emission requirements for new motor vehicles have been strengthened numerous times since the first federal rulemaking took effect in 1968. The most recent revision, referred to as the "Tier 2" standards, was promulgated in February 2000. Tier 2 required vehicle manufacturers to reduce tailpipe emissions of several common pollutants, including carbon monoxide (CO), formaldehyde (HCHO), nitrogen oxides (NO x ), non-methane organic gases (NMOG, a class of volatile organic compounds (VOCs)), and particulate matter (PM). The standards significantly targeted emissions of NMOG and NO x to help control the formation of ground-level ozone pollution. Relative to the prior Tier 1 standards, the fleet-average standard for NO x required vehicle manufacturers to reduce overall tailpipe emissions by 88% to 95% (based upon the vehicle type). However, manufacturers had the flexibility to average the NO x emissions of their vehicle fleets to demonstrate compliance with the standards instead of certifying each vehicle according to the same stringency. The Tier 2 standards also required at least an 80% reduction in PM emissions and a less stringent reduction in CO emissions. Tier 2 was phased in beginning in model year (MY) 2004 and required all new passenger cars and light trucks up to 8,500 pounds, and all new heavier passenger vehicles up to 10,000 pounds (including large sport-utility vehicles), to demonstrate full compliance by MY2009. To further the vehicle emissions requirements, standards were also set on the level of pollutants (or compounds that may lead to pollution) in the vehicles' fuels. Most prominently, oil refiners were required to limit sulfur levels in gasoline to an average of 30 parts per million (ppm) nationwide beginning in 2005, roughly 90% less than the previous national average of 340 ppm. Reducing sulfur in gasoline prevents the fouling of catalytic converters, restoring or preserving their effectiveness in reducing NMOG, NO x , and CO emissions. Vehicle fuel economy has been regulated by the National Highway Traffic Safety Administration (NHTSA) since 1975. Under the Obama Administration, EPA has begun regulating emissions of greenhouse gases (GHGs) from motor vehicles. Because fuel consumption and GHG emissions are closely linked, EPA has coordinated with NHTSA since 2010 to issue a series of joint rulemakings. Most recently, on August 28, 2012, NHTSA and EPA issued final rules to tighten passenger corporate average fuel economy (CAFE) and GHG standards for MY2017-2025. The agencies expect that combined new passenger car and light truck fuel economy standards will be nearly 41.0 miles per gallon (mpg) in MY2021 and 49.7 mpg in MY2025, up from 34.1 mpg in MY2016. GHG emissions from new vehicles will decline by about 50% as a result of the standards. While the joint CAFE/GHG standards are not directly associated with the Tier 2/Tier 3 standards, requirements in the latter would assist vehicle manufacturers in advancing technology that would help meet the requirements of the former. This relationship is discussed further in the next sections. In February 2011, EPA began scoping new emissions standards for conventional pollutants from cars and light trucks as mandated by a May 2010 memorandum from the White House. The memorandum directed EPA to "review for adequacy the current non-greenhouse gas emissions regulations for new motor vehicles, new motor vehicle engines, and motor vehicle fuels, including tailpipe emissions standards for nitrogen oxides and air toxics, and sulfur standards for gasoline," and to promulgate regulations as required. Through its investigation, EPA found that Over 149 million Americans are currently experiencing unhealthy levels of air pollution, which are linked with respiratory and cardiovascular problems and other adverse health impacts that lead to increased medication use, hospital admissions, emergency department visits, and premature mortality. Motor vehicles are a particularly important source of exposure to air pollution, especially in urban areas. By 2018, we project that in many areas that are not attaining health-based ambient air quality standards (i.e., "nonattainment areas"), passenger cars and light trucks will contribute 10-25 percent of total nitrogen oxides (NO x ) emissions, 15-30 percent of total volatile organic compound (VOC) emissions, and 5-10 percent of total direct particulate matter (PM 2.5 ) emissions. As a result of these findings, EPA proposed Tier 3 standards on May 21, 2013, released the final version on March 3, 2014, and published it in the Federal Register on April 28, 2014. The final rule is effective on June 27, 2014. As with Tier 2, the Tier 3 standards consider the vehicle and its fuel as an integrated system and include changes to both vehicle emission limits and fuel formulation rules, lowering the allowable sulfur content of gasoline. The Tier 3 standards have been controversial since at least a year prior to their proposal, with dueling studies from auto manufacturers, EPA, and the petroleum refining industry having been prepared in advance of their release. The proposed standards were reported to be nearly ready in the spring of 2012, but the controversy over their presumed content delayed the proposal for a year. Numerous Members of Congress weighed in on the standards during their development, with many expressing their concern over the potential cost, and urging delay to permit further study. As finalized, the Tier 3 standards set requirements on tailpipe emissions for the sum of non-methane organic gases (NMOG) and nitrogen oxides (NO x ), presented as NMOG+NO x , and for particulate matter (PM). They apply to all light-duty passenger cars and trucks as well as some medium and heavy-duty vehicles. Compared to current standards, the NMOG and NO x tailpipe standards for light-duty vehicles represent approximately an 80% reduction from Tier 2's fleet average and a 70% reduction in per-vehicle PM standards. Heavy-duty vehicle tailpipe standards represent about a 60% reduction in both fleet-average NMOG+NO x and per-vehicle PM standards. Consistent with the Tier 2 principle of vehicle and fuel neutrality, the Tier 3 standards apply to all light-duty vehicles and trucks, regardless of the fuel they use. That is, vehicles certified to operate on any fuel (e.g., gasoline, diesel fuel, ethanol blends, compressed natural gas, liquefied natural gas, hydrogen, and methanol) are all subject to the same standards. EPA is also extending the regulatory useful life period during which the standards apply from 120,000 miles to 150,000 miles. The tailpipe standards include different phase-in schedules that vary by vehicle class, but generally become effective between MY2017 and MY2025. In addition to the gradual phase-in schedules, several other provisions are designed to further ease manufacturers' paths to compliance. These flexibilities include credits for early compliance and the ability to offset some higher-emitting vehicles with extra-clean models. EPA is also finalizing more lead time for small businesses and small volume manufacturers as well as a hardship provision that allows for additional time to comply if a manufacturer cannot meet requirements after a good faith effort. The standards for NMOG+NO x are fleet-average standards, meaning that a manufacturer calculates the weighted average emissions of the vehicles it produces in each model year and compares that average to the applicable standard for that model year. The standards differ by vehicle class and test procedures (see Table A-1 for more detail). The PM standards are expressed on a per-vehicle basis, meaning the standards apply to each vehicle separately (i.e., not as a fleet average). PM standards also differ by vehicle class and test cycle (see Table A-2 for more detail). Tier 3 also sets standards designed to eliminate fuel vapor-related evaporative emissions from the vehicle's fuel system. The evaporative emissions program represents about a 50% reduction from current standards and applies to all light-duty and on-road gasoline-powered heavy-duty vehicles. As with the tailpipe standards, the evaporative emissions standards include phase-in flexibilities, credit and allowance programs, and more lead time and a hardship provision for small businesses and small volume manufacturers. EPA is also extending the regulatory useful life period during which the standards apply from 120,000 miles to 150,000 miles. See Table A-3 for more detail.