Despite steady improvements in air quality in many of the United States' most polluted cities, the goal of clean air continues to elude the nation. The most widespread problems involve ozone and fine particles. As of March 2006, 158 million people lived in areas classified "nonattainment" for the ozone National Ambient Air Quality Standard ; 88 million lived in areas that were nonattainment for fine particles (PM 2.5 ). Air quality has improved substantially since the passage of the Clean Air Act in 1970: annual emissions of the six most widespread ("criteria") air pollutants have declined 160 million tons (53%), despite major increases in population, motor vehicle miles traveled, and economic activity. Meanwhile, however, scientific understanding of the health effects of air pollution has caused EPA to tighten standards for ozone and fine particles. (Fine particles, as defined by EPA, consist of particulate matter 2.5 micrometers or less in diameter, abbreviated as PM 2.5 .) The agency attributes at least 33,000 premature deaths and millions of lost work days annually to exceedances of the PM 2.5 standard. Recent research has begun to tie ozone pollution to premature mortality as well. Thus, there is continuing pressure to tighten air quality standards: a tightening of the standard for fine particles was finalized September 21, 2006. Ozone standards are scheduled for review in 2007, with a final decision due by February 2008. And attention has focused on major sources of ozone and particulate pollution, such as coal-fired power plants and mobile sources. With this background in mind, the bulk of this report provides an overview of seven prominent air issues of interest in the 109 th Congress: revision of the particulate standards; multi-pollutant (or Clear Skies) legislation for electric power plants; mercury from power plants; New Source Review; the gasoline additives MTBE and ethanol; ozone nonattainment area deadlines; and the "conformity" of transportation and clean air planning. This report (formerly an Issue Brief) provides an overview: most of these issues are addressed at greater length in separate CRS reports, which contain more information and detailed sources. The CRS reports are referenced in the appropriate sections. On September 21, 2006, EPA Administrator Stephen Johnson signed revisions to the National Ambient Air Quality Standards (NAAQS) for particulate matter. (The standards appeared in the Federal Register on October 17. ) EPA reviewed 2,000 scientific studies on particulates in developing the revision, and found associations between particulates and numerous significant health problems, including aggravated asthma, chronic bronchitis, reduced lung function, irregular heart beat, heart attacks, and premature death in people with heart or lung disease. The revisions would strengthen the pre-existing standard for particulate matter 2.5 micrometers or less in diameter (known as fine particles or PM 2.5 ), but the standard will not be strengthened to the degree recommended by the agency's staff or scientific advisors. As shown in Table 1 , the new standard will cut the allowable concentration of PM 2.5 in the air averaged over 24-hour periods from 65 micrograms per cubic meter (µg/m 3 ) to 35 µg/m 3 ; the annual standard, currently set at 15 µg/m 3 , will not be changed. EPA's professional staff and the Clean Air Scientific Advisory Committee (CASAC), a group established by the Clean Air Act to provide independent scientific advice to the Administrator, had recommended stronger standards. CASAC endorsed a 24-hour standard in the range of 30 to 35 µg/m 3 and an annual standard in the range of 13 to 14 µg/m 3 . Twenty of the CASAC panel's 22 members concurred in the recommendation. In the Administrator's judgment, the science underlying this recommendation was not sufficient, relying primarily on two studies, neither of which "provide[s] a clear basis for selecting a level lower than the current standard...." The Administrator agrees with CASAC that the science shows a relationship between higher levels of PM 2.5 and an array of adverse health effects, but he believes there is too much uncertainty in the analysis to justify lowering the annual standard. He also noted that EPA is undertaking substantial research to clarify which aspects of PM-related pollution are responsible for elevated risks of mortality and morbidity, including a multi-million dollar research program whose timeline should permit the results to inform the Agency's next periodic reevaluation of the PM 2.5 standard, required by statute within five years. Thus, he concluded, "...it would be wiser to consider modification of the annual standard with a fuller body of information in hand than initiate a change in the annual standard at this time." The PM NAAQS also addresses slightly larger, but still inhalable particles, in the range of 10 to 2.5 micrometers. These are referred to as thoracic coarse particles, or PM 10-2.5 . In its last review of the particulate standards (in 1997), EPA had regulated these as particles 10 microns or smaller (PM 10 ), a category that overlapped the PM 2.5 category. Challenged in the D.C. Circuit Court of Appeals, the PM 10 standard was remanded to EPA, the court having concluded that PM 10 is a "poorly matched indicator" for thoracic coarse particles, because it includes the smaller PM 2.5 category as well as the larger particles. In response, earlier this year, EPA proposed a 24-hour standard for PM 10-2.5 . The standard would have been set at a level of 70 µg/m 3 , compared to the current 24-hour PM 10 standard of 150 µg/m 3 . The final standards signed by the Administrator September 21 reversed course, leaving in place the current form of the standard (PM 10 ) and the current level (150 µg/m 3 ). The only change to the PM 10 standard was revocation of its annual component. The agency argues that it has provided more thorough reasoning in support of the use of PM 10 as its coarse particle indicator, and believes that its explanation will satisfy the court. The Administrator's decisions represent the first time in CASAC's nearly 30-year history that the promulgated standards fall outside of the range of the scientific panel's recommendations. In a letter dated September 29, the 7 members of CASAC objected to the Administrator's actions, both as regards PM 10 and PM 2.5 . With regard to PM 2.5 , the letter stated: "CASAC is concerned that EPA did not accept our finding that the annual PM 2.5 standard was not protective of human health and did not follow our recommendation for a change in that standard." The letter noted that " there is clear and convincing scientific evidence that significant adverse human-health effects occur in response to short-term and chronic particulate matter exposures at and below 15 µg/m 3 , " and noted that 20 of the 22 Particulate Matter Review Panel members, including all seven members of the statutory committee, were in "complete agreement" regarding the recommended reduction. " It is the CASAC ' s consensus scientific opinion that the decision to retain without change the annual PM 2.5 standard does not provide an ' adequate margin of safety ... requisite to protect the public health ' (as required by the Clean Air Act) .... " With regard to PM 10 , the letter stated that CASAC was "completely surprised" at the decision to revert to the use of PM 10 as the indicator for coarse particles, noting that the option of retaining the existing daily PM 10 standard was not discussed during the advisory process and that CASAC views this decision as "highly problematic." The Administrator is not required by statute to follow CASAC's recommendations; the Act (in Section 307(d)(3)) requires only that the Administrator set forth any pertinent findings, recommendations, and comments by CASAC and the National Academy of Sciences, and, if his proposal differs in an important respect from any of their recommendations, provide an explanation of the reasons for such differences. Courts, in reviewing EPA regulations, also generally defer to the Administrator's judgment on scientific matters, focusing more on issues of procedure, jurisdiction, and standing. Nevertheless, CASAC's detailed objections to the Administrator's decisions and its description of the process as having failed to meet statutory and procedural requirements could play a role if the standards are challenged in court. EPA is prohibited from taking cost into account in setting NAAQS, but in order to comply with an Executive Order, the agency has produced a Regulatory Impact Analysis (RIA) analyzing in detail the costs and benefits of the new PM standards. The agency estimates that compliance with the new PM 2.5 standard will prevent 1,200 to 13,000 premature deaths annually, as well as substantial numbers of hospital admissions and missed work or school days due to illness. Critics of the rule argue that as many as 30,000 premature deaths could be avoided annually if the Administrator had chosen the more stringent standards endorsed by CASAC. The higher estimate is based on an "Expert Elicitation" conducted for EPA and released on the same day the Administrator signed the new NAAQS. The agency's RIA estimates the cost of meeting the new standards at $5.4 billion annually in 2020, and provides a range of benefit estimates (from $8 billion to $76 billion annually, depending on the number of avoided deaths, the choice of discount rate, and other factors). A more stringent alternative (reducing the annual standard to 14 µg/m 3 ) would increase the cost by about 50%, to $7.9 billion annually, according to the agency, but would nearly double the estimated benefits. Thus, the benefit-cost ratio would be more favorable, according to the agency's analysis, had the Administrator chosen the more stringent standard. Using the most recent available monitoring data, the agency identified 141 counties where air quality is worse than allowed under the new standards.