EPA has identified and promulgated National Ambient Air Quality Standards (NAAQS) under the Clean Air Act (CAA) for six principal pollutants commonly referred to as "criteria pollutants": particulate matter (PM), ozone (O 3 , a key measure of smog), nitrogen dioxide (NO 2 , or, inclusively, nitrogen oxides, NOx), sulfur oxides (SOx, or, specifically, SO 2 ), carbon monoxide (CO), and lead (Pb). On October 17, 2006, EPA published its revisions to the NAAQS for particulates to provide protection against potential health effects associated with short- and long-term exposure to particulate matter (including chronic respiratory disease and premature mortality). EPA's revised particulates NAAQS modified the standards established in 1987 that focused on particles smaller than 10 micrometers (PM 10 , or coarse particles), and standards for "fine" particles smaller than 2.5 micrometers (PM 2.5 ) introduced for the first time with the promulgation of the 1997 PM 2.5 NAAQS. Specifically, the agency lowered the allowable daily concentration averaged over 24-hour periods of PM 2.5 in the air from 65 micrograms per cubic meter (µg/m 3 ) to 35 µg/m 3 . The annual PM 2.5 standard, which is set in addition to the daily standard to address human health effects from chronic exposures to the pollutants, was unchanged. The 2006 particulates NAAQS also retained the 24-hour standard and revoked the annual standard for slightly larger, but still inhalable, particles less than or equal to 10 micrometers (PM 10 ). The 2006 revisions to the particulates NAAQS were the culmination of EPA's statutorily required periodic review, based on its evaluation and analysis of more than 2,000 scientific studies available between 1997 and 2002, and on determinations made by the Administrator. EPA's review of the particulates NAAQS and of the scientific criteria for setting the standards completed in 2006 was initiated not long after the 1997 promulgation. The 2006 particulates NAAQS generated national interest and debate, and oversight in Congress, as did the previous changes to the particulates standards promulgated in 1997. While the 2006 particulates NAAQS generally tightened the air quality standards for fine particulate matter, the action caused considerable controversy, including concerns that the standards were outside the range recommended by both EPA staff and by the scientific advisory panel (Clean Air Scientific Advisory Committee, or CASAC ) established by the CAA. Conversely, some contended that available data did not support the need for stricter standards or, in some cases, the standards as promulgated in 1997. A February 24, 2009, decision by the U.S. Court of Appeals for the D.C. Circuit to grant, in part, petitions challenging certain aspects of EPA's revised NAAQS generated renewed interest in the PM NAAQS. In December 2006, 13 states, the District of Columbia, and several state agencies had petitioned the U.S. Court of Appeals for the D.C. Circuit to review the 2006 particulates NAAQS. In addition, several groups representing various industry and agriculture interests (including coal, iron, steel, and corn refiners, oilseed processors, farmers, and cattle and pork producers), as well as environmental and public health organizations, filed petitions with the D.C. Circuit challenging the 2006 NAAQS. Petitioners, joined by several interveners and amici, challenged both the fine and coarse particulates standards. The court's decision did not vacate the PM standards but remanded certain aspects of the annual PM 2.5 standard to EPA for reconsideration; the petitions did not challenge the tightening of the daily (24-hour) standard. Although implementation of the 2006 PM NAAQS continues, EPA initiated its statutorily required review of the PM NAAQS, in part in response to the decision, not long after the 2006 promulgation of the PM NAAQS. EPA staff reassessed scientific studies considered in setting the 2006 PM NAAQS revisions, reviewed and analyzed extensive subsequent research, and considered public comments and recommendations of the CASAC. Based on the scientific evidence considered, EPA Administrator Lisa P. Jackson signed the proposal that would tighten the current standard primarily by lowering the annual health-based ("primary") standard for PM 2.5 . The proposal includes options for new secondary standards to address visibility impacts in urban areas associated with PM 2.5 , but did not modify the standards for inhalable coarse particles smaller than 10 micrometers (PM 10 ). EPA is to finalize its decision regarding the PM NAAQS by December 14, 2012. For an overview of the proposed revisions to the PM NAAQS published June 29, 2012, see CRS Report R42671, Air Quality: EPA's 2012 Proposed Changes to the Particulate Matter (PM) Standard , by [author name scrubbed]. The range of issues raised prior to and shortly after the promulgation of the 2006 PM 2.5 NAAQS could provide relevant insights as EPA proceeds with finalizing its decisions with regard to revising particulates NAAQS. This CRS report provides an analysis of the agency's final 2006 revisions to the particulates NAAQS, and an overview of the estimated costs and benefits of the revised standards and of more stringent alternatives analyzed. The report highlights concerns and issues raised regarding the 2006 revisions to the particulates standards, including those of the science advisory committee (CASAC), and actions in Congress. Establishing NAAQS does not directly limit emissions; rather, it represents the EPA Administrator's formal judgment regarding the level of ambient pollution that will protect public health with an adequate margin of safety . Under Sections 108-109 of the CAA, Congress mandated that EPA set national ambient (outdoor) air quality standards for pollutants whose emissions "may reasonably be anticipated to endanger public health (primary standards) or welfare (secondary standards)" and "the presence of which in the ambient air results from numerous or diverse mobile or stationary sources." The statute also requires that every five years EPA review the latest scientific studies and reaffirm or modify previously established NAAQS. EPA's review leading up to the 2006 promulgation of the revised PM NAAQS found that the scientific evidence since 1997 reinforced the associations between exposure to particulates and numerous cardiovascular and respiratory health problems, including aggravated asthma, chronic bronchitis, reduced lung function, irregular heartbeat, nonfatal heart attacks, and premature death. The CASAC found that the numerous epidemiological studies EPA reviewed "… have shown statistically significant associations between the concentrations of ambient air PM 2.5 and PM 10 (including levels that are lower than the 1997 particulates NAAQS) and excess mortality and morbidity." Further, EPA concluded, and most of the CASAC panel concurred, that the scientific evidence supported modifying the particulates standards. The 1997 primary NAAQS for both PM 2.5 and PM 10 included an annual and a daily (24-hour) limit. To attain the annual standard, the three-year average of the weighted annual arithmetic mean PM concentration at each monitor within an area must not exceed the maximum limit set by the agency. The 24-hour standards were a concentration-based percentile form, indicating the percentage of the time that a monitoring station can exceed the standard. For example, a 98 th percentile 24-hour standard indicates that a monitoring station can exceed the standard 2% of the days during the year. As modified and published in the October 17, 2006, Federal Register Notice, the primary PM 2.5 and PM 10 standards are as follows: PM 2.5 : strengthened the daily (24-hour) standard, which had allowed no more than 65 micrograms per cubic meter (µg/m 3 ), by setting a new limit of 35 µg/m 3 , based on the three-year average of the 98 th percentile of 24-hour PM 2.5 concentrations; retained the annual standard at 15 µg/m 3 . PM 10 : retained the daily standard at 150 µg/m 3 but changes from the 99 th percentile to no more than one exceedance per year on average over three years; eliminated the annual maximum concentration (50 µg/m 3 ) standard for PM 10 . For PM 2.5 and PM 10 , the secondary (welfare) NAAQS are the same as the primary standards. Table 1 below provides a comparison of the revised primary NAAQS with those previously promulgated for both PM 2.5 and PM 10 . EPA's final revisions to the standards for fine particulates (PM 2.5 ) were the same as those the agency had proposed in January 2006. However, the 2006 EPA revisions to the PM 2.5 NAAQS, while tightening the standards, were not as stringent as those recommended by the CASAC and by EPA staff. With regard to coarse particulates, EPA had proposed replacing the current particle size indicator of PM 10 with a range of 10 to 2.5 micrometers (PM 10-2.5 ), referred to as inhalable (or thoracic) coarse particles, and setting a PM 10-2.5 daily standard of 70 µg/m 3 rather than the current PM 10 daily standard of 150 µg/m 3 . The proposal also included narrowing the focus of the PM 10-2.5 standard to "urban and industrial" sources and excluding particles typical to rural areas, including "windblown dust and soils and particulates generated by agricultural and mining sources." The range of alternative standards considered and proposed and issues associated with EPA's final decisions are discussed later in this report. Promulgation of NAAQS in 2006 set in motion a process under which the states and EPA first identified geographic nonattainment areas, those areas failing to comply with the NAAQS based on monitoring and analysis of relevant air quality data.