Under Sections 108-109 of the Clean Air Act (CAA), Congress mandated that the Environmental Protection Agency (EPA) set national ambient (outdoor) air quality standards (or NAAQS) for pollutants whose emissions (1) "may reasonably be anticipated to endanger public health or welfare"; and (2) "the presence of which in the ambient air results from numerous or diverse mobile or stationary sources." The statute further requires that EPA review the latest scientific studies and either reaffirm or modify previously established NAAQS every five years. EPA has identified and promulgated NAAQS 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, or 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 particulate matter (PM) to provide protection against potential health effects associated with short- and long-term exposure to particulates (including chronic respiratory disease and premature mortality). The 2006 particulates NAAQS primarily tightened the pre-existing (1997) standard for "fine" particulate matter 2.5 micrometers or less in diameter (PM 2.5 ). The standard for slightly larger, but still inhalable, particles less than or equal to 10 micrometers (PM 10 ) established in 1987 was not similarly changed. 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 . Promulgation of NAAQS sets in motion a process under which the states and EPA first identify geographic nonattainment areas, those areas failing to meet the NAAQS based on monitoring and analysis of relevant air quality data. States then have three years from the date of EPA's final designations to submit nonattainment area State Implementation Plans (SIPs), which identify specific regulations and emission control requirements that will bring an area into compliance. EPA published the final designations for the 2006 PM 2.5 NAAQS on November 13, 2009. The final designation rule became effective December 14, 2009 (30 days from the date of publication). The EPA final designations reflected a change from those announced by the agency in December 22, 2008. The publication of final designations—and thus the effective date of the final designations—had been delayed since January 2009 pending review by the incoming Administration. EPA initiated review of several previous agency decisions, including those covered by the White House January 20, 2009, memorandum, and the Office of Management and Budget's subsequent January 21, 2009, memorandum , on regulatory review. The 2006 particulates NAAQS generated national interest and debate, and oversight in Congress. Additionally, several states and industry, agriculture, business, and public advocacy groups petitioned the U.S. Court of Appeals for the District of Columbia Circuit, challenging certain aspects of EPA's 2006 revisions. A February 24, 2009, decision by the D.C. Circuit granted the petitions in part, denying other challenges, and remanded the standards to EPA for further consideration. While the court did not specifically vacate the 2006 PM standards, and their implementation proceeded, the decision and EPA's ongoing actions have prompted renewed interest in PM NAAQS among Members of Congress. Simultaneously with the ongoing implementation of the 2006 PM NAAQS, EPA initiated its next round of statutorily required periodic review of the PM NAAQS not long after the October 2006 promulgation of the standards, in part in response to the D.C. Circuit Court's decision. On January 15, 2013, EPA published a final rule further revising the PM NAAQS. The January 2013 revisions changed the existing (2006) annual health-based ("primary") standard for "fine" particulate matter 2.5 micrometers or less in diameter (or PM 2.5 ), lowering the allowable average concentration of PM 2.5 in the air from the current level of 15 micrograms per cubic meter (µg/m 3 ) to a limit of 12 µg/m 3 . The existing primary daily (24-hour) standard for PM 2.5 that was reduced from 65 µg/m 3 to 35 µg/m 3 in 2006 was retained, as was the existing standard for larger, but still inhalable, "coarse" particles less than 10 micrometers in diameter, or PM 10 . "Secondary" standards that provide protection against "welfare" (non-health) effects, such as ecological effects and material deterioration, are identical to the primary standards and the same as in 2006. The proposed rule regarding the PM NAAQS published June 29, 2012, solicited comments on two options for a 24-hour PM 2.5 standard to improve visibility that were not adopted in the final rule. For an overview of the revisions to the PM NAAQS published January 15, 2013, see CRS Report R42934, Air Quality: EPA's 2013 Changes to the Particulate Matter (PM) Standard , by [author name scrubbed]. The following sections of this report focus primarily on the NAAQS implementation process for designating geographical nonattainment areas with respect to the revisions to the PM 2.5 standards under the 2006 particulates NAAQS, including comparisons with the final designations under the 1997 PM 2.5 NAAQS. EPA did not require new nonattainment designations for PM 10 based on the 2006 revisions to the standard. Also included is a brief overview of states' subsequent obligations for developing and submitting implementation plans (SIPs) for attaining or maintaining compliance with the NAAQS. Appendix A contains a comparative time line for implementing the 2006 and 1997 PM 2.5 NAAQS. Appendix B provides a state and county breakdown of EPA's November 13, 2009, final designated nonattainment areas and areas identified by EPA on December 22, 2008, areas proposed by the states for the 2006 24-hour PM 2.5 NAAQS, as well as the final EPA nonattainment designations for the 1997 PM 2.5 NAAQS. Appendix C identifies tribes included in EPA's final nonattainment designations 2006 24-hour PM 2.5 NAAQS. The Clean Air Act (CAA) provides for two types of NAAQS: primary standards, "the attainment and maintenance of which in the judgment of the [EPA] Administrator ... are requisite to protect the public health," with "an adequate margin of safety"; and secondary standards, necessary to protect public welfare, a broad term that includes visibility impairment as well as damage to crops, vegetation, property, building materials, etc. The primary NAAQS include a daily (24-hour) limit for both PM 2.5 and PM 10 , and an annual limit for PM 2.5 (the previous annual limit for PM 10 was revoked). 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 are 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. For PM 2.5 and PM 10 , the secondary (welfare) NAAQS are the same as the primary standards. 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 : tightened the daily (24-hour) standard, which had allowed no more than 65 micrograms per cubic meter (µg/m 3 ) under the 1997 PM 2.5 NAAQS, 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 set in 1987 but changed 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 . As discussed in more detail in the following section, strengthening the daily (24-hour) standard for PM 2.5 had implications for those counties and partial counties designated nonattainment for the 1997 PM 2.5 NAAQS. With only a few exceptions, most of the designated nonattainment areas for the 1997 PM 2.5 NAAQS were not meeting the annual standard , but were meeting the daily (24-hour) standard . EPA is not requiring new nonattainment designations for PM 10 since the standards were not strengthened. To the contrary, a number of counties previously designated nonattainment for PM 10 have been determined by EPA to be in attainment since the 2006 revisions of the particulates NAAQS were promulgated. Designating geographical areas not achieving the established NAAQS based on monitoring and analysis of relevant air quality data is a critical step in NAAQS implementation. Section 107(d) of the CAA (42 U.S.C. 7407) establishes the process for designating "attainment" and "nonattainment" areas and setting their boundaries, but allows the EPA Administrator some discretion in determining what the final boundaries of the areas will be. Areas are identified as nonattainment when they violate or contribute to the violation of NAAQS.