Introduction

On January 15, 2013, the Environmental Protection Agency (EPA) published a final rule in the Federal Register to strengthen the National Ambient Air Quality Standard (NAAQS) for particulate matter (PM) . The standards, set pursuant to the Clean Air Act (CAA), address potential health e ffects (including chronic respiratory disease and premature mortality) associated with short- and long-term exposure to particulate matter. The CAA, enacted in 1970 and amended in 1990, requires EPA to set minimum NAAQS standards for pollutants anticipated to endanger public health and welfare, and where their presence in ambient air results from numerous and diverse mobile or stationary sources. EPA has identified six "criteria air pollutants" under this authority: ozone ("smog"), particulate material ("soot"), sulfur dioxides, nitrogen oxides, carbon monoxide, and lead. The law also requires EPA to evaluate each NAAQS every five years to determine whether it is adequately protective of human health and the environment, based on the most recent science.
The EPA Administrator signed the final rule revising the PM NAAQS on December 14, 2012. EPA's most recent review and process generated controversy and national debate among a variety of stakeholders including industry groups, health and environmental advocacy groups, and states, as well as oversight in Congress. EPA reportedly received and considered more than 230,000 written comments in determining the final PM standard. Similar controversy and debate transpired during the previous changes leading up to the PM NAAQS promulgated October 2006, and those established in 1997.
As published, January 2013 final PM NAAQS rule was the culmination of EPA's statutorily required review of the NAAQS under the CAA based on studies available through mid-2009 and recommendations of EPA staff and a scientific advisory panel (Clean Air Scientific Advisory Committee, or CASAC) established by the CAA. The agency initiated the statutorily required periodic review 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. EPA has already initiated the next five-year review of the PM NAAQS.
Based on the scientific evidence and comments considered, EPA Administrator Lisa P. Jackson signed the final rule that would change the current standard primarily by lowering the annual health-based ("primary") standard for fine particles smaller than 2.5 microns (PM 2.5 ). In the final rule, the "secondary" standards that provide protection against "welfare" (nonhealth) effects, such as ecological effects and material deterioration, are identical to the primary standards, the same as in 2006. The final rule relies on the existing secondary 24-hour standard to protect against visibility impairment, and did not adopt a separate standard included among options in the June 2012 proposal. Also, as proposed, the final rule did not modify the standards for inhalable "coarse" particles larger than 2.5 but smaller than 10 microns (PM 10 ). Some stakeholders in the agricultural community and some Members maintained a particular interest in EPA's consideration of the PM 10 standards and potential impacts that revising the NAAQS may impose on the agricultural operations.
In its Regulatory Impact Analysis (RIA) accompanying the final rule assessing the costs and benefits of proposed revisions to the PM NAAQS, EPA estimated that tightening the PM 2.5 annual standard would add further health benefits beyond those anticipated with the promulgation of the 2006 PM NAAQS. Others have suggested that potential health benefits of tightening the PM NAAQS might be higher than EPA's estimates. On the other hand, tighter standards could impose additional compliance requirements on communities, states, industry, and others, at what some stakeholders and Members contend will be a substantial economic cost. EPA expects that requirements and emission reductions associated with existing and recently promulgated federal regulations under the CAA will allay impacts of complying with the revised PM standards, and anticipates that virtually all counties will meet the standards as promulgated in 2020.
On January 15, 2015, EPA published its classification of 14 areas as "Moderate" nonattainment for the revised 2013 primary annual PM 2.5 standard. The areas include 38 counties or portions of counties in six states—California, Idaho, Indiana, Kentucky, Ohio, and Pennsylvania. EPA also deferred the designation period for 11 other areas (including 2 entire states and all but 3 counties of another) by up to one year, and designated all other areas as "unclassifiable" or as "unclassifiable/attainment." 
On April 7, 2015, EPA published amendments to the final designations for the 2013 PM 2.5 NAAQS that were published on January 15, 2015. The changes to area designations were the result of air quality data for 2014 recently submitted by affected states. The final area designations as amended and published in the April 7, 2015, Federal Register encompass 20 counties or portions of counties (nine areas) in four states as nonattainment only for the 2013 revised annual PM 2.5 standard. EPA also designated five areas in Georgia (including two adjacent counties in Alabama and South Carolina) as unclassifiable. 
The effective date of April 15, 2015 (90 days from the date of publication) based on the January 15, 2015, final designation rule remained unchanged. CAA section 188(c)(1) of Subpart 4 requires Moderate areas achieve attainment as expeditiously as practicable, but no later than the end of the sixth year after the effective date of final designation as nonattainment. For a more detailed discussion of the designation of nonattainment areas for the 2013 PM 2.5 NAAQS see CRS Report R43953, 2013 National Ambient Air Quality Standard (NAAQS) for Fine Particulate Matter (PM2.5): Designating Nonattainment Areas .
Several recent and pending EPA regulations implementing the various pollution control statutes enacted by Congress garnered vigorous oversight during the 112 th Congress. Members expressed concerns in hearings, through bipartisan letters commenting on proposed regulations, and through introduced legislation that would delay, limit, or prevent certain EPA actions. Particular attention was focused on EPA's implementation of the CAA. Because of health and cost implications, NAAQS decisions historically have been the source of significant concern to some in Congress.
The evolution and development of the PM NAAQS, in particular, has been the subject of extensive oversight. During the 112 th Congress, some Members expressed concerns in hearings, letters to the Administrator, and proposed legislation in anticipation of potential changes to the PM NAAQS, and the January 2013 final rule is expected to generate further oversight. Some Members and industry stakeholders had urged EPA to delay the final rule, while conversely, others, including some states and various environmental and public health advocacy groups, urged timely completion of a tighter standard. Changes to the NAAQS historically have triggered litigation alleging the standards are too stringent or not stringent enough, and often resulted in delays in implementation.
This CRS report summarizes EPA's January 15, 2013, final and June 2012 proposed changes to the PM NAAQS and includes comparisons with previous (1997 and 2006) promulgated and proposed standards. Key actions leading up to the agency's determination, and potential issues and concerns associated with changing the PM 2.5 annual standard, are also highlighted.

	Background

Particulate matter is one of six "criteria pollutants" for which EPA has promulgated NAAQS under the CAA. The others are ozone ("smog"), nitrogen oxides (NO x ), sulfur oxides (SO x , or, specifically, SO 2 ), carbon monoxide (CO), and lead (Pb).
PM 2.5 can be emitted directly from vehicles, smokestacks, and fires but can also form in reactions in the atmosphere from gaseous precursors, including sulfur oxides, nitrogen oxides, and volatile organics occurring naturally or as emissions typically associated with gasoline and diesel engine exhaust, and from utility and other industrial processes. PM 10 (or coarse PM) is an indicator used in the NAAQS to provide protection from slightly larger (in the range of 2.5 to 10 microns or thoracic "coarse" particles), but still inhalable particles that penetrate into the trachea, bronchi, and deep lungs. These particles are often associated with dust from paved and unpaved roads, construction and demolition operations (including mining), and sometimes with certain industrial processes and agriculture operations, as well as biomass burning.
Establishing NAAQS does not directly limit emissions; rather, it represents the EPA Administrator's formal judgment regarding the concentration of a pollutant in ambient air 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 process for setting and revising NAAQS consists of the statutory steps incorporated in the CAA over a series of amendments. Several other steps have also been added by EPA, by executive orders, and by subsequent regulatory reform enactments by Congress.
Section 109(d)(1)) of the CAA requires EPA to review the criteria that serve as the basis for the NAAQS for each covered pollutant every five years, to either reaffirm or modify previously established NAAQS. Prior to the January 2013 revisions, EPA has revised the PM NAAQS three times, in 1987, 1997, and October 2006, to ensure that the standards continue to provide adequate protection for public health and welfare.
A February 24, 2009, decision by the U.S. Court of Appeals for the District of Columbia Circuit had remanded elements of EPA's decisions as promulgated in October 2006, in particular the decision not to tighten the primary annual NAAQS for PM 2.5 , to the agency for further consideration but did not vacate the revised standard nor set a specific timeline. The decision was in response to petitions filed in the D.C. Circuit by 13 states, industry, agriculture, business, and environmental and public health advocacy groups, challenging certain aspects of EPA's revisions for both PM 2.5 and PM 10 . The D.C. Circuit granted the petitions in part with regard to the PM 2.5 annual standard and the secondary standards for PM 2.5 and PM 10 (including visibility impairment), denying other challenges.
Concerned with delays in EPA's schedule for proposing revisions to the 2006 PM NAAQS, the American Lung Association and the National Parks Conservation Association, and nine states separately filed petitions with the D.C. Circuit in November 2011 urging the court to order EPA's immediate compliance with the February 2009 remand. Subsequently, in February 2012 the two organizations sued EPA in the D.C. Circuit for failing to fulfill their statutory duty to review the October 2006 PM NAAQS within five years, and a coalition of 11 states filed a similar suit with the U.S. District Court Southern District of New York. In response, the D.C. Circuit initially directed EPA to sign a proposed rule concerning its decision regarding revisions to the PM NAAQS by June 7, 2012, and following a motion filed by the agency, amended the deadline to June 14, 2012. As part of a September 4, 2012, consent decree, EPA agreed to finalize revisions to the PM NAAQS by December 14, 2012.
Promulgation of a revised NAAQS, such as the PM NAAQS, initiates a series of statutorily required actions, ultimately culminating in issuance of permits pursuant to state regulations in a State Implementation Plan (SIP). The first step is designation of attainment and nonattainment areas, based on the accumulated results of ambient air monitoring and modeling data. States first propose to designate certain geographic areas (e.g., counties) as either "attainment" or "nonattainment," depending on whether the data indicate the concentrations of pollutants will be below or above the NAAQS. After extensive dialogue with state officials, EPA either approves the proposed attainment and nonattainment areas, or sends back to states proposed revisions. EPA and states generally come to an agreement about the area designations. Following this designation, approved by EPA, states then develop a SIP, which consists essentially of state regulations to be implemented by states that would affect the state emissions inventory, and therefore the expected or modeled concentrations of air pollutants. After approval of the SIP as being adequate to control air pollution and reduce the ambient air pollutant concentrations in designated nonattainment areas, the states then issue permits (new or modified) for facilities whose emissions affect the air in designated nonattainment areas.

	EPA's January 2013 Final Changes to the PM NAAQS

EPA's 1997 revisions to the PM NAAQS revised the standards focused on particles smaller than 10 microns (PM 10 or coarse particles) established in 1987, and introduced standards for "fine" particles smaller than 2.5 microns (PM 2.5 ) for the first time. The primary (health protection) PM NAAQS as revised in 2006 include an annual and a daily (24-hour) limit for PM 2.5 , but only a daily limit for PM 10 . To attain the PM 2.5 annual standard, the three-year average of the weighted annual arithmetic mean PM 2.5 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 percent of the time that a monitoring station can exceed the standard. For instance, a 98 th percentile 24-hour standard indicates that a monitoring station can exceed the standard 2% of the time during the year. For PM 2.5 and PM 10 , the secondary NAAQS, which are set at a level "requisite to protect the public welfare," are the same as the primary standards.
In the final rule published by EPA on January 15, 2013, the PM 2.5 and PM 10 standards and other implementation changes are as follows:
Primary (Public Health) PM Standards
PM 2.5 : EPA revised the annual standard, which currently is 15 micrograms per cubic meter (µg/m 3 ), by setting a new limit of 12 µg/m 3 (the proposal included an optional limit of 13 µg/m 3 and solicited comment for 11 µg/m 3 ); compliance with the "annual" standard is determined by whether the three-year average of its annual average PM 2.5 concentration (at each monitoring site in the area) is less than or equal to 12 µg/m 3 ; as proposed, EPA retained the daily (24-hour) standard at 35 µg/m 3 based on the current three-year average of the 98 th percentile of 24-hour PM 2.5 concentrations as established in 2006. PM 10 : As proposed, EPA retained the current daily standard of no more than one exceedance of concentrations of 150 µg/m 3 per year on average over three years; there is no current annual standard for PM 10 (the previous annual maximum concentration standard of 50 µg/m 3 was eliminated by EPA in 2006).
Secondary (Welfare) PM Standards
PM 2.5 and PM 10 : As proposed, secondary (welfare) NAAQS are the same as the primary standards, the same correlations as the 2006 PM NAAQS, with the exception of visibility impairment associated with PM 2.5 . PM 2.5 Visibility Impairment: The final rule did not add a distinct secondary standard as proposed, defined in terms of a PM 2.5 visibility index based on speciated PM 2.5 mass concentrations and relative humidity data to calculate light extinction on a deciview (dv) scale similar to the current Regional Haze Program. Specifically, the proposal would have set a 24-hour averaging time of 30 or 28 deciviews (dv) based on a 90 th percentile form over three years. EPA also sought comment on alternative levels (down to 25 dv) and averaging times (e.g., 4 hours). Based on public comment and further analysis of air quality monitoring data, EPA concluded that the current secondary standard would provide visibility protection greater than or equal to 30 dv.
Implementation Changes
Monitoring: 41 As proposed, updates several aspects of monitoring regulations including requiring relocating a small number of PM 2.5 monitors to be collocated with measurements of other criteria pollutants (e.g., nitrogen dioxide (NO 2 ) and carbon monoxide (CO)) near-roadway monitoring so as to ensure these monitors are at one location in each urban area with a population of 1 million or more, and to be phased in starting with the largest areas (2.5 million or more populations) by January 1, 2015, and extended to the remainder of areas by January 1, 2017. Includes the use data from existing Chemical Speciation Network or the EPA/National Park Service IMPROVE monitoring network to determine whether an area meets the proposed secondary visibility index standard for PM 2.5 . No changes to PM 10 monitoring. Air Quality Index (AQI): As proposed, updates the AQI (EPA's color-coded tool for informing the public about air quality and associated measures for reducing risks of exposure) for PM 2.5 by changing the upper end range for "Good" category (an index value of 50) on the overall scale (0 to 500 based on conversion of PM 2.5 concentrations) to the level of the revised annual PM 2.5 standard (12 µg/m 3 ). Also as proposed, EPA is setting the 100 value of the index scale ("Moderate") at the level of the current 24-hour PM 2.5 standard, which is 35 µg/m, 3 and the AQI of 150 ("Unhealthy Sensitive Groups") at 55 µg/m 3 . The current upper ends for the "Hazardous" (500), "Unhealthy" (200) and "Very Unhealthy" (300) AQIs are retained. Prevention of Significant Deterioration (PSD): EPA revised the PSD permitting program (rules) with respect to the revised PM NAAQS so as not to "unreasonably delay" pending permits and establish a "grandfather" provision for permit applications if: the permitting agency deems an application complete by December 14, 2012; or public notice for a draft permit or preliminary determination has been published (for public comment) no later than the effective date of revised PM NAAQS (60 days after January 15, 2013, publication in the Federal Register ). This provision would not apply to NAAQS for other criteria pollutants and permits not meeting these criteria would have to demonstrate compliance with the revised standards once they are finalized.

		Comparison of the January 2013 Revised PM2.5 Standards with Previous Promulgated and Proposed Alternative PM Standards

The final PM 2.5 daily standard established in 2006 was among the less stringent within the range of alternative levels recommended by EPA staff, and the annual standard is not as stringent as the standard recommended by the CASAC. The decision to retain the annual PM 2.5 standard was also less than recommended. Table 1 below shows the January 2013 revised PM 2.5 annual standard in comparison to the June 2012 proposed options and to the annual and daily standards for 1997 and 2006 promulgated standards, and alternative levels recommended prior to the 2006 final revisions.

	Review Process Leading Up to the January 2013 Revised PM NAAQS

The CAA as enacted includes specific requirements for a multistage process to ensure the scientific integrity under which NAAQS are set, laying the groundwork for the Administrator's determination of the standard, and the procedural process for promulgating the standard. Primary NAAQS, as described in Section 109(b)(1), were to be "ambient air quality standards the attainment and maintenance of which in the judgment of the Administrator, based on such criteria and allowing an adequate margin of safety, are requisite to protect the public health."
Based on this premise, the CAA specifies the criterion to be used by the Administrator in deciding on the final standard, including preparation of a "criteria document" that summarizes scientific information assessed. The act also requires the establishment and role of an independent advisory committee (CASAC) to review EPA's supporting scientific documents, and the timeline for completing specific actions. EPA administratively added the preparation of a "staff paper" that summarizes the criteria document and lays out policy options. This EPA document typically serves as the basis for CASAC review and comment. EPA revised certain aspects (not including reinstating the closure letter) of the CASAC review process most recently in May 2009. In addition, Executive Order 12866 requires a Regulatory Impact Analysis (RIA), although the economic impact analysis is essentially only for informational purposes and cannot be directly considered as part of the decision in determining the NAAQS.
Beginning June 2007 with its general call for information, EPA initiated the current PM NAAQS review, which culminated in assessments of the scientific research and risk analyses, and ultimately the April 2011 publication of the staff's final Policy Assessment for the Review of the Particulate Matter National Ambient Air Quality Standards (or PM Policy Assessment) . The staff paper presented the staff conclusions and recommendations on the elements of the PM standard based on evaluation of the policy implications of the scientific evidence contained in the criteria document and the results of quantitative analyses (e.g., air quality analyses, human health risk assessments, and visibility analyses) of that evidence. Table B-1 in Appendix B provides a chronological listing of EPA's supporting documents leading up to the June 2012 proposed PM NAAQS.
Supplemental to public comments solicited in the Federal Register , the CASAC reviewed EPA's drafts and final documents supporting the science and policy behind the Administrator's decisions in the June 2012 PM NAAQS proposal. The CASAC conducted meetings and consultations, and submitted written overviews, providing their views of the validity and completeness of the agency's assessments and findings, and recommending improvements. CASAC's final product, its review of EPA's second external review draft of the "PM Policy Assessment," was completed June 2010.
 Table B-2 in Appendix B provides a chronological summary of CASAC consultations and reviews of the supporting documents for the June 2012 proposal.
The April 2011 EPA policy assessment ("staff paper") concluded, and the CASAC panel concurred in its final recommendations, that the scientific evidence supported modifying the PM 2.5 primary standard and considering options for revising the secondary standard for reducing visibility impairment associated with PM. Recognizing certain limitations of the data, the policy assessment included a range of alternatives for consideration by the Administrator for modifying the current PM NAAQS. These recommendations were the core basis for the June 2012 proposal and the Administrator's final decision to revise the PM NAAQS, taking into account other factors including public comments received in response to the June 2012 proposal.
The EPA staff paper included possible modifications to strengthen certain aspects of the PM 10 standard. However, staff and CASAC placed considerable emphasis on continuing uncertainties and lack of sufficient data to initiate relevant quantitative risk assessment to support such modifications to the standard. As presented in the June 2012 Federal Register notice, the Administrator provisionally concluded that the growing evidence continued to support the appropriateness of the existing primary 24-hour PM 10 standard's protection of short-term health effects, and proposed to retain the existing PM 10 standard.
A perennial issue in conducting NAAQS reviews is whether the agency is basing its decisions on those studies that reflect the latest science, and that the scientific basis is rigorous and unbiased. In reviewing thousands of studies, the agency staff ultimately needs to establish a cutoff date, or be faced with the need for a continuous review. The current review was based on studies completed by mid-2009, but in the June 29, 2012, Federal Register notice EPA indicated that it
is aware that a number of new scientific studies on the health effects of PM have been published since the mid-2009 cutoff date for inclusion in the Integrated Science Assessment. As in the last PM NAAQS review, the EPA intends to conduct a provisional review and assessment of any significant new studies published since the close of the Integrated Science Assessment, including studies that may be submitted during the public comment period on this proposed rule in order to ensure that, before making a final decision, the Administrator is fully aware of the new science that has developed since 2009. In this provisional assessment, the EPA will examine these new studies in light of the literature evaluated in the Integrated Science Assessment. This provisional assessment and a summary of the key conclusions will be placed in the rulemaking docket.
Publication of the proposed PM NAAQS rule in the Federal Register on June 29, 2012, started a nine-week public comment period that ran through August 31, 2012. EPA also held two public hearings for the proposal on July 17, 2012, in Philadelphia, PA, and July 19, 2012, in Sacramento, CA. EPA's final determinations for revising the PM NAAQS published on January 15, 2013, were based on information provided in the two public hearings, the more than 230,000 written public comments received, and EPA's consideration of and analysis in response to this information. EPA also revised its Regulatory Impact Analysis (RIA), in large part in response to comments received.

	Implementing the Revised PM2.5 NAAQS

Promulgation of NAAQS sets in motion a process under which the states and EPA first identify geographic nonattainment areas, those areas failing to comply with the NAAQS based on monitoring and analysis of relevant air quality data. The CAA is specific with regard to the timelines for determining areas in noncompliance, submission of plans for achieving (or maintaining) compliance, and when noncompliant areas must achieve the established or revised NAAQS.
Typically, within three years of issuance of a NAAQS, states are required to submit "infrastructure" plans demonstrating that they have the basic air quality management components necessary to implement the NAAQS. Following states' proposed and EPA's final designations of attainment and nonattainment areas, states (and tribes if they choose to do so) must submit their plans (State Implementation Plans, or SIPs) for how they will achieve and/or maintain attainment of the standards. These often include new or amended state regulations and new or modified permitting requirements.
If new, or revised, SIPs for attainment establish or revise a transportation-related emissions allowance ("budget"), or add or delete transportation control measures, they will trigger "conformity" determinations. Transportation conformity is required by the CAA, Section 176(c) (42 U.S.C. 7506(c)), to prohibit federal funding and approval for highway and transit projects unless they are consistent with ("conform to") the air quality goals established by a SIP, and will not cause new air quality violations, worsen existing violations, or delay timely attainment of the national ambient air quality standards.
Areas designated nonattainment, as well as those designated unclassifiable or unclassifiable/attainment for the NAAQS, are also subject to new source review (NSR) requirements. Enacted as part of the 1977 CAA Amendments and modified in the 1990 CAA Amendments, NSR is designed to ensure that newly constructed facilities, or substantially modified existing facilities, do not result in violation of applicable air quality standards. NSR provisions outline permitting requirements both for construction of new major pollution sources and for modifications to existing major pollution sources. The specific NSR requirements for affected sources depend on whether the sources are subject to "Prevention of Significant Deterioration" (PSD) or nonattainment provisions. As discussed earlier (see " EPA's January 2013 Final Changes to the PM NAAQS "), the January 2013 final PM NAAQS includes revisions to the PSD permitting program (rules) with respect to the revised PM NAAQS so as not to "unreasonably delay" pending permits and establish a "grandfather" provision for permit applications if a draft permit or preliminary determination has been issued for public comment by the date the revised PM NAAQS go into effect.
In addition to the CAA requirement for states to submit implementation plans, EPA acts to control NAAQS pollutants through national regulatory programs. These may be in the form of regulations of products and activities that might emit the pollutants (particularly fuels and combustion engines, such as automobiles and trucks) and in the form of emission standards for new stationary sources (e.g., utilities, refineries). Often these national regulations reflect aspects of state rules previously issued by various states. EPA anticipates that recent CAA rules, including rules to reduce pollution from power plants, clean diesel rules for vehicles, and rules to reduce pollution from stationary diesel engines, would help states meet the revised PM NAAQS.

		Nonattainment Area Designation Process

The process of designating nonattainment areas is intended as a cooperative federal-state-tribal process in which states and tribes provide initial designation recommendations to EPA for consideration. In Section 107(d)(1)(A) (42 U.S.C. 7407), the statute states that the governor of each state shall submit a list to EPA of all areas in the state, "designating as ... nonattainment, any area that does not meet ( or that contributes to ambient air quality in a nearby area that does not meet ) an air quality standard" (emphasis added). Areas are identified as "attainment/unclassifiable" when they meet the standard or when the data are insufficient for determining compliance with the NAAQS.
Following state and tribal recommended designation submissions, the EPA Administrator has discretion to make modifications, including to the area boundaries. As required by statute (Section 107(d)1(B)(ii)), the agency must notify the states and tribes regarding any modifications, allowing them sufficient opportunity to demonstrate why a proposed modification is inappropriate, but the final determination rests with EPA.
Measuring and analyzing air quality to determine where NAAQS are not being met is a key step in determining an area's designation. Attainment or nonattainment designations are made primarily on the basis of three years of federally referenced monitoring data. EPA began developing methods for monitoring fine particles at the time the PM 2.5 NAAQS were being finalized in 1997, and operation of the network of monitors for PM 2.5 was phased in from 1999 through 2000. The network of monitors and their locations have been modified over time. Most recently, in a separate action in conjunction with the October 2006 publication of the revised particulates NAAQS, EPA amended its national air quality monitoring requirements, including those for monitoring particle pollution. The amended monitoring requirements were intended to help federal, state, and local air quality agencies by adopting improvements in monitoring technology. Additional modifications to the PM NAAQS monitoring network were included in the final January 2013 rule, as discussed earlier in this report.
In addition to air emission and air quality data, EPA considers a number of other relevant factors when designating nonattainment areas, and recommends that states apply these factors in their determinations in conjunction with other technical guidance. Examples of these factors include population density and degree of urbanization (including commercial development), growth rates, traffic and commuting patterns, weather and transport patterns, and geography/topography. States and tribes may submit additional information on factors they believe are relevant for EPA to consider.
Nonattainment areas include those counties where pollutant concentrations exceed the standard as well as those that contribute to exceedance of the standard in adjoining counties. Entire metropolitan areas tend to be designated nonattainment, even if only one county in the area has readings worse than the standard. In addition to identifying whether monitored violations are occurring, states' or tribes' boundary recommendations for an area are to also show that violations are not occurring in those portions of the recommended area that have been excluded, and that they do not contain emission sources that contribute to the observed violations.

			January 2013 Final Revised PM2.5 Annual NAAQS: Area Designations68

The January 2013 final rule revising the PM 2.5 annual standard, as expected, resulted in an increased number of areas (typically defined by counties or portions of counties) designated nonattainment. Similar to the revisions to the PM 2.5 daily (24-hour) standard in 2006, the January 2013 revised concentrations for the PM 2.5 annual standard were expected to affect primarily areas currently in nonattainment for the 2006 standards, but also included counties that have not been previously designated as nonattainment. EPA did not require new nonattainment designations for the PM 2.5 24-hour standard or the PM 10 primary NAAQS since the standards were not changed in the January 2013 final rule.
Section 107(d)(1) of the CAA requires states to submit area designation recommendations no later than one year following the promulgation of a NAAQS standard. For the 2013 PM NAAQS, state recommendations were due by December 13, 2013. The CAA requires EPA to make its final area designations within one year of the state and tribal recommendations. EPA is required to notify states and tribes of its intended modifications to their recommendations 120 days prior to promulgating final designations. EPA responded to the states with its proposed modifications to their area designation recommendations for the 2013 PM 2.5 NAAQS for the annual standard in letters on or about August 19, 2014. As per the CAA, states were provided the opportunity to submit additional relevant information to demonstrate why EPA's modifications to the states' recommendations are inappropriate prior to the agency's final designations.
EPA's final area designations as amended and published in the April 2015 Federal Register encompass 20 counties or portions of counties (nine areas) in four states—California, Idaho, Ohio, and Pennsylvania—as nonattainment only for the 2013 revised annual PM 2.5 standard. The remaining nearly 3,000 counties in the United States were classified as unclassified or unclassified/nonattainment for the 2013 revised annual standard. Comparatively, EPA's final designations for nonattainment of the 2006 and 1997 PM 2.5 NAAQS (those areas with or contributing to air quality levels exceeding the annual and 24-hour PM 2.5 standards or both) included all or part of 242 counties in 28 states and the District of Columbia. Counties included in EPA's amended final nonattainment area designations for the 2013 PM 2.5 NAAQS are indicated in the map in Figure 1 .
The April 2015 modified designations supersede EPA's January 15, 2015, final area designations that classified 14 areas as nonattainment for the revised 2013 primary annual PM 2.5 standard. The areas included 38 counties or portions of counties in six states—California, Idaho, Indiana, Kentucky, Ohio, and Pennsylvania. EPA also had deferred the designation period for 11 other areas composed of 2 entire states (Florida and Tennessee, excluding 3 counties), and 24 counties in three states (Alabama, Georgia, and South Carolina) by up to one year.
In accordance with a January 4, 2014, decision by the Circuit Court of the District of Columbia, EPA classified all nonattainment areas as "Moderate" under section 188(a) of Subpart 4 of Title I of the CAA as specified in its April 2013 area designation guidance for states. EPA may reclassify as "Serious" those nonattainment areas that EPA determines cannot practicably attain the PM 2.5 NAAQS by the applicable attainment date, or if the agency determines that any area has not in fact attained the PM 2.5 NAAQS after each area's applicable attainment date has passed. EPA had previously implemented the PM 2.5 NAAQS, including nonattainment determinations for the 1997 and 2006 revisions, under the general implementation provisions in Subpart 1 of Part D of Title I of the act. However, in the January 4, 2013 decision, the Circuit Court of the District of Columbia determined that EPA had erred in implementing the PM 2.5 NAAQS under Subpart 1, and required the Agency to implement the PM 2.5 NAAQS under Subpart 4.
Based on EPA's April 7, 201,5 amended final designations for the 2013 PM 2.5 annual standard, two counties (or portions) of the 20 counties are designated nonattainment for PM 2.5 for the first time, but the majority of the counties overlap with EPA's final nonattainment designations for the 2006 and 1997 PM 2.5 annual and/or 24-hour standards. Of the remaining 18 counties designated as nonattainment for the 2013 revised annual PM 2.5 standard, two have not been previously designated for the annual standard but were designated as nonattainment for the 24-hour standard, and the remaining 16 counties have been previously designated as nonattainment only for the annual standard or for the annual and the 24-hour PM 2.5 standards.
The 2006 revisions to the PM NAAQS tightening the 24-hour standard, which are currently being implemented, primarily affected urban areas. EPA final designations published on November 13, 2009, for the 2006 PM NAAQS included 31 areas in 18 states, comprising 120 counties (89 counties and portions of 31 additional counties) for nonattainment of the revised 2006 24-hour PM 2.5 standard. The designations, based on 2006 through 2008 air quality monitoring data, included a few counties that were designated nonattainment for PM 2.5 for the first time, but the majority of the counties identified overlapped with EPA's final nonattainment designations for the 1997 PM 2.5 NAAQS. It is important to note that most of the 1997 PM 2.5 nonattainment areas were only exceeding the annual standard; thus, tightening the 24-hour standard resulted in an increased number of areas being designated nonattainment based on exceedances of both the 24 - hour and the annual standard. The majority of the roughly 3,000 counties throughout the United States (including tribal lands) were designated attainment/unclassifiable, and are not required to impose additional emission control measures to reduce PM 2.5 .

		State Implementation Plans (SIPs)

Under the CAA, EPA sets the nationwide standard for criteria pollutants, and EPA and states are responsible for placing limits on emissions that contribute to criteria pollution and for regulating entities emitting criteria pollutants. Areas designated as attainment/unclassifiable will not have to take steps to improve air quality, but under the statute they must take steps to prevent air quality from deteriorating to unhealthy levels. For those areas designated nonattainment, state, local, and tribal governments must outline detailed control requirements in plans demonstrating how they will meet the 2013 PM 2.5 annual standard. These plans are defined as State Implementation Plans, and referred to as SIPs (TIPs for tribal implementation plans).
As discussed previously in this CRS report, all initial nonattainment area designations for the 2013 PM 2.5 annual standard will be classified by EPA as Moderate as provided under Subpart 4 of Part D of Title I of the CAA (section 188(a)), not under the general implementation provisions in Subpart 1 of the act. For the most part, the Subpart 4 SIP requirements for areas classified as Moderate are comparable to those of Subpart 1. However, under Subpart 4 states have 18 months from the date of EPA's final designations to submit SIPs. Implementing the PM 2.5 NAAQS under Subpart 1 required submission of SIPs three years from the date of EPA's final designations. The EPA Moderate nonattainment designations for the 2013 PM 2.5 NAAQS will be effective 90 days after publication in the Federal Register ; thus EPA anticipates that states will need to submit their plans by fall of 2016.
Under Subpart 4, EPA may reclassify as "Serious" any nonattainment area that the agency determines cannot practicably attain the PM 2.5 NAAQS by the applicable attainment date or those areas classified as Moderate that do not attain the PM 2.5 NAAQS after their applicable attainment date has passed. Subpart 4 introduces additional statutory SIP planning requirements for areas classified as "Serious." These additional requirements must be reflected in the states' initial SIP submissions.
Subpart 4 requires states to achieve attainment for Moderate areas as expeditiously as practicable, but no later than six years after designation; Serious areas must achieve attainment no later than 10 years from designation as nonattainment. Under the general provisions in Subpart 1, which has no classifications, attainment must be achieved no later than five years from the effective designation date. Both Subpart 4 and Subpart 1 included provisions for extensions.

		National Regulations

EPA anticipates that in many cases, stationary and mobile source controls and additional reductions currently being adopted to attain the 2006 PM 2.5 standards in conjunction with expected emission reductions from implementing national regulations and strategies will help states meet the proposed standards. These national actions EPA referenced include the
Cross-State Air Pollution Rule (CSAPR); Mercury and Air Toxics Standards (MATS); Light-Duty Vehicle Tier 2 Rule; Heavy Duty Diesel Rule; Clean Air Nonroad Diesel Rule; Regional Haze Regulations and Guidelines for Best Available Retrofit Technology Determinations; NOx Emission Standard for New Commercial Aircraft Engines; Emissions Standards for Locomotives and Marine Compression-Ignition Engines; Control of Emissions from Nonroad Spark Ignition Engines and Equipment; Category 3 Oceangoing Vessels; Reciprocating Internal Combustion Engines (RICE) National Emissions Standards for Hazardous Air Pollutants (NESHAPS); and New Source Performance Standards and Emissions Guidelines for Hospital/Medical/Infectious Waste Incinerators Final Rule Amendments.
Stakeholders and some Members of Congress are skeptical about EPA's expectations with respect to the corollary benefits associated with some of these regulations, and raise concerns about pending efforts to delay some of the more recent programs and historical delays of others. Of particular concern are the Cross-State Air Pollution Rule ("Cross-State Rule," or CSAPR), which was to have gone into effect in 2012 but was stayed in December 2011, then vacated on August 21, 2012, by the D.C. Circuit Court of Appeals, and the Mercury and Air Toxics Standards (MATS), which EPA itself has stayed with regard to new plants, pending reconsideration. On October 5, 2012, the U.S. Department of Justice filed a petition seeking en banc rehearing of the D.C. Circuit's August 21, 2012, decision regarding the CSAPR. The D.C. Circuit denied requests for both a panel and the en banc rehearing on January 24, 2013. To date, EPA has not made a decision regarding its response to the Court's denial for a rehearing. 
Other rules remanded or reconsidered include the hazardous air pollutant ("MACT") standards for boilers and cement kilns. EPA has delayed implementation of the boiler MACT rules for more than a year and a half while considering changes to the requirements. The agency has also extended the compliance deadline for the cement kiln MACT by two years.

	Potential Impacts of More Stringent PM Standards

The impacts of the revising PM NAAQS can be both potentially far-reaching and indirect. As discussed earlier in this report, the NAAQS by itself does not compel any specific direct pollution control measures. Rather it starts a process that could result in significant required investments by emitting sources in control measures. In addition to these costs, the eventual result is projected by EPA to be potentially significant health benefits. Estimates of health and welfare risk reductions and costs associated with control strategies for areas potentially not in compliance provide some insights into potential impacts of the June 2012 proposed and January 2013 final revisions to the PM NAAQS.
The Clean Air Act requires that NAAQS be set solely on the basis of public health and welfare protection, while costs and feasibility are generally taken into account in implementation of the NAAQS (a process that is primarily a state responsibility). As discussed previously, in setting and revising the NAAQS, the CAA directs the EPA Administrator to protect public health with an adequate margin of safety . This language has been interpreted, both by the agency and by the courts, as requiring standards be based on a review of the health impacts, without consideration of the costs, technological feasibility, or other nonhealth criteria.
Nevertheless, coinciding with the PM NAAQS final rule released on December 14, 2012, and proposed rule in the June 29, 2012, Federal Register , EPA released regulatory impact analyses (RIA) assessing the costs and benefits of setting the standard at the proposed and other alternative levels, to meet its obligations under Executive Order 12866 and in compliance with guidance from the White House Office of Management and Budget. EPA emphasized that the RIA is for informational purposes and that decisions regarding revisions to the PM NAAQS are not based on consideration of the analyses in the RIA in any way. In addition, the expected costs are more difficult to predict than for many other regulations because the ultimate pollution control requirements, which are the primary costs, will depend on a variety of factors, such as state regulatory decisions and the results of monitoring and modeling analysis of designated areas that are not fully knowable at this time.
In part in response to comments received and considered following the June 2012 proposal, EPA revised its RIA for the final rule. Table 2 below presents a range of EPA's estimated economic costs, monetized benefits, and net benefits (subtracting total costs from the monetized benefits) associated with achieving the revised PM 2.5 standards in the final rule published in January 2013, and other alternatives considered as presented in EPA's revised RIA.
As shown in Table 2 , EPA estimated that the monetized benefits associated with the January 2013 final revised PM 2.5 annual standard of 12 µg/m 3 would range $4.0 billion to $9.1 billion per year in 2020 (2010 $), compared to annual costs ranging from $53.0 million to $350.0 million. EPA also noted that a full accounting of benefits would include additional environmental and societal benefits that were not quantified in the analysis. The basis for the benefits calculations is health and welfare impacts attributable to reductions in ambient concentration of PM 2.5 resulting from a reasonable, but "speculative," array of known state implementation emission control strategies selected by EPA for purposes of analysis. The analysis does not model the specific actions that each state will undertake or emerging technologies in implementing the alternative PM 2.5 NAAQS. EPA notes that reductions in annual premature deaths represent a substantial proportion of total monetized benefits (over 90%).
EPA estimated total costs under partial and full attainment of several alternative PM standards. The engineering costs generally include the costs of purchasing, installing, and operating the referenced control technologies. The technologies and control strategies selected for analysis are illustrative of one way in which nonattainment areas could meet a revised standard. EPA anticipates that in actual SIPS, state and local governments will consider programs that are best suited for local conditions as there are various options for potential control programs that would bring areas into attainment with alternative standards. EPA includes a detailed discussion of the limitations and uncertainties associated with the benefits assumptions and analyses.
While recognizing the need to adequately protect against potential health concerns associated with PM, some Members and stakeholders are apprehensive that EPA has underestimated potential costs and are concerned with the potential monetary consequences associated given the current economic environment. In particular, some stakeholders question the validity of EPA's reliance on the associated impacts of other national regulations in reducing the potential burdens. Critics are concerned that this results in underestimating the number of areas (counties) likely to be affected in terms of their ability to attain the proposed alternative PM NAAQS and the expected associated costs of necessary measures that will be required in the form of SIPs.

	Reaction to the Revised PM NAAQS

Prior to EPA's June 2012 proposed rule to revise the PM NAAQS, stakeholders were providing evidence and arguments in letters, press releases, at public hearings and other forums for their preferred recommendations, and EPA received numerous comments during various stages of development of the criteria and policy documents. In general, business and industry opposed more stringent standards particularly in light of the current national and global economic environment; and public health and environmental advocacy groups advocated support for more stringent standards based on the continuing evidence of health effects from ongoing scientific research. As mentioned earlier, several states petitioned EPA, and subsequently filed suit in the D.C. Circuit Court urging timely completion of EPA's review of the PM NAAQS in response to the February 2009 remand. Other state air quality regulators recognized the need to ensure adequate health protection from PM, but expressed concerns about the impacts of more stringent PM NAAQS on already strained state budgets.
Proponents of more stringent standards generally stress that
the PM 2.5 standards should be at least as stringent as the more stringent combined daily and annual levels recommended in the 2006 EPA staff paper, and those recommended by the CASAC; scientific evidence of adverse health effects is more compelling than when the standards were revised in 2006; more stringent standards ensure continued progress toward protection of public health with an adequate margin of safety as required by the CAA; and welfare effects, particularly visibility, should be enhanced.
Critics of more stringent PM NAAQS stress that
more stringent (and in some cases the existing) standards are not justified by the scientific evidence; the proposal does not take into account studies completed since the 2009 cutoff; requiring the same level of stringency for all fine particles without distinguishing sources is unfounded; costs and adverse impacts on regions and sectors of the economy are excessive; EPA has potentially overstated the expected benefits and underestimated expected costs; revising the standards could impede implementation of the existing (2006) PM NAAQS and the process of bringing areas into compliance, given the current status of this process; the benefits (and costs) associated with implementation of the 2006 PM NAAQS, as well as compliance with other relatively recent EPA air quality regulations that are being implemented, have not yet been realized; and revisions to PM NAAQS are unnecessary as shown by EPA's trends data that annual and 24-hour measured PM national concentrations have declined 24% and 28% respectively from 2001 to 2010.

	Congressional Activity

Not long after EPA's release of its PM NAAQS proposal, the House Committee on Energy and Commerce Subcommittee on Energy and Power held a hearing on June 28, 2012, on the potential impacts of tightening the PM 2.5 NAAQS. The focus of the debate was the regulatory costs and burdens associated with the implementation of the revised standards, and potential impacts on economic growth, employment and consumers. Just prior to EPA's release of the June 2012 proposal, several Members urged the Administrator to include retaining the current (as of 2006) PM 2.5 standard as an option for consideration in the agency's proposal. In November 2012, some Members urged EPA to consider delaying the final rule, while conversely, others, along with some state attorneys general, supported timely completion of the agency's review. As mentioned earlier in this report, also in November 2012, some Members recommended EPA reconsider its calculations of costs and benefits supporting the proposed rule. Also, although the January 15, 2013, final rule did not modify the standards for inhalable "coarse" particles larger than 2.5 but smaller than 10 microns (PM 10 ), nor were modifications proposed in June 2012, some Members maintained a particular interest in EPA's consideration of the PM 10 standards.
During the second session of the 111 th and during the first session of the 112 th Congress, some Members raised concerns in letters to the EPA Administrator and during oversight hearings about EPA's staff draft reports and CASAC recommendations regarding changes to the PM NAAQS leading up to the June 2012 proposal. Some Members expressed their concerns of potential impacts that the options for changing PM NAAQS standards could have on industry and on agricultural operations. In letters to the EPA Administrator, several Members also communicated their particular concerns with the agency's consideration of stricter standards for coarse particulates (PM 10 ), including apprehensions of how changes may affect the agricultural community. 
Additionally, during the 112 th Congress, the House-passed Farm Dust Regulation Prevention Act of 2011 ( H.R. 1633 ) would have prohibited EPA from proposing, finalizing, implementing, or enforcing any regulation revising primary or secondary NAAQS applicable to PM "... with an aerodynamic diameter greater than 2.5 micrometers ..." for one year. Further, the House-passed bill would have amended the CAA to exempt "nuisance dust" from the act and would have excluded nuisance dust from references in the act to particulate matter "... except with respect to geographic areas where such dust is not regulated under state, tribal, or local law.... " Nuisance dust was defined in the bill as particulate matter that
(1) is generated primarily from natural sources, unpaved roads, agricultural activities, earth moving, or other activities typically conducted in rural areas; (2) consists primarily of soil, other natural or biological materials, windblown dust, or some combination thereof; (3) is not emitted directly into the ambient air from combustion, such as exhaust from combustion engines and emissions from stationary combustion processes; (4) is not comprised of residuals from the combustion of coal; and (5) does not include radioactive particulate matter produced from uranium mining or processing.
A general provision included in FY2012 House-reported EPA appropriations language ( H.R. 2584 , Title IV, and §454) would have restricted the use of FY2012 appropriations "to modify the national primary ambient air quality standard or the national secondary ambient air quality standard applicable to coarse particulate matter (generally referred to as "PM 10 ")." No comparable provision was retained in the Consolidated Appropriations Act, 2012 ( P.L. 112-74 ), enacted December 23, 2011, which ultimately included EPA's FY2012 appropriation.
NAAQS decisions have often been a source of significant concern to many in Congress. The evolution and development of the PM (and ozone) NAAQS, in particular, have been the subject of extensive oversight. For example, following promulgations of the 1997 NAAQS Congress held 28 days of hearings on the EPA rule. Congress enacted legislation specifying deadlines for implementation of the 1997 standard, funding for monitoring and research of potential health effects, and the coordination of the PM (and ozone) standard with other air quality regulations. During the 109 th Congress, hearings were held regarding implementation and review of the PM NAAQS leading up to promulgations of the 2006 PM NAAQS.
Because of the potential impacts PM NAAQS could have on both public health and the economy, EPA's final rule published on January 15, 2013, modifying these standards generated mixed reactions from some Members, and the issue will likely be of continued interest in the 114 th Congress, as will EPA's next five-year review of the PM NAAQS, which the agency commenced at the beginning of December 2014.

	Conclusions

EPA's changes to the PM NAAQS in its final rule published on January 15, 2013, following completion of its statutorily required review, have continued to garner attention and conflicting concerns among a diverse array of stakeholders, and in Congress. As evidenced by the history of the PM NAAQS, the level of scrutiny and oversight could increase in the coming months. Because both the health and economic consequences of particulate matter standards are potentially significant, the PM NAAQS are likely to remain a prominent issue in the 113 th Congress.
EPA asserts that its review and analyses of scientific evidence showed that revising the PM NAAQS could potentially result in fewer adverse health effects for the general population and particularly sensitive populations such as children, asthmatics, and the elderly, as well as improved welfare effects. Nonetheless, concerns remain with regard to the potential associated costs. In its assessment of the impacts of revising the PM NAAQS, EPA expected that relatively few additional areas (counties) would be in nonattainment and require more stringent pollution controls to achieve compliance. Industry, some Members, and some state representatives remained concerned that the January 2013 revised PM NAAQS would result in more areas than anticipated by EPA being classified as nonattainment and needing to implement new controls on particulate matter. Further, they are concerned that stricter standards may mean more costs for the transportation and industrial sectors, including utilities, refineries, and the trucking industry, affected by particulate matter controls. Others stress that related ongoing control efforts from prior and recently promulgated actions are expected to reduce the potential number of nonattainment areas, or at least facilitate compliance.
EPA's review and establishment of the 1997 PM NAAQS was the subject of litigation and challenges, including a Supreme Court decision in 2001. EPA's 1997 promulgation of standards for both coarse and fine particulate matter prompted critics to charge EPA with over-regulation and spurred environmental groups to claim that EPA had not gone far enough. Not only was the science behind the PM NAAQS challenged, but EPA was also accused of unconstitutional behavior. More than 100 plaintiffs sued to overturn the standard. Although EPA's decision to issue the standards was upheld unanimously by the Supreme Court, for the most part, stakeholders on both sides of the issue continued to advocate their recommendations for more stringent and less stringent PM standard. Several states and industry, agriculture, business, and environmental and public health advocacy groups petitioned the U.S. Court of Appeals for the District of Columbia Circuit, challenging certain aspects of EPA's revisions of the PM NAAQS as promulgated December 2006. 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. The court did not specifically vacate the 2006 PM NAAQS and implementation is currently underway.
EPA received considerable (more than 230,000 written) comments in response to the June 2012 proposal. Concerned stakeholders may return to the courts or initiate challenges in response to the final standards published on January 15, 2013, thus potentially furthering delays in designating nonattainment areas, and states' development and implementation of SIPs.
Chronological Summary of Key Milestones Subsequent to the January 2013 PM NAAQS Final Rule
As part of the D.C. Circuit's decision and a related Consent Agreement, EPA agreed to issue final revised PM NAAQS by December 14, 2012. The timeline presented in Table A-1 below reflects the most recent projected milestone dates subsequent to the January 15, 2013, publication of the final rule revising the PM NAAQS. These milestones are driven primarily by statutory requirements under the CAA, and are based on milestones identified in the June 29, 2012, Federal Register and EPA fact sheets accompanying the agency's proposed and final regulatory actions. The CAA does not specify a timeframe with regard to when states must meet secondary PM standards; relevant milestones are determined by EPA and states through the implementation planning process.
Supporting EPA Scientific and Policy Documents, and CASAC Review
Comparison of Potential Nonattainment Areas for the January 2013 Final Revised PM 2.5 Annual Standard with the Final Designations for the 2006 and 1997 PM 2.5 NAAQS
The nonattainment designations presented in this appendix are as predicted by EPA at the time of the release of the final PM NAAQS rule on December 14, 2012. For details regarding EPA's initial final designations for the 2013 PM NAAQS revisions published January 15, 2015, and the subsequent April 7, 2015, modified designations, see CRS Report R43953, 2013 National Ambient Air Quality Standard (NAAQS) for Fine Particulate Matter (PM2.5): Designating Nonattainment Areas , by [author name scrubbed].
Based on anticipated reductions associated with several other existing national air pollution control regulations and programs (see discussion in " National Regulations " section), EPA had predicted that seven counties in California would be the only areas unable to meet the new PM 2.5 primary standard by 2020. Additionally, for illustrative purposes, EPA identified 66 counties with monitors that showed concentrations of PM 2.5 that would exceed the revised limit of the primary annual standard of 12 µg/m 3 based on 2009-2011 air quality monitoring data. According to EPA, 47 of these counties were determined nonattainment areas previously for PM 2.5 NAAQS based on earlier monitoring data available at the time and other factors considered.
The map in Figure C-1 below depicts the potential nonattainment areas (counties) as identified by EPA for the revised PM 2.5 annual standards based solely on the 2009-2011 air quality monitoring. The areas are depicted in the map for illustration purposes as a rough approximation of the potential areas that may be designated nonattainment, as they do not take into account other factors generally considered in making final designation determinations. The specific counties based on the 2009-2011 monitoring data are shown in Table C-1 , which also shows the overlap of those nonattainment areas for the existing (2006) PM 2.5 annual and/or daily (24-hour) standards, and indicates those areas not previously designated nonattainment. EPA predicted that data from future monitoring, 2011-2013, will possibly show continued decline in levels of PM and their precursors, resulting in fewer nonattainment areas than shown by the 66 counties approximated.

Summary:
On January 15, 2013, the Environmental Protection Agency (EPA) published a final rule revising the National Ambient Air Quality Standard (NAAQS) for particulate matter (PM). The revised air quality standards were completed pursuant to the Clean Air Act (CAA) and, in part, in response to a court order and consent agreement. Based on its review of scientific studies available since the agency's previous review in 2006, EPA determined that evidence continued to show associations between particulates in ambient air and numerous significant health problems, including aggravated asthma, chronic bronchitis, nonfatal heart attacks, and premature death. Populations shown to be most at risk include children, older adults, and those with heart and lung disease, and those of lower socioeconomic status. EPA's review of and revisions to the PM NAAQS have generated considerable debate and oversight in Congress.
The January 2013 revisions change the existing (2006) annual health-based ("primary") standard for "fine" particulate matter 2.5 micrometers or less in diameter (or PM2.5), lowering the allowable average concentration of PM2.5 in the air from the current level of 15 micrograms per cubic meter (µg/m3) to a limit of 12 µg/m3. The annual PM2.5 NAAQS is set so as to address human health effects from chronic exposures to the pollutants. The existing "24-hour primary standard" for PM2.5 that was reduced from 65 µg/m3 to 35 µg/m3 in 2006 was retained, as was the existing standard for larger, but still inhalable, "coarse" particles less than 10 micrometers in diameter, or PM10. As it did in 2006, EPA set "secondary" standards that provide protection against "welfare" (nonhealth) effects, such as ecological effects and material deterioration, identical to the primary standards.
EPA revised the Regulatory Impact Analysis (RIA) accompanying its June 2012 proposed rule in part in response to comments received regarding the agency's cost and benefit estimates. In its December 2012 RIA, EPA estimated that the potential "quantifiable" health benefits (2010 $) associated with attaining the PM standard would range from $4.0 billion to $9.1 billion, and costs (2010 $) would range from $53.0 million to $353.0 million. Some stakeholders and some Members continue to express concerns that cost impacts would be more significant than those estimated by EPA for those areas out of compliance with the new standards.
EPA's revisions to the PM NAAQS do not directly regulate emissions from specific sources, or compel installation of any pollution control equipment or measures, but indirectly could affect operations at industrial facilities and other sources throughout the United States. Revising PM NAAQS starts a process that includes a determination of areas in each state that exceed the standard and must, therefore, reduce pollutant concentrations to achieve it. Following determinations of these "nonattainment" areas based on multiple years of monitoring data and other factors, state and local governments must develop (or revise) State Implementation Plans (SIPs) outlining measures to attain the standard. These include promulgation of new regulations by states, and the issuance of revised air permits. The process typically takes several years.
On January 15, 2015, EPA published its classification of 14 areas as "Moderate" nonattainment for the revised 2013 primary annual PM2.5 standard. EPA subsequently published amended designations on April 7, 2015. EPA's April 2015 modifications to the final area designations for the 2013 PM NAAQS resulted in nine areas consisting of 20 counties in four states designated as nonattainment. All but two of the 20 counties have been previously designated as nonattainment for the 2006 and/or the 1997 PM2.5 NAAQS. For a Moderate area, CAA Section 188(c)(1) of Subpart 4 establishes an attainment deadline of as expeditiously as practicable but no later than the end of the sixth calendar year after designation as nonattainment.