The movement of air pollutants across state lines, known as interstate transport, has posed a decades-long challenge to air quality protection. The Clean Air Act (CAA) assigns responsibility to states to limit emissions from sources within their borders as needed to attain federal health-based air quality standards. A state's air quality, however, may be affected by emissions from upwind sources located in a different state. Hence, controlling emissions within the border of a state may not be sufficient to attain the air quality standard. The downwind state lacks authority to limit emissions from the sources in the upwind state(s) but is nonetheless responsible for attaining the federal standards. Interstate transport has made it difficult for some downwind states to attain federal standards for ozone and fine particulate matter (PM 2.5 ). Both of these pollutants are formed by precursor emissions that can travel long distances. Specifically, sulfur dioxide (SO 2 ) and nitrogen oxide (NO x ) contribute to the formation of PM 2.5 in the air. NO x and volatile organic compounds (VOCs) react in sunlight to form ground-level ozone, the main component of smog. Studies have shown that these precursor emissions, as well as ozone and PM 2.5 , are regional pollutants, meaning that they can travel hundreds of miles through the atmosphere. For example, Bergin et al. 's study of the eastern United States concluded that regional transport affected air quality in most eastern states. They attributed an average of 77% of each state's ozone and PM 2.5 concentrations to emissions from upwind states. These regional emissions are associated with health impacts and are therefore of concern. For example, research shows that ground-level ozone is associated with aggravated asthma, chronic bronchitis, heart attacks, and premature death. Studies have also linked exposure to particulate matter to respiratory illnesses, such as aggravated asthma, as well as heart attacks and premature death. The CAA's "Good Neighbor" provision recognizes such interstate issues and requires states to prohibit emissions that significantly contribute to air quality problems in another state (Section 110(a)(2)(D)). It requires each state's implementation plan—a collection of air quality regulations and documents—to include adequate provisions to prohibit emissions that either "contribute significantly" to nonattainment or "interfere with maintenance" of federal air quality standards in another state. Since the 1990s, the U.S. Environmental Protection Agency (EPA) and the states have implemented various regional programs to address interstate air transport. Many of these programs have since concluded. The current program—the Cross State Air Pollution Rule (CSAPR, pronounced "Casper")—is an emissions trading program for 28 states in the eastern part of the United States. EPA established CSAPR to limit interstate transport of power sector SO 2 and NO x emissions and help states comply with the 1997 and 2006 PM 2.5 standards as well as the 1997 and 2008 ozone standards. EPA has attributed emission reductions to CSAPR and the agency's other emissions trading programs, such as the Acid Rain Program: annual SO 2 emissions from power plants participating in CSAPR were 1.2 million tons in 2016, an 87% reduction from 2005 levels. CSAPR power plants also emitted 420,000 tons of NO x in the 2016 ozone season, roughly an 80% reduction from the 1990 ozone season NO x emissions. Emissions reduction progress notwithstanding, some areas of the country do not meet federal air quality standards for pollutants like ozone and particulate matter. In 2018, EPA designated 52 areas with approximately 200 counties or partial counties as "nonattainment" with respect to the 2015 ozone standard. Members of Congress representing both downwind and upwind states may have an interest in how EPA and states implement the CAA's Good Neighbor provision, particularly as states begin to develop plans for nonattainment areas to come into compliance with the 2015 ozone standards. Some downwind states with nonattainment areas have attributed their ozone violations—at least in part—to emission sources from upwind states. Downwind states have also expressed concerns that transported air pollution contributes to harmful human health impacts and adversely affects economic growth. For example, a Maryland state agency reported that transport of emissions from upwind states has required Maryland's sources to compensate with "deeper in-state emissions reductions," thereby adding economic costs to the state's business community. Upwind states have disagreed with the approach used by EPA to determine whether emissions from upwind sources contribute to downwind air quality problems. For instance, Ohio's state environmental agency described EPA's transport approach as "deeply flawed," concluding that it would place "an unfair amount of responsibility" on upwind power plants to reduce emissions. To assist Members and staff in understanding interstate transport issues, this report presents background information about the CAA's interstate transport provision, provides a brief history of regional programs leading up to CSAPR, discusses key aspects of the CSAPR program and program results, summarizes the status of Good Neighbor determinations with respect to ozone standards, and concludes with issues for congressional consideration. The CAA requires EPA to establish national standards for air pollutants that meet the criteria in Section 108(a)(1). These pollutants—the "criteria pollutants"—are those that EPA has determined "may reasonably be anticipated to endanger public health or welfare" and whose presence in "ambient air results from numerous or diverse mobile or stationary sources." EPA must design two types of National Ambient Air Quality Standards (NAAQS) for the criteria pollutants. Primary NAAQS must protect public health with an "adequate margin of safety," and secondary NAAQS must "protect public welfare from any known or anticipated adverse effects." The NAAQS are concentration standards measured in parts per million (ppm) by volume, parts per billion (ppb) by volume, and micrograms per cubic meter of air (µg/m 3 ). The NAAQS do not set direct limits on emissions but rather define what EPA considers to be clean air for the pollutant in question. Section 109(d) of the act requires periodic NAAQS reviews. Every five years, EPA must review the NAAQS and the science upon which the NAAQS are based and then revise the NAAQS if necessary. This multi-step process is rarely completed within the five-year review cycle and is often the subject of litigation that results in court-ordered deadlines for completion of NAAQS reviews. Since January 1997, EPA has completed at least one review for each of the six criteria pollutants (carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide), with standards being made more stringent for five of the six. Most of the revisions finalized in this time period were for the ozone and particulate matter standards. The CAA assigns responsibility to states to establish procedures to attain and maintain the NAAQS within their borders. In particular, the act requires each state to submit a new or revised state implementation plan (SIP) to EPA within three years of a NAAQS promulgation or revision. This SIP submission, also known as an "infrastructure SIP," outlines how the state will implement, maintain, and enforce the NAAQS . The infrastructure SIP allows EPA to "review the basic structural requirements of [a state's] air quality management program in light of each new or revised NAAQS." Examples of the basic structural requirements include enforceable emission limits, an air monitoring program, an enforcement program, air quality modeling capabilities, and "adequate personnel, resources, and legal authority." The state's SIP must also address its interstate transport obligations under the CCAA. EPA refers to this section of the SIP submission as the "Good Neighbor SIP." The Good Neighbor SIP must prohibit "certain emissions of air pollutants because of the impact they would have on air quality in other states." Specifically, the state's Good Neighbor SIP must prohibit sources in that state from "emitting any air pollutant in amounts which will … contribute significantly to nonattainment in, or interfere with maintenance" of a NAAQS in another state. EPA reviews SIPs to ensure they meet statutory requirements. The agency also has authority to require states to revise their SIPs. Furthermore, the act requires EPA, under certain conditions, to impose sanctions and to issue a Federal Implementation Plan (FIP) if a state fails or declines to submit or implement an adequate SIP. Recognizing ongoing challenges with ozone transport, the 1990 CAA Amendments established regional planning provisions specific to ozone. For example, CAA Section 184 created a multi-state ozone transport region, known as the Ozone Transport Region (OTR), and established the northeast Ozone Transport Commission (OTC) to advise EPA about ozone controls in the OTR. The OTR is comprised of 12 Northeastern and Mid-Atlantic states (Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, certain counties in Northern Virginia) and Washington, DC. The CAA required states in the OTR to impose controls on sources in all specified areas, regardless of attainment status. Such controls included enhanced vehicle inspection and maintenance programs and reasonably available control technology for sources of VOCs. In addition, CAA Section 176A allows EPA to establish transport regions to address regional pollution problems contributing to violations of a primary NAAQS. The agency must establish a commission, comprised of EPA and state officials, for each transport region that makes recommendations to EPA on appropriate mitigation strategies. The CAA provides two independent statutory authorities to facilitate compliance with the Good Neighbor provision: (1) the SIPs process under Section 110 and (2) a petition process under Section 126(b). While these authorities are separate, they each address the same objective—that is, the Good Neighbor provision in Section 110(a)(2)(D)(i).