Under the Clean Air Act, EPA regulates two primary types of air pollutants. The first category—the so-called “criteria pollutants” for which EPA has established air quality criteria that limit the allowable concentrations in the ambient air—includes carbon monoxide, ground- level ozone (smog), lead, nitrogen oxides, particulate matter, and sulfur dioxide. EPA sets these standards at a level it believes protects public health and the needs of sensitive populations such as asthmatics, children, and the elderly. EPA and the states use air quality monitoring to measure compliance with the standards and develop pollution control strategies to help bring areas with poor air quality into compliance. The second category consists of hazardous air pollutants (or “air toxics”) for which no ambient air quality standards exist, and includes 187 chemicals that cause a variety of adverse health effects, including cancer. A variety of sources emit one or more of these air toxics (see fig. 1). In 2002, mobile sources emitted 41 percent of all air toxics, small stationary sources emitted 30 percent, major stationary sources emitted 20 percent, and other sources, such as fires, emitted 9 percent, according to EPA’s most recent data. Table 1 identifies the most widely emitted air toxics, the primary sources of these pollutants, and some of the adverse health effects associated with exposure to these substances. It is important to note that the health risks posed by air toxics vary considerably. Thus, small quantities of more harmful pollutants can pose greater health threats than large quantities of less harmful pollutants. Prior to 1990, the Clean Air Act required EPA to list air toxics it deemed hazardous and then promulgate regulations for them. However, by 1990, EPA had regulated only seven such pollutants. In 1990, Congress dramatically changed the program. Instead of requiring EPA to develop ambient standards for air toxics as it does for the six criteria pollutants, the Clean Air Act Amendments of 1990 listed the air toxics to be controlled and directed EPA to control them by, among other things, (1) developing technology-based emissions limits (MACT standards) for major stationary sources, such as incinerators and chemical plants; (2) regulating emissions from smaller sources, such as dry cleaners and gas stations; and (3) evaluating the need for and feasibility of regulations from mobile sources, such as cars, and regulating these sources based on this evaluation. The standards for major stationary sources generally require the use of available control technologies to achieve emissions reductions without the explicit consideration of a chemical’s toxicity or potential risk. To develop MACT standards, the 1990 amendments directed EPA to group emissions points at industrial facilities into categories of similar sources and then develop regulations for each “source category.” Examples of source categories include cement manufacturing, hazardous waste combustion, and semiconductor manufacturing. The next step consisted of evaluating the level of emissions control achieved by the best-performing facilities in each source category and using this as the minimum level of control required throughout the entire source category. Additionally, the amendments required EPA to review the MACT standards every 8 years to evaluate any remaining, or residual, health risks from these sources and identify developments in control technologies. EPA has combined the residual risk assessments and technology reviews into a concurrent process. Thus, the agency simultaneously evaluates the remaining risks from each source category and the availability of new pollution control technologies. The risk assessment process seeks to estimate the cancer and other health risks faced by individuals exposed to toxic emissions. As shown in table 2, the four steps of risk assessment include hazard identification, dose-response assessment, exposure assessment, and risk characterization. The risk assessment process is limited by scientific uncertainty about the health effects associated with exposure to air toxics. Nonetheless, the Clean Air Act’s residual risk program seeks to determine whether the most exposed individuals face excess cancer risk of more than 1 in 1 million. In cases where estimated risks exceed this threshold, EPA develops a residual risk standard that seeks to provide an ample margin of safety for affected individuals. Figure 2 provides an overview of the regulatory process for major stationary sources of air toxics, including MACT standards and 8-year technology and residual risk reviews. In addition to requirements for major sources, the act required EPA to develop a comprehensive strategy to control emissions of air toxics in urban areas, including identifying at least 30 small stationary source categories that account for 90 percent of the risk from these sources, and issue regulations by November 2000. EPA has listed 70 small stationary source categories for regulation. The act also required EPA to assess the need for and feasibility of air toxics standards for motor vehicles and fuels, and, based on that assessment, issue regulations to control air toxics from motor vehicles and fuels. Table 3 summarizes the 453 actions required of EPA under the air toxics provisions of the 1990 amendments. Because these actions range in scope from developing MACT standards to issuing reports, they vary in their potential to reduce emissions. EPA’s Office of Air and Radiation has primary responsibility for completing air toxics actions required under the Clean Air Act. Within the Office of Air and Radiation, responsibility for implementing the air toxics requirements of the Act rests primarily with the Office of Air Quality Planning and Standards and, to a lesser extent, with the Office of Transportation and Air Quality. The responsibility for analyzing the health, economic, and other effects of individual air toxics programs also rests with these offices. The Office of Policy Analysis and Review supplements these program-specific analyses by conducting periodic assessments of the health, ecological, and economic effects of the overall Clean Air Act, including the air toxics provisions, and coordinating these studies as appropriate with other EPA offices. In conducting these broader studies, the Office of Policy Analysis and Review also works with the Advisory Council for Clean Air Act Compliance Analysis, an independent, multi- disciplinary panel of outside experts organized under the auspices of EPA’s Science Advisory Board. The agency’s Office of Research and Development performs scientific research on air toxics to support regulatory efforts. The Office of Enforcement and Compliance Assurance directs efforts to ensure compliance with air toxics requirements. In most cases, state and local air pollution control agencies implement the standards developed by EPA. Additionally, the act generally allows these agencies to impose more stringent requirements than the federal standards, although some states have enacted laws or rules prohibiting air pollution control agencies from adopting more stringent requirements. Nonetheless, some state and local programs have developed innovative air toxics programs. EPA has completed issuing emissions standards for major stationary sources but issued most of these standards late and has made limited progress toward completing the remaining air toxics requirements. In particular, EPA has made little progress and is behind schedule in completing residual risk and technology reviews and in issuing emissions standards for small stationary sources and mobile sources. EPA’s limited progress and program implementation challenges have resulted primarily from the program’s lower priority relative to other clean air programs. Furthermore, the agency lacks a program implementation strategy. Stakeholders we interviewed—including EPA, state and local agency officials, environmental groups, and industry representatives—provided additional perspective on EPA’s implementation of the air toxics program and highlighted data limitations and inadequate funding as major challenges. EPA has completed issuing the MACT standards for major stationary sources but has made limited progress in addressing requirements related to residual risk and technology reviews, and in issuing standards for small stationary sources and mobile sources. As a result of the limited progress in implementing these requirements, EPA has not reduced human health risks from air toxics to the extent and in the time frames envisioned in the act. Table 4 summarizes EPA’s overall progress in implementing air toxics requirements under the Clean Air Act. To meet the act’s requirements for major stationary sources, EPA had to identify a list of major source categories and then issue standards beginning in 1992, with all standards due by November 2000. In response, EPA identified 158 major source categories and issued 96 standards covering these categories between 1993 and 2004. Table 5 summarizes the timeliness of EPA’s MACT standards relative to the act’s deadlines. While the agency missed most of the deadlines, a senior EPA official said that issuing the 96 standards represented a major achievement and that the agency had never previously issued so many standards for one program in such a limited period of time. Because EPA issued most of the MACT standards well behind schedule, the residual risk and control technology reviews, which EPA is to complete 8 years after issuing each standard, have been pushed back commensurately, thereby delaying any additional public health protection that these reviews may provide. Specifically, instead of completing the initial residual risk assessments and technology reviews for all of the MACT standards by 2008 as specified by the act, EPA is not required to complete all of the initial reviews until 2012 because it issued many MACT standards behind schedule. For example, because EPA issued the MACT standard for industrial boilers in 2004 rather than 2000, as required, the residual risk assessment and technology review for this source category become due in 2012, almost 4 years later than the act’s intended timeline. Furthermore, EPA is behind schedule on the residual risk assessments and technology reviews. As of April 2006, EPA had finalized only five of these reviews, and all of these were late. Three additional reviews have court- ordered deadlines and will be completed by the end of 2006, according to EPA. The act required EPA to develop regulations for small stationary sources by November 2000. However, the agency has not met this schedule. In July 2000, EPA outlined its plans for issuing standards for small stationary sources in a report to Congress describing its strategy for reducing threats from air toxics in urban areas. This report identified 16 categories of small stationary sources that it described as “already subject to regulation” or “will be subject to regulation.” The report also identified 13 additional categories for which it planned to issue standards by 2004. In 2002, EPA expanded the list to include a total of 70 source categories. However, as of April 2006, EPA has issued standards for only 16 categories of sources, leaving standards for 54 source categories past due.