In the past several Congresses, critics of the Environmental Protection Agency (EPA) have focused much of their attention on the agency's regulatory actions under the Clean Air Act (CAA). (For a summary of those actions, see CRS Report R41561, EPA Regulations: Too Much, Too Little, or On Track? , by [author name scrubbed] and [author name scrubbed].) During this time, the Obama Administration promulgated numerous CAA regulations. In general, these regulations came in response to congressional authority or mandates under the Clean Air Act Amendments of 1970, 1977, and 1990. A number were also in response to court decisions that remanded to the agency regulations that the agency had promulgated—often under previous administrations. Critics have maintained that many of these regulatory actions would be too costly, harming a wide range of industries, and would not be justified by the benefits obtained. The Clean Air Act is not consistent in whether it allows or requires the consideration of costs and benefits in setting standards. (See " Clean Air Act Authorities ," below.) The act requires or authorizes the EPA Administrator to promulgate regulations or set standards in more than 60 sections or subsections. In 25 of these sections or subsections, cost is not mentioned or implied as a factor to be considered. In the remaining sections and subsections where the Administrator is required or authorized to promulgate regulations—more than 40 of them in all—cost is either identified explicitly or implied as a factor to be considered. Whether or not the statute requires a consideration of cost, EPA has prepared cost estimates for all economically significant rules since the Carter Administration as the result of executive orders. Under Executive Order (E.O.) 12866, each economically significant regulatory action taken by Executive Branch agencies (under any statutory authority) must include estimates of the cost and benefits of the action in a Regulatory Impact Analysis (RIA) before it is proposed, and again before it is promulgated. RIAs play a major role in the interagency review process overseen by the Office of Management and Budget, which precedes the publication of significant rules in the Federal Register . Thus, there is a process for considering the costs and benefits of all Clean Air Act (and other) economically significant regulations. How well this process works is the question. One issue raised by EPA's critics is whether the agency underestimates the cost and other negative impacts of rules in these RIAs by considering them individually, and not considering cumulative impacts. A second criticism is that the agency relies for most of its CAA benefit assessments on the effects of reducing a single category of pollutants, particulate matter (PM). Research has tied PM to tens of thousands of premature deaths, and EPA often finds that reductions in PM emissions justify regulation, even where the target of the regulations is a different pollutant. A third issue critics raise is whether the methodology used to place monetary value on the avoidance of premature death—a technique referred to as calculating the "value of a statistical life"—inflates the estimated benefits of regulation. A fourth issue, not generally raised by critics, but often noted by regulators, concerns the difficulty of identifying and estimating the full range of costs and benefits of environmental regulations. This report examines these issues in the context of Clean Air Act (CAA) regulations. It begins with a review of EPA's CAA authorities and the role of cost considerations in CAA standard-setting. The Clean Air Act, originally enacted in 1955 and amended numerous times since then, gives EPA sweeping powers to "protect and enhance the quality of the Nation's air resources so as to promote the public health and welfare and the productive capacity of its population." In the statute, Congress directs the EPA Administrator to, among other things set national ambient air quality standards; set emission standards for both stationary and mobile sources of air pollution; reduce emissions of 187 hazardous air pollutants that Congress itself listed in the statute; protect air quality in relatively pristine areas from significant deterioration; regulate fuels and fuel additives, both to protect public health and welfare and to prevent the impairment of emission control devices; require the use of renewable transportation fuels; control acid deposition; protect the stratospheric ozone layer by requiring the phase-out of ozone-depleting substances; issue permits and enforce the act's emission limits; and develop and enforce Federal Implementation Plans in states that fail to implement the act's requirements. The specific authorities given to the Administrator are established in more than 60 different sections and subsections of the act, which range from broad authority to protect public health with an adequate margin of safety to detailed requirements that specify numerical emission limits or require that standards be at least as stringent as the emission limitation achieved by the best controlled similar source. Because the act's authorities are so fragmented, it can be difficult to generalize regarding the role of cost considerations in setting air quality standards. Many of the act's authorities allow or require the Administrator to "take into account" or "take into consideration" the cost or technical feasibility of specific emission requirements. A review of 67 sections, subsections, or provisions of the act that authorize regulations indicates that about half (34) specifically mention cost or economic considerations. Among the 34 sections are several major regulatory authorities, including the authority to set emission standards for new stationary sources (power plants, refineries, etc.) in Section 111; go "beyond the floor" in emission standards for sources of 187 hazardous air pollutants, under Section 112(d); set emission standards for motor vehicles (beyond the standards specifically listed in the act), under Sections 202(a) and 202(i); control mobile source air toxics, under Section 202(l); control or prohibit the manufacture and sale of fuels and fuel additives under Section 211(c); require the sale of reformulated gasoline in nonattainment areas, under Section 211(k); set emission standards for nonroad vehicles and engines, including construction equipment, recreational equipment, agricultural machinery, electric generators, and other sources, under Section 213; and set emission standards for locomotives, buses, and aircraft, under Sections 213, 219, and 231. In eight other cases, consideration of cost is implied by the act, e.g., where it requires a standard that is "practicable" or "reasonably achievable." These sections of the act direct the EPA Administrator to consider the "remaining useful life of the existing source" to which an emission standard will apply, under Section 111(d); provide for the use of "generally available control technologies" to control area sources of hazardous air pollutants, under Section 112(d)(5); promulgate "reasonable regulations and appropriate guidance to provide, to the greatest extent practicable, for the prevention and detection of accidental releases" of extremely hazardous substances and take into consideration "the concerns of small business," under Section 112(r)(7); consider "the availability and feasibility of pollution control measures" in classifying nonattainment areas under Section 172; consider "such other factors as he [the Administrator] deems pertinent" and take into consideration "the restraints of an adequate leadtime for design and production" in setting vapor recovery standards for gasoline under Section 202(a)(5) impose emissions standards or emissions control technology requirements that "reflect the best retrofit technology and maintenance practices reasonably achievable" for retrofit of urban buses under Section 219(d); decide whether a requirement is "practicable, taking into account technological achievability, safety, and other relevant factors" in establishing an accelerated schedule for phasing out production and consumption of ozone-depleting substances under Section 606; and consider "the purpose or intended use of the product, the technological availability of substitutes ..., safety, health, and other relevant factors" in regulating nonessential products that release class I ozone depleting substances under Section 610 (except for two specific categories of products that are listed in the statute). A full list of the 42 provisions that mention or imply consideration of cost is provided in Table 1 . In 25 CAA sections or subsections where regulatory authority is conferred on the Administrator (identified in Table 2 ), cost is not mentioned or implied as a factor to be considered. These statutory authorities tend to fall into one of four categories: (1) provisions in which Congress itself set the standards; (2) provisions where Congress directed the agency to set health-based standards, without mentioning cost; (3) provisions in which Congress gave the agency broad authority to promulgate regulations to achieve an objective that Congress determined was necessary (generally protecting public health directly or indirectly, or protecting the environment), but the specifics of which Congress could not anticipate; or (4) a provision requiring EPA to promulgate federal requirements in cases where states have failed to develop or implement adequate regulations to meet a federal mandate. These authorities are discussed briefly in the sections that follow. In the late 1980s, when the most recent major CAA amendments were being drafted, Congress, frustrated with the slow progress being achieved under earlier versions of the statute and by the delays caused by litigation, limited EPA's discretion in setting emission standards in a number of cases by writing detailed regulatory requirements into the statute. Perhaps the most specific of these requirements appear in Section 202 of the act, which addresses motor vehicle emissions. Here, Congress listed the pollutants to be controlled, mandated specific numerical standards for their emission, and set schedules for implementation. EPA still needed to promulgate regulations to implement these standards, but the standards themselves were set by Congress to take effect on a date certain. Another major example can be found in Section 112, where Congress addressed emissions of hazardous air pollutants. Here, Congress listed 187 pollutants the emissions of which were to be controlled; defined the threshold quantity of emissions that would require sources to meet the most stringent standards; and required that sources meet emission limits at least as stringent as the emissions of the best controlled similar sources. The act mandated a 10-year schedule for promulgating standards. The cornerstone of the Clean Air Act consists of health-based standards for widespread air pollutants identified by EPA under Sections 108 and 109 of the act.