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Clean Air Act Overview

Setting Emissions Standards for Major Sources of Toxic Air Pollutants

The Clean Air Act requires EPA to set emissions standards based on technology performance for major sources of "hazardous air pollutants," also known as "air toxics." After implementation, EPA is required to evaluate the remaining health and environmental risks at these facilities and to set additional standards if necessary to protect human health and the environment.

Hazardous air pollutants, also known as "air toxics," are those pollutants that are known or suspected to cause cancer or other serious health effects, such as reproductive effects or birth defects, or adverse environmental effects. Major sources are plant sites that emit or have the potential to emit 10 tons per year or more of a single hazardous pollutant or 25 tons per year or more of a combination of hazardous pollutants. Examples are petroleum refineries and chemical plants.

Phased Regulatory Approach to Address Air Toxics

To control air toxics emissions and reduce risks from major stationary sources, the Clean Air Act sets out a phased regulatory approach.

First Phase: Emissions Standards Based on the Best Performers

In the first phase, standard-setting focuses on what the best performers in a particular industry are already doing in practice. The agency sets national emissions standards with numerical limits reflecting available control technologies or work practice requirements for major stationary sources of air toxics. This phase, commonly referred to as the maximum achievable control technology (MACT) program, is intended to achieve emission reductions in a timely manner.

New source standards are based on the top-performing similar source. For existing sources, EPA must set standards that require at least the level of performance already achieved by the average of the top-performing 12 percent of similar sources. These provisions, known as the “MACT floor,” are designed to ensure that the higher-emitting sources in an industry make improvements to bring emissions in line with levels already achieved in practice by the lower-emitting sources. The resulting performance standards give all sources the flexibility to decide the most cost-effective way to comply.

Second Phase: Health and Environmental Risk Evaluation and Possible Risk-Based Standards

In the second regulatory phase, known as the residual risk program, EPA evaluates the health and environmental risks from sources within 8 years after MACT standards are established. The Clean Air Act requires EPA to set additional, risk-based standards if more emission reductions are necessary “to provide an ample margin of safety to protect public health” or, “to prevent, taking into account costs and other relevant factors, an adverse environmental effect.”

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Risk Assessment

To carry out the risk evaluation in the second phase, EPA develops a risk assessment for each source category, relying on peer-reviewed tools to determine the magnitude of potential impacts from air toxics exposures. EPA uses methods laid out by the National Academy of Sciences National Research Council, recommendations of the Commission on Risk Assessment and Risk Management (mandated under the Clean Air Act), and newer methodologies and guidelines developed by EPA's Office of Research and Development, which are updated periodically to reflect advancements in science.


For both human health and ecological risk assessments, EPA implements a tiered approach, beginning with a screening analysis and moving, as warranted, to more detailed analyses. The screening analysis uses existing data, defined decision criteria and models with simplifying, conservative assumptions as inputs. By using these default assumptions, EPA ensures that a lack of data does not result in overlooking a source category that may pose significant risk. A more refined analysis requires more resources and data, but the results are more certain and less likely to overestimate risk. EPA also conducts demographic analyses to better understand the potential impacts of the rule on low income, minority or indigenous communities.

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Technology Review

In addition to the residual risk review, the Clean Air Act requires EPA to conduct a technology review for major sources every eight years. EPA evaluates whether there have been developments in technologies or other air toxics emission reduction approaches since issuance of the initial MACT. This analysis includes, but is not limited to, evaluating whether technologies available at the time of the initial MACT have changed to more efficient, cost-effective methods warranting tighter standards. The Risk and Technology Review (RTR) is a combined effort to evaluate risk in conjunction with the first technology review required by the CAA after promulgation of a MACT standard. Current information regarding proposals and final actions on RTR rules can be found on the EPA's RTR website.

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