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MACTs and Other NESHAPs:
Air Toxics Lead and Asbestos Section (ATLAS)

Maximum Achievable Control Technology (MACT) Rules

The 1990 Clean Air Act (CAA) Amendments established a new and fairly complex program to regulate emissions of 188 hazardous air pollutants (HAPs) from particular industrial sources. The Act required the US Environmental Protection Agency (USEPA) to regulate emissions of these HAPs by developing and promulgating technology-based standards based on the best-performing similar facilities in operation. The national emission standards for hazardous air pollutants (NESHAPs) established by USEPA are commonly called maximum achievable control technology (MACT) standards. MACT standards are designed to reduce HAP emissions to a maximum achievable degree, taking into consideration the cost of reductions and other factors.

When developing a MACT standard for a particular source category, USEPA looks at the current level of emissions achieved by best-performing similar sources through clean processes, control devices, work practices, or other methods. These emissions levels set a baseline, often referred to as the "MACT floor" for the new standard. At a minimum, a MACT standard must achieve, throughout the industry, a level of emissions control that is at least equivalent to the MACT floor. USEPA can establish a more stringent standard when it makes economic, environmental, and public health sense to do so.

The MACT floor differs for existing sources and new sources.

  • For existing sources, the MACT floor must equal the average current emissions limitations achieved by the best-performing 12% of sources in the source category, if there are 30 or more existing sources. If there are fewer than 30 existing sources, the MACT floor must equal the average current emissions limitation achieved by the best-performing five sources in the category.
  • For new sources, the MACT floor must equal the current level of emissions control achieved by the best-controlled similar source.

Wherever feasible, USEPA writes the final MACT standard as an emissions limit, a percent reduction in emissions or a concentration limit that regulated sources must achieve. Emissions limits provide flexibility for industries to determine the most effective ways to comply with the standards. Sources subject to MACT standards are classified as either major sources or area sources.

  • Major sources are sources that emit 10 tons per year of any of the listed HAPs, or 25 tons per year of a mixture of HAPS. These sources may release HAPs from equipment leaks, when materials are transferred from one location to another, or during discharge through emission stacks or vents.
  • Area sources consist of smaller-size facilities that release lesser quantities of HAPs into the air. Area sources are sources that emit less than 10 tons per year of a single HAP, or less than 25 tons per year of a combination of HAPs. Though emissions from individual area sources are often relatively small, collectively their emissions can be of concern, particularly where large numbers of sources are located in heavily populated areas.

MACT Rules and Information

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