National Emission Standards for Hazardous Air Pollutants (NESHAPs) and Maximum Achievable Control Technology (MACT) Standards

What are national emission standards for hazardous air pollutants (NESHAPs) and maximum achievable control technology (MACT) standards?

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 hazardous air pollutants (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 the U.S. Environmental Protection Agency (EPA) are commonly called maximum achievable control technology (MACT) standards. Maximum achievable control technology (MACT) standards are designed to reduce hazardous air pollutants (HAPs) emissions to a maximum achievable degree, taking into consideration the cost of reductions and other factors. After the Environmental Protection Agency (EPA) adopts a maximum achievable control technology (MACT) standard at the federal level, the Regulatory and Compliance Support Unit proposes the same standard for adoption at the state level by the Air Quality Control Division on a semi-annual basis.

When developing a maximum achievable control technology (MACT) standard for a particular source category, the Environmental Protection Agency (EPA) 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 "maximum achievable control technology (MACT) floor" for the new standard. At a minimum, a maximum achievable control technology (MACT) standard must achieve, throughout the industry, a level of emissions control that is at least equivalent to the maximum achievable control technology (MACT) floor. The Environmental Protection Agency (EPA) can establish a more stringent standard when it makes economic, environmental, and public health sense to do so.

The maximum achievable control technology (MACT) floor differs for existing sources and new sources:

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

Wherever feasible, the Environmental Protection Agency (EPA) writes the final maximum achievable control technology (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 maximum achievable control technology (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 hazardous air pollutants (HAPs), or 25 tons per year of a mixture of hazardous air pollutants (HAPs). These sources may release hazardous air pollutants (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 hazardous air pollutants (HAPs) into the air. Area sources are sources that emit less than 10 tons per year of a single hazardous air pollutant (HAP), or less than 25 tons per year of a combination of hazardous air pollutants (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.

***Note: On December 9, 2005, Environmental Protection Agency (EPA) finalized permanent exemptions from the Title V operating permit program for five categories of non-major (area) sources that are subject to national emission standards for hazardous air pollutants (NESHAPs).***