Department of
Environmental Quality

Hazardous air pollutants are pollutants that cause or may cause cancer or other serious health problems, such as reproductive effects or birth defects, or adverse environmental and ecological effects.

The Clean Air Act (CAA) requires the U.S. Environmental Protection Agency (EPA) to regulate emissions of 188 hazardous air pollutants (HAPs).

• Original List of Hazardous Air Pollutants

EPA has identified "source categories" that must meet technology requirements to control HAP emissions and is required to develop standards for all industries that emit one or more of the HAPs in significant quantities. These standards are called the National Emissions Standards for Hazardous Air Pollutants (NESHAPs) or Maximum Achievable Control Technology (MACT) standards.

Understanding 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. The standards are based on emissions levels already achieved by best-performing similar facilities. This straightforward, performance-based approach yields standards that are both reasonable and effective in reducing toxic emissions. It also provides a level economic playing field by ensuring that facilities with good controls are not disadvantaged relative to competitors with poorer controls.

Classification of Sources

Industries 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.

The MACT Floor

When developing a MACT standard for a particular source category, 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 "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. EPA 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, EPA 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.

Applicability of MACT Standards

All source categories and subcategories that include major sources and area sources of HAP are listed within Section 112(c) of the CAA. To find which MACT standards apply to your company, first select the best matched source category from the MACT promulgated standard list source description. Then, from this list, you can obtain further information about what processes are regulated by the MACT standard, and the compliance deadlines in the standard.

• List of Promulgated MACT Standards

MACT Sources and Tier I Operating Permits

Facilities subject to a MACT standard may also be subject to Tier I air quality operating permit requirements. EPA has delegated authority to DEQ for issuing Tier I operating permits to industries in Idaho. Timelines for submitting Tier I permit applications depend upon whether the facility is classified as a major or area source.

All major sources that existed before May 1, 1994, were required to apply for a Tier I permit no later than June 1, 1996. All major sources that came into existence after May 1, 1994, are required to submit a Tier I application to DEQ within 12 months of beginning operations. Contact DEQ for assistance.

Minor or area sources of air pollution may be subject to Tier I requirements. Permitting requirements and exemptions are outlined in the source category MACT.

Regulation of MACT Sources in Idaho

DEQ is the state agency delegated by EPA to implement and enforce MACT standards for Title V facilities only. Implementation and enforcement of MACT standards for non-Title V sources is the responsibility of EPA. 

• EPA NESHAP Delegation Letter