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 requires the U.S. Environmental Protection Agency (EPA) to regulate emissions of 187 hazardous air pollutants (HAPs) from a published list of industrial sources called "source categories." EPA has identified source categories that must meet technology requirements to control HAP emissions and is required to develop regulations 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).

> View list of federally-regulated Hazardous Air Pollutants.

The NESHAPs are air quality standards, issued under Section 112 of the Clean Air Act (40 CFR Part 63), which regulate 187 HAPs from particular industrial sources. These industry-based NESHAPs are also 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.

EPA's MACT 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. 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.

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.

Check the federal rule to determine if a MACT standard has been developed for your industry and whether the requirements apply to your facility or, if you are a small business owner or operator, contact DEQ's Small Business Environmental Solutions Program located at Boise State University's Small Business Development Center.

All NESHAP or MACT standards were promulgated after 1992. Pre-existing sources still must comply with the applicable industry standard, but under an extended deadline, as long as they have not reconstructed. An extended compliance date is listed in each MACT. In setting the standards, EPA is allowed to distinguish between new and existing sources and to subject new or reconstructed major sources to stricter controls.

"Reconstructed" means component parts of a stationary source have been replaced to the extent that the capital cost of the new components exceeds 50% of the capital cost to construct a comparable new source, if it is technologically and economically feasible for the reconstructed source to meet the MACT standard.

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.

> Link to Tier I operating permit application form and instructions.

DEQ is the state agency delegated by EPA to regulate all major and certain area sources at Tier I major sources, with the following exceptions:

• Dry Cleaning (40 CFR Part 63, Subpart M)
• Decorative Chromium Electroplating, Chromic Acid Anodizing, Hard Chrome Electroplating (40 CFR Part 63, Subpart N)
• Ethylene Oxide emissions from Sterilization Facilities (40 CFR Part 63, Subpart O)
• Halogenated Solvent Cleaning (40 CFR Part 63, Subpart T)
• Secondary Lead Smelters
• Sources subject to MACT standards that emit radionuclides