• The EPA regulates major and minor stationary sources of HAPs.
• Major sources that take steps to upgrade their facilities or install pollution-reduction equipment can potentially be reclassified as area sources.
• While individual area sources have much lower emissions than individual major sources, these sources can be numerous and widespread.

The Environmental Protection Agency (EPA) regulates two types of stationary sources of hazardous air pollutants (HAPs):

• Major sources are those that emit 10 tons per year (tpy) of any of the 187 listed toxic air pollutants, or 25 tpy of a mixture of air toxics. These sources may release air toxics from equipment leaks, during the transfer of materials, or during discharge through emissions stacks or vents.
• Area sources (or minor sources) are smaller-sized facilities that release lesser amounts of toxic pollutants into the air. Area sources emit less than 10 tpy of a single air toxic or less than 25 tpy of a combination of air toxics.

The 1990 Clean Air Act (CAA) amendments directed the EPA to set standards for all major sources of air toxics and some area sources that are of particular concern.

Major sources

For major sources, the EPA establishes maximum achievable control technology (MACT) standards. The EPA applies MACT standards for most source categories based on the emission controls that are being achieved by the best-controlled sources in that category. While MACT standards for existing major sources may be less stringent than for new sources, existing sources must meet one of the following:

1. For sources in categories or subcategories containing 30 or more sources, at least as stringent as the average emission limitation met by the best performing 12 percent of existing sources.
2. For source in categories containing less than 30 sources, at least as stringent as the average emission limitation met by the best performing five sources.

MACT can be limits on emissions of HAPs, or it can involve technology requirements for controlling emissions. When setting MACTs, the EPA must consider the cost, energy consumption, and other environmental consequences such as waste disposal and water quality.

If the company is a major source of HAPs, they must meet certain emission performance standards, which involves:

1. Meeting air pollution control limits set by the federal government based on performance standards that have been achieved in practice by a similar industry, process, or equipment.
2. Obtaining a Title V operating permit.

In the past, once a business qualified as a major source, the business had to continue as a major source, even if it reduced its emissions to below major source levels. However, a January 2018 memo from the EPA reversed this long-standing policy known as “once in, always in.” This means that major sources can take steps to upgrade their facilities or install pollution-reduction equipment and potentially be reclassified as area sources.

Area (or minor) sources

A facility that cannot emit a level of criteria pollutants or HAPs equal to a major source is called an area source (or minor source). To be an area source, the business must:

• Have a potential to emit (PTE) below major source levels, or
• Have a PTE at or above major source levels, but limit its operations or emissions. These businesses must obtain an operating permit from the state air pollution control agency which places limits on the facility’s operations or emissions. This could include specific restrictions or requirements on:
  • The use, operation, and maintenance of air pollution control equipment;
  • Operating time;
  • The types and amounts of input materials used (e.g., fuel or solvents); and
  • Recordkeeping and reporting.

While individual area sources have much lower emissions than individual major sources, these sources can be numerous and widespread. The collective emissions from area sources can add up to more than the emissions from major sources. Examples of area sources are gas stations and dry cleaners, and these area sources may also be subject to regulation.

The EPA says that to use a limitation to reduce the maximum capacity and be considered an area source, the limitation must be an unchanging and unavoidable physical constraint. The constraint(s) must result in predictable upper limits on the facility operations and capacity.

Area sources must meet generally available control technology (GACT). These are less stringent standards than those required by major sources and are usually easier to meet.