Jump to main content or area navigation.

Contact EPA Pacific Southwest Air Program

Pacific Southwest, Region 9


Air Permits

Air Permits Quick Finder

Overview of the Prevention of Significant Deterioration Program

The EPA encourages public involvement in its permitting process. The following information is intended to assist the public in understanding the Prevention of Significant Deterioration process.

On this page:

What is Prevention of Significant Deterioration (PSD)?

The Prevention of Significant Deterioration (PSD) permitting program is a Clean Air Act permitting program for new and modified major sources of air pollution such as power plants, manufacturing facilities, and other facilities that emit air pollution. PSD applies to all pollutants that do not exceed the National Ambient Air Quality Standards (NAAQS) in an area. The NAAQS establish maximum pollution concentration levels to protect public health and welfare from harmful levels of pollutants. Pollutants covered by the NAAQS are nitrogen oxides, volatile organic compounds (which are precursors to ground-level ozone), sulfur dioxide, fine particulate, carbon monoxide, and lead. These pollutants are criteria pollutants.

PSD also applies to other pollutants that do not have a NAAQS. These non-criteria pollutants are listed in EPA's regulations and include fluorides, sulfuric acid mist, hydrogen sulfide, total reduced sulfur, and certain contaminants from municipal solid waste plants.

What is the purpose of PSD?

PSD does not prevent pollution sources from increasing emissions. Instead, PSD is primarily designed to achieve the following:

  • prevent violations of the Clean Air Act National Ambient Air Quality Standards (NAAQS) and protect the environment;
  • protect the air quality and visibility in national parks, national wilderness areas and other areas of special natural, recreational, scenic, or historic value;
  • allow economic growth while limiting impacts on good air quality;
  • require the Best Available Control Technology (BACT) for new or modified major sources of air pollution to minimize air pollution; and
  • inform the public of EPA's proposed PSD permitting decision and allow the public to comment on these decisions.

What is a PSD permit?

A PSD permit is a legal document that limits the amount of air pollution that may be released by a source (i.e., a plant or facility). A PSD permit is required before a "major" new source constructs, or before changes or modifications that are "major" or "significant" are made at an existing "major" source of air pollution. The permit may be issued by EPA or the designated permitting authority. The permit specifies what construction is allowed, what emission limits must be met, and often how the equipment that is causing the air pollution must be operated.

Who needs a PSD permit?

The program applies to a new plant that will have "major" and "significant" amounts of air pollution for any criteria pollutant. It also applies to an existing plant that plans to modify its operations such that the modification leads to increases of air pollution that will be "major" or "significant".

"Major" means emitting or having the potential to emit 100 tons per year (tpy) or more of any criteria pollutant for the specific source categories listed in the PSD regulations. There are 28 listed source categories, which include power plants that use steam to generate electricity, petroleum refineries and glass fiber processing plants. If a plant does not fall into one of the listed source categories, then a threshold of 250 tpy applies.

"Significant" refers to thresholds assigned to each criteria pollutant and certain non-criteria pollutants (e.g., the significance threshold is 40 tpy for NOx and 15 tpy for PM-10).

What does PSD require?

PSD requires (1) installation of the Best Available Control Technology; (2) an air quality analysis; (3) an additional impacts analysis; and (4) public participation.

1. Best Available Control Technology
Best Available Control Technology (BACT) is an emissions limitation which is based on the maximum degree of control that can be achieved at a major stationary source. It is a case-by-case decision that considers energy, environmental, and economic impact. BACT can be add-on control equipment or modification of the production processes or methods.

2. Air Quality Impact Analysis
The main purpose of the air quality analysis is to demonstrate that new air pollution that will be emitted from a proposed major stationary source or major modification, in conjunction with other applicable emissions increases and decreases from existing sources, will not cause or contribute to a violation of any applicable NAAQS or PSD increment. (PSD increment is the amount of pollution an area is allowed to increase. PSD increments prevent the air quality in clean areas from deteriorating to the level set by the NAAQS.) Generally, the air quality analysis will involve (1) an assessment of existing air quality, which may include ambient monitoring data and air quality dispersion modeling results, and (2) predictions, using dispersion modeling, of ambient concentrations that will result from the applicant's proposed project and future growth associated with the project.

3. Additional Impact Analysis
The additional impacts analysis assesses the impacts of air, ground and water pollution on soils, vegetation, and visibility caused by any increase in emissions of any regulated pollutant from the source or modification under review, and from associated growth. Associated growth is industrial, commercial, and residential growth that will occur in the area due to the source.

4. Public Participation
Public participation is citizens being involved in the permitting process. Any person may comment on the permit or request a public hearing, if one has not been scheduled, during the public comment period.

Top of page


Jump to main content.