Prevention Of Significant Deterioration

Prevention of Significant Deterioration in the United States

Prevention of Significant Deterioration (PSD) in Environmental Law

A program under the Clean Air Act designed to preserve the air quality of an area after it has met or exceeded the national standards for various pollutants. If an area is unclassifiable as either attainment or nonattainment, the PSD requirements also apply. Preconstruction review and permitting of new major sources and major modifications of existing sources are central to the PSD program. See major source; National Ambient Air Quality Standards; New Source Review; nonattainment areas.

The PSD requirements generally pertain to sources that are capable of discharging 250 tons per year of a regulated pollutant. However, if the facility fits into one of 28 categories listed by Congress, it is considered major if it can emit 100 tons per year. Pollutants regulated by the PSD provisions include the criteria pollutants lead, ozone, sulfur dioxide, nitrogen dioxide [see nitrogen oxides], and small particulates but also cover sulfuric acid mist, other oxides of nitrogen, other compounds containing sulfur, and larger particulates. Initially, the program focused on only sulfur dioxide and particulates. States must develop their own regulations to make certain the Clean Air Act requirements for Prevention of Significant Deterioration are followed.

If a source within an air quality area that has attained the national standards plans to make a modification that will significantly increase pollution, it must also go through preconstruction review. The EPA has issued regulations to specify what levels of increase it considers significant. For example, forty tons per year of sulfur dioxide or volatile organic compounds is a significant increase.


When an air quality control region has met the National Ambient Air Quality Standards for criteria air pollutants, it is designated an attainment area. Attainment areas may be further described as Class I, Class II, or Class III areas. Of the classes, Class I has the tightest restrictions on degradation of air quality. Other than national monuments, parks, recreation areas, scenic or wilderness areas, Class I areas are rare. Class III areas are even more uncommon, since attainment areas are automatically classified as Class II. Moving an area to a different category involves affirmative steps by the state or federal government. The amount of deterioration allowed in a Class I area is only two percent, Class II areas may increase emissions by as much as twenty-five percent, and Class III areas are permitted to increase air pollutants by up to fifty percent. However, the increments may not be used to allow the air quality control region to fall below the national standards.

States establish baselines by measuring the air quality at the time the air district attains the standards. Each class is allowed a percentage increase from the baseline, as long as the increase will not cause the region to violate the National Ambient Air Quality Standards. The increase is called an increment. To determine what remains of the increment, all of the pollutants in the air quality district must be considered, not just the ones coming from major emitting facilities. For example, new small sources may emit a regulated pollutant like carbon monoxide. Although they do not have to go through preconstruction review and permitting, they do discharge pollutants that use up part of the allowable increment.

Features of Preconstruction Permitting

Before a major source of emissions begins construction, it must go through a lengthy review. The source will be required to perform air monitoring and modeling based on the proposed location, collecting data over a year in most situations. It will have to demonstrate that building the new facility or making the modification will not cause the area to exceed the allowable increment of deterioration. It must also prove that it will not cause a violation of the national standards in any air quality control region. That means the source must consider not only what will happen to its pollution within the same area, but also how the pollutants may travel and affect another area.

Finally, the new source or modification will have to use best available control technology (BACT) to control all pollutants it may emit in significant amounts. The state will be the final authority on whether the controls proposed for use in the facility will meet the requirements. To assist sources in deciding what is necessary, the EPA publishes information on various technologies.
Based on “Environment and the Law. A Dictionary”.



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