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Ozone Implementation

Guidance for Determining Boundaries of 8-Hour Ozone Attainment and Nonattainment Areas

Information provided for informational purposes onlyNote: EPA no longer updates this information, but it may be useful as a reference or resource.




  • In another step to ensure that Americans breathe cleaner air, EPA has issued guidance for states to use in recommending areas to be designated as attainment and nonattainment for the 8-hour ozone standard.
  • The non-binding guidance outlines how states should determine appropriate boundaries for the attainment and nonattainment areas. The term "nonattainment" means an area has had, or has contributed to, nearby ozone violations over a three-year period - generally 1997 through 1999.
  • Under the process EPA outlined today, the soonest designations could become effective is early 2001. EPA will ensure that state and local governments have ample time to comply with any requirements as the Agency decides when to finalize designations and make them effective.
  • EPA issued the 8-hour ozone standard in 1997. Two laws - the Clean Air Act and the Transportation Act for the 21st Century - require EPA to designate areas this year as attainment or nonattainment for the 8-hour standard.
  • In general, areas with monitors showing violations of the 8-hour level during 1997-1999 would be designated as nonattainment. Surrounding counties contributing to those violations should be included in the nonattainment area.
  • Today's guidance encourages states to base attainment and nonattainment area boundaries on Metropolitan Statistical Areas or Consolidated Metropolitan Statistical Areas. These boundaries will ensure that states consider population density, traffic and commuting patterns, commercial development and area growth when recommending areas for attainment and nonattainment designation.
  • The guidance also recommends that if an area to be classified as nonattainment for the 8-hour standard also has been designated as nonattainment for the 1-hour ozone standard, the boundary would be the larger of : a) the current consolidated/metropolitan statistical area boundary; or b) the 1-hour nonattainment area boundary.
  • States may suggest different boundaries. However, states must provide a rationale for any boundary changes and explain how they meet Clean Air Act requirements.


  • Once nonattainment designations take effect, they become an important component of state and local governments' efforts to control ground-level ozone, or smog.
  • The Clean Air Act requires state and local governments to take steps to reduce smog-causing pollution in nonattainment areas. Among these is a measure known as "transportation conformity," which means local transportation and air quality officials must coordinate planning to ensure that transportation projects, such as road construction, are consistent with air quality goals.
  • Once designated, nonattainment areas also are subject to new source review requirements, to ensure that new and modified sources of pollution do not impede progress toward cleaner air.
  • Once nonattainment areas are designated, states must develop plans demonstrating how their nonattainment areas will meet the ozone standard. These plans are known as state implementation plans, or SIPs.


  • States will have until June 30, 2000, to recommend to EPA areas that should be designated as nonattainment. EPA will review and consider those recommendations, and respond to the states by late summer.
  • EPA will not make final designations until at least four months after it responds to states' recommendations. States will have the opportunity during this time to comment on EPA's responses.
  • Based on this schedule, EPA could not make final designations before late December. EPA will set an effective date when it makes the final designations. The earliest designations could become effective would be early 2001.
  • Indian tribes that have their own air quality programs may submit their own recommendations for designations; however, they are not required to do so. Because air quality data is lacking in some tribal areas, EPA will work with tribes to determine the appropriate designations.


  • Two laws require EPA to designate areas as attainment or nonattainment for the 8-hour ozone standard. Those laws are the Clean Air Act and the Transportation Act for the 21st Century (known as TEA-21). EPA is required to designate attainment and nonattainment areas by July 2000.
  • In May 1999, and as modified in October 1999, the U.S. Court of Appeals for the D.C. Circuit remanded 8-hour ozone standard and issued an opinion limiting the manner in which EPA can implement it. However, the court affirmed EPA's authority to make designations.
  • The federal government has asked the U.S. Supreme Court to review aspects of the D.C. Circuit decision on the 8-hour ozone standard. The Court of Appeals did not question the need for the new standard or the science behind it. That standard, based on 8-hour averages of ozone rather than the previous 1-hour average, reflects a more realistic measure of people's exposure and is more protective of public health.
  • EPA provided initial guidance on designations in June 1999. Today's guidance supplements that information.


  • Ground-level ozone, the primary component of smog, is formed when emissions of nitrogen oxides and volatile organic compounds react in the sun. Sources for these emissions include power plants, factories, motor vehicles, chemical solvents and consumer products.
  • When inhaled - even at very low levels - ground-level ozone can: cause acute respiratory symptoms, such as coughing and discomfort in the throat and chest; aggravate asthma; reduce lung function; inflame and damage lung tissue; and impair the body's immune response to respiratory infection.
  • Children - especially those with asthma - are at the greatest risk from ozone pollution. During the summer, when concentrations of ground-level ozone are highest, children playing outside are most likely to experience respiratory symptoms and effects.


  • Today's guidance may not be immediately available on EPA's Internet site, depending on the status of the site at this time. For more information about today's guidance, call Sharon Reinders at the Office of Air Quality Planning & Standards, 919-541-5284.
  • To read the June 1999 guidance, go to the EPA's World Wide Web site at


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