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Area Designations for 2006 24-Hour Fine Particle (PM2.5) Standards

Factors EPA Will Consider as the Basis for Nonattainment Area Boundaries

EPA believes that certain factors are appropriate to consider in making nonattainment area boundary recommendations and final boundary determinations. EPA will consider these same factors,1 along with any other relevant information, in evaluating modifications to the boundary recommendations from States and Tribes. EPA recommends that States and Tribes consider the following nine factors in assessing whether to include an area in the designated nonattainment area boundary:

  • Emission data
  • Air quality data
  • Population density and degree of urbanization (including commercial development)
  • Traffic and commuting paterns
  • Growth rates and patterns
  • Meteorology (weather/transport patterns)
  • Geography/topography (mountain ranges or other air basin boundaries)
  • Jurisdictional boundaries (e.g., counties, air districts, Reservations, metropolitan planning organizations (MPOs))
  • Level of control of emission sources
This list of recommended factors is not intended to be exhaustive, and States and Tribes may submit additional information on factors they believe are relevant for EPA to consider. In general, a State’s or Tribe’s demonstration supporting the boundary recommendation for an area should show that: 1) violations are not occurring in the excluded portions of the recommended area, and 2) the excluded portions do not contain emission sources that contribute to the observed violations. A State or Tribal submittal that only addresses whether monitored violations are occurring in an area will not suffice as the sole justification for designating the boundaries of a nonattainment area.

1An explanation of each of these nine factors is provided in Chapter 5 of the Technical Support Document for December 17, 2004 designations and April 2005 modifications, available at:

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