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National Ambient Air Quality Standards (NAAQS) are set by the United States Environmental Protection Agency (U.S. EPA) for six criteria pollutants, in order to protect human health and the environment as required by the federal Clean Air Act (CAA). NAAQS can be “primary” or “secondary” and several different units of measure are used to communicate NAAQS; parts per million (ppm) by volume, parts per billion (ppb) by volume, and micrograms per cubic meter of air (µg/m3). It is NAAQS that set in motion a number of activities performed by the Indiana Department of Environmental Management’s (IDEM’s) Office of Air Quality (OAQ).

Area Designations

State and local governments monitor ambient air to determine the level of pollution in it. The federal CAA requires that, no later than one year after promulgation of a new or revised NAAQS for any of the six criteria pollutants, the governor of each state must submit a list of all areas within their state indicating how each would be classified under the new or revised standard. The classification choices are:

  • Attainment:
    • If air quality in an area meets the current NAAQS it is considered to be in “attainment”, unless it contributes ambient air quality in a nearby area that does not meet the current NAAQS.
  • Nonattainment:
    • If air quality in an area exceeds the current NAAQS, or contributes to ambient air quality in a nearby area that does not meet the current NAAQS, it is considered to be in “nonattainment”.
  • Unclassifiable:
    • If air quality data does not exist that enables an area to be deemed “attainment” or “nonattainment”, it is considered to be “unclassifiable”.

Within one year of the governor’s submittal, U.S. EPA is to announce a list of federally designated nonattainment areas. States may contest the designation of any areas within their borders that were not included in their own submissions. The public has the opportunity to comment on proposed plans before U.S. EPA makes a final decision.

Once ambient (outdoor) air quality standards are announced as final, the federal CAA sets a basic schedule for achieving the standards and, in some cases, requirements to meet the standards. The states have primary responsibility for selecting programs most likely to improve air quality.

State Implementation Plan (SIP)

A State Implementation Plan (SIP) is the vehicle by which states accomplish implementation, maintenance and enforcement of NAAQS.