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National Ambient Air Quality Standards

Sections 108 and 109 of the federal Clean Air Act (CAA) require United States Environmental Protection Agency (U.S. EPA) to set National Ambient Air Quality Standards (NAAQS) for six common and widespread “criteria” air pollutants that are harmful to human health and the environment. IDEM provides information, including health and environmental effects, for each of the pollutants, which include: carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone (O3), particulate matter (PM), and sulfur dioxide (SO2).

States are responsible for implementing, maintaining and enforcing the NAAQS. Section 110 of the CAA requires states to maintain state implementation plans (SIPs) detailing how they will accomplish this. IDEM maintains Indiana’s SIP.

The CAA identifies two types of NAAQS, including:

  • Primary standards, which set limits on concentrations of the pollutants in the outdoor air to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly, and;
  • Secondary standards, which are designed to protect public welfare from adverse effects, including decreased visibility (regional haze) and damage to animals, crops, vegetation, and buildings.

As such, the NAAQS are health-based regulations that define the allowable level of criteria pollutants in ambient (outdoor) air, and U.S. EPA sets the primary and secondary standards based on scientific studies showing at what level exposure may cause harm. The primary and secondary standards for a pollutant may be set at the same level. U.S. EPA’s NAAQS table contains primary and/or secondary standards for each pollutant, including the level, averaging time, and form (approved method for averaging concentrations and allowable exceedances for a specified timeframe).

IDEM operates a statewide ambient outdoor air monitoring network to measure pollutants according to current NAAQS requirements and works with U.S. EPA to develop clean air strategies, which may affect citizens, businesses, and governments.

Periodic NAAQS Review and Revision

Sections 108 and 109 of the CAA govern U.S. EPA’s periodic NAAQS review and potential revisions to these standards. U.S. EPA is responsible for examining the science upon which the regulations are based and making appropriate revisions. U.S. EPA’s review process includes an extensive Integrated Science Assessment (ISA), Risk/Exposure Assessment (REA), and Policy Assessment (PA). During this process, U.S. EPA provides documents to the Clean Air Scientific Advisory Committee (CASAC), which is a congressionally mandated, independent federal advisory committee that provides scientific and technical advice to the U.S. EPA Administrator concerning the NAAQS program. The CASAC reviews U.S. EPA documents and provides advice and recommendations concerning the adequacy of the existing standards or revisions that may be appropriate to consider. U.S. EPA considers information in the ISA, REA(s), and PA, and the advice of CASAC, to develop appropriate rule revisions. U.S. EPA publishes announcements concerning proposed rules, opportunities for the public participation, and final rules in the Federal Register.

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