Two landmark events in 1970 helped to establish the basic framework for managing air quality in the United States: the enactment of the CAA Amendments and the creation of EPA. The CAA and its subsequent amendments (such as those in 1977 and 1990) endeavor to protect and promote public health and public welfare by pursuing the following goals:

  • Mitigate potentially harmful ambient concentrations of six so-called criteria pollutants: carbon monoxide (CO), nitrogen dioxide (NO2), sulfur dioxide (SO2), ozone (O3), particulate matter (PM), and lead (Pb).

  • Limit the sources of exposure to hazardous air pollutants (HAPs), also called “air toxics.”

  • Protect and improve visibility in wilderness areas and national parks.

  • Reduce emissions of substances that cause acid deposition, specifically sulfur dioxide and nitrogen oxides (NOx).

  • Curb use of chemicals that have the potential to deplete stratospheric ozone.4

The nation’s AQM system operates through three broad kinds of activities (Figure ES-1): (1) setting standards and objectives, (2) designing and implementing control strategies, and (3) assessing status and measuring progress. The committee’s detailed assessments of the strengths and limitations of these activities are presented in Chapter 2 (Setting Standards and Objectives), Chapter 3 (Implementation Planning), Chapter 4 (Mobile-Source Controls), Chapter 5 (Stationary-Source Controls), and Chapter 6 (Measuring Progress). Overall, the committee found that the AQM system has made substantial progress, especially in the following ways:

  • Setting National Ambient Air Quality Standards (NAAQS) for criteria pollutants, designing and implementing state implementation plans (SIPs) to comply with the NAAQS, and implementing other CAA programs to address hazardous air pollutants, acid rain, and other issues have all promoted enhanced technologies for pollution control and have contributed to substantial decreases in pollutant emissions.

  • Air quality monitoring networks have documented decreases in ambient concentrations of the criteria pollutants, especially in urban areas, and despite growth in power production and transportation uses. The NAAQS for sulfur dioxide, nitrogen dioxide, and carbon monoxide have


The NRC charged the committee only to address air quality in the troposphere (lower atmosphere). The NRC has elsewhere provided treatment of issues related to stratospheric ozone depletion and global climate change, see NRC (1998a, 2001a,b, 2003a) and NAE/ NRC (2003).

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