and stakeholders and examined the operation, successes, and limitations of the many components of the nation’s AQM system.


The committee concluded that implementation of the CAA has contributed to substantial decreases in emissions of several pollutants. Regulations for light-duty vehicles, light-duty trucks, and fuel properties have greatly reduced emissions per mile traveled. Programs for stationary sources, such as power plants and large factories, have also achieved substantial reductions of pollutant emissions. However, most of the reductions have been accomplished through regulations on new facilities, while many older, often higher-emitting facilities can be a substantial source of emissions. Emission “cap and trade” has also provided a mechanism for achieving emission reductions at reduced costs. Air quality monitoring networks have confirmed that ambient pollutant concentrations, especially in urban areas, have decreased over the past three decades, and monitoring has documented a reduction in sulfate deposition in the eastern United States. Economic assessments of the overall costs and benefits of AQM in the United States indicate, despite uncertainties, that implementation of the CAA has had and will probably continue to have substantial net economic benefits.


Despite the progress, the committee identified scientific and technical limitations in the current AQM system that will hinder future progress, especially as the nation attempts to meet the following key challenges in the coming decade:

  • Meeting new standards for ozone, particulate matter, and regional haze.

  • Understanding and addressing the human health risks from exposure to air toxics.

  • Responding to the evidence that, for some pollutants, there may be no identifiable threshold exposure below which harmful effects cease to occur.

  • Mitigating pollution effects that might disproportionately occur in minority and low-income communities in densely populated urban areas.

  • Enhancing understanding and protection of ecosystems affected by air pollution.

  • Understanding and addressing multistate and international transport of pollutants.

  • Adapting the AQM system to a changing (and most likely warmer) climate.

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