critical analysis of specific aspects of the standard-setting procedure, especially those aspects relating to the scientific basis for the standards and the procedures used to set them.


The CAA sets standards in a number of ways:

  • The setting of National Ambient Air Quality Standards (NAAQS) for six principal pollutants (known as criteria pollutants).

  • The setting of emission standards for a variety of stationary and mobile sources for substances that are the criteria pollutants, their precursors, or hazardous air pollutants (HAPs).

  • Promulgate additional emission standards for HAPs that continue to pose a significant residual risk following the implementation of the first round of emission standards.

  • The setting of fuel and product reformulation standards (for example, reformulated gasoline and requirements for chlorinated fluorocarbons).

  • The setting of reduced caps for emissions of certain pollutants from certain industries (for example, the sulfur dioxide [SO2] cap-and-trade program).

The CAA also contains many provisions for attaining and maintaining these standards. In this chapter, the committee focuses on, and critiques, the process by which many of these standards are set. Subsequent chapters discuss how they are implemented.

The CAA begins by addressing two major categories of pollutants for which standards are set differently: criteria pollutants and HAPs. The principal difference between the two arises from the specification in the CAA that the presence of criteria pollutants “in the ambient air results from numerous or diverse mobile or stationary sources.” No such requirement is stated for HAPs.1 Thus, presumably, criteria pollutants are more ubiquitous, pose a risk to a larger fraction of the general population, and have more widespread impacts on ecosystems and natural resources than HAPs. Criteria pollutants and HAPs are managed through fundamentally different regulatory frameworks. Criteria pollutants are regulated primarily through the setting of ambient-air-concentration and time standards, known as the NAAQS, and taking action to attain these standards. HAPs are regulated through the promulgation of standards that limit the release or emissions of such compounds (as opposed to their ambient concentrations), followed in


The only specific limitation currently placed on HAPs in the CAA is that they cannot be criteria pollutants.

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