Section 109(a) of the Clean Air Act (CAA) requires the Administrator of the Environmental Protection Agency (EPA) (defendant) to promulgate National Ambient Air Quality Standards (NAAQS) for certain air pollutants. Section 109(b)(1) of the CAA directs the EPA to set ambient air quality standards, the attainment and maintenance of which in the judgment of the Administrator are necessary to protect the public health. In July 1997, the Administrator revised the NAAQS for particulate matter and ozone. The American Trucking Associations, Inc. and others (plaintiffs) challenged the new standard in federal court. The U.S. Court of Appeals for the District of Columbia found that § 109(b)(1) of the CAA delegated legislative power to the Administrator in contravention of Article I, § 1 of the United States Constitution because the EPA had interpreted the CAA to provide no “intelligible principle” to guide the agency’s exercise of authority. The appellate court further held that Whitman is prohibited from considering the costs of implementation in setting NAAQS under the CAA. The appellate court remanded the NAAQS to the EPA. The case then came before the United States Supreme Court.