The Clean Air Act: Regulating America's Atmosphere
The Clean Air Act (CAA), most significantly reformed in 1970 and 1990, is the primary federal statute regulating air pollution in the United States. It establishes a multi-layered regulatory framework that addresses pollution from stationary sources (factories, power plants), mobile sources (vehicles, aircraft), and hazardous air pollutants.
The cornerstone of the CAA is the National Ambient Air Quality Standards (NAAQS) system. The EPA is required to set NAAQS for six 'criteria pollutants': particulate matter (PM2.5 and PM10), ground-level ozone (smog), carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. Two tiers of standards are set: primary standards to protect public health (with an 'adequate margin of safety' including for sensitive populations — children, elderly, those with asthma), and secondary standards to protect public welfare (crops, visibility, buildings, ecosystems). Once EPA sets NAAQS, each state must develop a State Implementation Plan (SIP) demonstrating how it will attain and maintain these standards.
For new and modified major stationary sources, the CAA requires pre-construction permits and technology-based emissions controls. New Source Review (NSR) and Prevention of Significant Deterioration (PSD) programs ensure that new industrial facilities do not degrade air quality below NAAQS. These programs have generated substantial litigation, particularly regarding the interpretation of 'modification' — whether upgrading existing facilities triggers NSR requirements.
The 1990 CAA Amendments were transformative, establishing the landmark Acid Rain Program (Title IV), the first national cap-and-trade system, which dramatically reduced sulfur dioxide emissions from power plants. The 1990 Amendments also significantly strengthened requirements for hazardous air pollutants (Section 112, establishing Maximum Achievable Control Technology standards for 187 listed toxic pollutants) and for mobile source emissions and vehicle fuel standards. The CAA is administered by EPA and implemented through a cooperative federalism structure: states have primary implementation responsibility, but EPA can set standards and take enforcement action if states fail to act.