What is Prevention of Significant Deterioration (PSD) under the Clean Air Act?

Amendments to the CAA in 1997 established a PSD structure that requires permits for areas that have achieved better air quality standards than required under NAAQS. These areas are known as attainment areas. The state administers this permitting process with EPA approval. This system includes ambient (increment & NAAQS compliance) and Best Available Control Technology (BACT) components. The BACT standards tend to be more stringent than NSPS. As with the BAT standards, BACT standards do not require a particular technology; rather, they provide a process for choosing what control technology to employ.