Areas subject to PSD regulation are divided into three classes, and maximum allowable increases in ambient concentrations of air pollutants, above initial baseline concentrations, are defined for each class.

• 服从《防止大气质量严重恶化》规章的地区可被划为三类，大气污染物密度提高的最大允许值以及超过基线密度的，都被划归各类等级。