Over the past three decades of the Clean Air Act, EPA state,
local, and tribal agencies, other major stakeholders in the
process, including the private sector, have worked to
implement programs aimed at reducing emissions of those
pollutants that contribute to poor air quality (Fig. 1;
Bachmann 2007). The national-level trends in criteria
pollutants and selected hazardous air pollutants (HAP)
shown in Fig. 2 (EPA 2008a) indicate the progress in air
quality resulting from these programs. Of the six pollutants
for which national ambient air quality standards (NAAQS)