Does attainment status for the PM₁₀ National Air Ambient Quality Standard change the trend in ambient levels of particulate matter?

Despite increasingly stringent and cost-demanding national, state, and local air quality regulations, adverse health effects associated with ambient exposure to air pollution persist. Accountability research, aimed at evaluating the effects of air quality regulation on health outcome, is increasingly viewed as an essential component of responsible government intervention. In this paper, we focused on assessing the impact of air quality regulations on ambient levels of air pollution. We considered two groups of counties: the first group (A) includes counties that in 1991 were designated as in attainment or unclassifiable with respect to the 1987 National Ambient Air Quality Standards (NAAQS) and maintained their status through 2006; the second group (Ā), includes counties that in 1991 were designated as nonattainment and were subsequently redesignated as in attainment. We hypothesized that if air pollution control programs adopted to meet the NAAQS are effective in reducing air pollution levels, counties in group Ā will experience a sharper decrease in PM₁₀ levels than counties in group A. To provide evidence to support this hypothesis, Bayesian hierarchical models were developed for estimating 1) the yearly percentage change in ambient PM₁₀ levels for 100 counties and the entire USA during the period 1987-2007 and 2) the change in PM₁₀ ambient levels in counties in group Ā compared with counties in group A. We found statistically significant evidence of variability across counties in trends of PM₁₀ concentrations. We also found strong evidence that counties transitioning from nonattainment to attainment status during the period 1987-2007 experienced a sharper decline in PM₁₀ when compared with counties that were always in attainment.