Citizens and businesses in nonattainment areas and near-nonattainment areas are concerned about their "ppm" numbers. 

Those air-pollution readings determine whether a city stays in its nonattainment category, achieves attainment, or moves into a more severe category.  Under the law, the federal government can impose measures, such as mandating tailpipe emissions testing, requiring the use of cleaner fuels, or withholding funding for highways, if an area fails to follow the schedule outlined in its state plan.