If the concentration of one or more criteria pollutants in a geographic area is found to exceed the regulated or ‘threshold’ level for one or more of the NAAQS, the area may be classified as a nonattainment area. Areas with concentrations of criteria pollutants that are below the levels established by the NAAQS are considered either attainment or unclassifiable areas.
To add to @liz 's answer, the "criteria pollutants" are lead, ozone, sulfur dioxide, nitrogen oxides, carbon monoxide, and particulate matter. The National Ambient Air Quality Standards (NAAQS) are the upper limits of ambient air concentrations for each of these pollutants based on risks to human and ecological health. More information about the NAAQS can be found here: https://publiclab.org/wiki/frac-sand-legislation#National+Ambient+Air+Quality+Standards.