Table 2: National ambient air quility standards (NAAQS) for criteria air pollutants [59].

Criteria air pollutant Primary standard Secondary standard Level Averaging Time Level Averaging Time (1) Ozone 0.075 ppm (2008 std) 8 hours Same as primary (2) Particulate Matter Same as primary  PM2.5 15.0 μg/m3 Annual 35 μg/m3 24 hours Same as primary  PM10 150 μg/m3 24 hours (3) Carbon Monoxide 9 ppm (10 mg/m3) 8 hours None 35 ppm (40 mg/m3) 1 hour (4) Nitrogen Dioxide* 53 ppb Annual Same as primary 100 ppb 1 hour None (5) Sulfur Dioxide 75 ppb 1 hour 0.5 ppm 3 hour (6) Lead 0.15 μg/m3 Rolling 3 month average Same as primary

*Although NAAQS cover the entire group of NOX, NO2 is use as indicator for this group.