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Regulatory Framework The Clean Air Act (CAA), which was last amended in 1990, requires the Environmental Protection Agency (EPA) to set NAAQS (40 CFR part 50) for criteria pollutants. Criteria pollutants are air contaminants that are commonly emitted from the majority of emissions sources and include carbon monoxide (CO), lead (Pb), sulfur dioxide (S02), particulate matter smaller than 10 & 2.5 microns (PM 10 & PM2,5), ozone (03), and nitrogen dioxide (NO2). The CAA established 2 types of NAAQS: Primary standards: – Primary standards set limits in order to protect public health, including the health of "sensitive" populations (such as asthmatics, children, and the elderly). Secondary standards: – Secondary standards set limits in order to protect public welfare, including protection against decreased visibility, and damage to animals, crops, vegetation, and buildings. The EPA regularly reviews the NAAQS (every five years) to ensure that the latest science on health effects, risk assessment, and observable data such as incidence rates are evaluated in order to re- propose any NAAQS to a lower limit if the data supports the finding. The Colorado Air Pollution Control Commission, by means of an approved State Implementation Plan (SIP) and /or delegation by EPA, can established state ambient air quality standards for any criteria pollutant that are at least as stringent as, or more so, than the federal standards. Ambient air quality standards must not be exceeded in areas where the general public has access. Table 3 lists the federal and state ambient air quality standards. Table 3. Ambient Air Quality Standards (EPA, 2011) Pollutant [final rule cite] Primary/ Secondary Averaging Time Level Form Carbon Monoxide primary 8 -hour 9 ppm Not to be exceeded 1 -hour 35 ppm [76 FR 54294, Aug 319 more than once per 20-1-11 year Lead [73 FR 66964 Nov 12 primary and Rolling 3 month average 0.15 µg /m3 Not to be exceeded 20081 secondary Nitrogen Dioxide primary 1 -hour 100 ppb 98th percentile, [75 FR 6474 Feb 9 2010] averaged over 3 years primary Annual 53 ppb Annual Mean [61 FR 52852 Oct 8 1996] and secondary Ozone primary 8 -hour 0.075 Annual fourth- highest [73 FR 16436 Mar 27 and ppm daily maximum 8 -hr 2008] secondary concentration, averaged over 3 years Particle PM2.5 primary Annual 15 µg/M3 annual mean, averaged Pollution and over 3 years 24 -hour 35 µg/M3 98th percentile, [71 FR 61144, secondary Oct 17, 2006] averaged over 3 years PM10 primary 24 -hour 150 µg/M3 Not to be exceeded and more than once per secondary year on average over 3 DOI- BLM -CO- 110 - 2012 - 0023 -EA 15