per unit of energy generated were lower than 75% of all U.S. coal fired power plants. The plant's low CO2 emissions per electric output, compared to other coal fired plants, were attributable to a relatively low heat rate combined with exclusive use of bituminous coal, which creates less CO2 per heat output than other types of coal. Mercury emissions in 2011 were or (). Air quality effects Northern Arizona and the Colorado Plateau have consistently met National Ambient Air Quality Standards (NAAQS) established to protect public health. The EPA's Air Quality Index (AQI), showing air quality on a daily basis, lists no unhealthy days for the general population in all of northern Arizona and southern Utah counties. Unhealthy days for ozone-sensitive groups – those with asthma or lung disease – are also rare in these counties despite high natural background levels in the Intermountain West. Coconino County, with the highest occurrences, averages less than two unhealthy days per year, these occurring between March and June, suggesting a correlation with spring stratospheric ozone intrusions. NGS emissions are unlikely to have contributed to high ozone occurrences, as the plant is located at the northern border of Coconino County, and prevailing southwesterly winds in the spring blow toward counties to the north and east, which have not reported any days with unhealthy ozone levels. Moreover, Coconino County's two ozone monitoring sites – Grand Canyon Hance Camp and Page – show significant differences within the county (the sites being 70 miles apart). During periods when ozone readings have been highest, peak concentrations in Page were noted to be more than 10 percent lower than those at the Grand Canyon. Nitrogen dioxide (NO2) levels in the Page area average about 3 ppb, 94 percent lower than the NAAQS standard of 53 ppb. Carbon monoxide (CO) levels have also been far below the standards. Fine particulate (PM2.5) levels in the Grand Canyon region have been among the lowest in the nation since before NGS completed installation of SO2 scrubbers and low NOx-SOFA burners. Annual mean levels for PM2.5 in the Page area are about 3 micrograms/cubic meter (µg/m3) – one-quarter of the NAAQS standard of 12 µg/m3 and as low or lower than the cleanest U.S. cities listed by the World Health Organization. Visibility in the Grand Canyon region has also been among the best in the nation since before NGS installed scrubbers and low NOx-SOFA