The Tucson Metropolitan Statistical Area (MSA) ranked eighth out of 12 comparable western MSAs in the percent of good air quality in 2017. Tucson’s air quality was rated “good” 39.7 percent of the time, a decline from 54.9 percent in 2016. Phoenix was last among the 12 western metropolitan areas, with only 8.8 percent of days in 2017 considered “good”. Days are rated “good”, according to the U.S. EPA, when air quality is satisfactory and air pollution poses little or no threat. The annual share of good air quality days in Tucson was at an eighteen-year low in 2017.