Arizona’s Phoenix-Mesa-Scottsdale metro area has the worst air quality in the country. In 2021, the area experienced unhealthy air quality on 64.9% of days, while only having good air quality 3% of the time.
In contrast, three US metros: Urban Honolulu and Kahului, both in Hawaii, and Lake Havasu City/Kingman, Arizona, had healthy air quality on more than 99% of days in 2021.
Air quality ratings are determined by an area’s air quality index (AQI). A higher AQI means the air has more pollutants and is less healthy. The Environmental Protection Agency (EPA) uses AQI ranges to grade air quality from good to hazardous.
Good air quality days are relatively common nationwide. Around 75% of all metro areas in the US had good air quality for at least two-thirds of 2021. And 90%had good air quality on more than half of days that year.
In a few metro areas, however, good air quality days are few and far between. In Riverside-San Bernardino-Ontario, California and the greater Phoenix metro area, less than 10% of days have an AQI in the “good” range.
While good air quality is common throughout the country, unhealthy air quality is concentrated in the western and southwestern US. In 2021, 11 cities had unhealthy air quality more than 10% of the year. Of these 11, eight were in California, with the rest in Arizona, Colorado, and Utah.
Four metro areas had multiple days of hazardous air: Las Cruces, New Mexico, and Sacramento-Roseville-Arden-Arcade; El Centro; and Riverside-San Bernardino-Ontario, in California. With hazardous air quality, the entire population is likely to be impacted by “serious health effects,” including reduced lung function and heart failure in extreme cases.
Median AQI is another way of comparing air quality — it reflects an average day of air quality in a location. The results tell a similar story as good and bad AQI days.
Hawaii and the Pacific Northwest have metro areas with the best average AQIs. Phoenix-Mesa-Scottsdale is the only area with an average AQI in the “unhealthy” range, at 123. Riverside-San Bernardino-Ontario; Bakersfield; and Visalia-Porterville, all in California, have the next worst air quality median AQI.
The EPA has data on air quality from 336 metropolitan statistical areas (MSAs). Each of these MSAs has air quality monitoring stations that report data an average of 349 days out of the year. These stations measure the presence of all pollutants: CO, Pb, NO2, Ozone, PM10, PM2.5, and SO2. Together, these pollutant measurements form an air quality index.
Learn more about the US environment, how air quality has changed since the Clean Air Act, and get the facts every week by signing up for our newsletter