Exposure to unhealthy levels of ozone air pollution continues to make breathing difficult for millions of Americans all across the country. In the years 2017, 2018 and 2019, more than 123.2 million people lived in the 163 counties that earned an F for ozone. That is fewer than in the 2020, 2019 and the 2018 reports, but more than in the 2017 report. There are still a lot of vulnerable people, including 28.1 million children and 18.2 million people age 65 or older, exposed to ozone air pollution and increased risk of harm.
The list of 25 cities with the worst ozone pollution in “State of the Air” 2021 remains the same as in last year’s report, although a few moved up or down a bit in the rankings (see Figure 1). Cities in the West and the Southwest continue to dominate the most-ozone-polluted list. California retains its historic distinction of having the most cities on the list, with 10 of the 25 most-polluted cities. The Southwest fills most of the remaining slots, with eight of the 25 cities on the list. Only seven of the worst 25 cities are east of the Mississippi River. Once again this year, no metropolitan areas in the Southeast rank among the 25 worst cities most polluted by ozone.
Overall, the 25 most ozone-polluted cities in the U.S. experienced fewer bad air days on average from 2017 to 2019 than did those in last year’s report covering 2016 to 2018. Four California cities on the list recorded their fewest bad ozone days on average in the report’s 22-year history, although they are still among the ten most ozone-polluted cities in the nation.
The geographical distribution of cities with the worst ozone problems confirms a trend seen over the past five reports: fewer Eastern cities and more Western cities. The increased oil and gas extraction in the Southwest and the cleanup of power plants in the East have shifted the cities that experienced the greatest number of unhealthy ozone days. However, there are still problems in the East with transported pollution when ozone and ozone precursors enter from upwind sources in the Midwest. For example, Fairfield County, Connecticut is the county with the highest ozone in the eastern half of the nation, in part because of pollution transported from other states.
NOAA National Centers for Environmental Information, Global Climate Report--Annual 2017, published online January 2018. Accessed at https://www.ncdc.noaa.gov/sotc/global/201713