Ground-level ozone air pollution, also known as smog, is making breathing difficult for more people living in the U.S. than any other single pollutant. In the years 2022, 2023 and 2024, 38% of the population, some 129.1 million people, were exposed to levels of ozone that put their health at risk. That is the highest number since our report six years ago. This is an increase of 3.9 million people over last year’s figure, and includes tens of millions of infants and children, people age 65 or older and others whose health conditions make them especially vulnerable to health harm from air pollution.
More than 129 million people live in counties with F grades for ozone pollution.
After several years of successful reductions in ozone pollution in many parts of the country – thanks to clean up measures enacted under the Clean Air Act – the results in both “State of the Air” 2025 and 2026 show a distressing reversal, demonstrating that past progress is fragile. The increase in ozone in this year’s report was especially remarkable given that the previous report already showed a drastic worsening and that more counties in this year’s report reported incomplete data. The places that earned an “F” grade for ozone in this year’s report were spread across 219 counties in 36 states and Washington DC, the highest number of counties with failing ozone grades since the 2016 report. There were 77 metro areas across the country where the worst-performing county earned a failing grade for ozone.
A changing climate is making the job of cleaning up the air more difficult
Extreme heat, drought and wildfires are contributing to unhealthy levels of air pollution across much of the U.S., exposing a growing proportion of the population to ozone that puts their health at risk. The regions of the country most seriously impacted by high levels of ozone are southwestern states from California to Texas as well as most of the Midwest. The worsening was due to two factors:
- Extensive wildfires in Canada in 2023 generated ozone-forming pollutants (also called precursor emissions) that blew across the border.
In both 2023 and 2024, high temperatures and other weather conditions were ideal for ozone formation, especially in the South, while regional sources of precursor emissions have also become stronger and more widespread.
Even after last year’s report revealed an unprecedented decline in air quality, this year’s findings show conditions worsening further. In seven states—Arizona, Connecticut, Louisiana, New Jersey, New Mexico, Oklahoma and Texas—more counties experienced increases in unhealthy ozone days than saw improvement or no change.
Climate change contributed to worsening ozone pollution by increasing precursor emissions (found in worsening wildfire smoke) and by creating atmospheric conditions, including higher temperatures and lower wind speeds, that allow pollutants to accumulate and ozone to form.
In contrast, about four of every ten states showed overall improvement in ozone. Altogether, seven states (Colorado, Michigan, Nebraska, Nevada, Rhode Island, Utah and Wyoming) showed more of their counties posting improvements compared to those with unchanged or worsening ozone pollution from last year. Nevertheless, in many of the counties even in these states, ozone levels continued to be unhealthy on enough days to earn failing grades.
Of the 25 metropolitan areas that ranked worst for ozone pollution, 18 worsened compared to last year’s report while only 7 improved. Sacramento (for the second year in a row) was the only city of these 25 that recorded its fewest-ever number of unhealthy days for ozone. Rankings remained relatively stable, with the worst five staying unchanged from last year’s report, and most being within two places of last year’s position. Only three metro areas worsened enough to be added to the Worst 25 list for ozone (Boise City, Idaho, worsening significantly from 46th to 24th worst; Louisville, Kentucky, from 29th to 22nd worst; and Milwaukee, Wisconsin, from 26th to 22nd worst). Three metropolitan areas had improved enough in ozone levels to leave this Worst list (Redding-Red Bluff, California, from 24th to 91st worst; Colorado Springs, Colorado, from 23rd to 54th worst; and San Antonio, Texas, from 20th to 28th worst).
In one small piece of good news, for the second report in a row, none of the cities on the Worst 25 list reported a worst-ever average number of days of ozone pollution.
The geographical distribution of cities on the Worst 25 list repeats the pattern seen over
the last decade, with the highest levels of ozone air pollution continuing to occur in the West. California retains its position of being the state with the most metro areas on the list with 8 of the 25 most-polluted cities. Arizona, Colorado, Idaho, Nevada, New Mexico, Oklahoma, Texas and Utah account for the other 11 metro areas. They are joined this year by six metro areas in four more easterly states— Connecticut, Illinois, Kentucky, and Wisconsin.
Although cleanup of ozone precursor pollutants from industrial, power generation, and mobile source has helped reduce ozone concentrations, the impact of climate change has contributed to widespread wildfire disasters along with higher temperatures, dry, sunny skies and more frequent air stagnation events. Taken together, these conditions produced much higher numbers of unhealthy ozone days than would otherwise be the case.