Ozone Pollution

More than 4 in 10 people in the United States live in areas with unhealthful levels of ozone.

What Is Ozone?

Ozone is a molecule of three oxygen atoms. Ozone attacks lung tissue by reacting chemically with it. To learn more, go to Health Risks.

Counties that received an F for ozone levels are home to more than 128.7 million people, or 40.7 percent of the U.S population. These people live in the 254 counties where the monitored air quality places them at risk for premature death, aggravated asthma, difficulty breathing, cardiovascular harm and lower birth weight. The actual number who breathe unhealthy levels of ozone is likely much larger, since this number does not include people who live in adjacent counties in metropolitan areas where no monitors exist.

Many cities suffered fewer ozone days (by weighted average) than in the 2014 report, but almost as many of the most polluted cities suffered more days. Steps to reduce ozone continue to pay off in many ozone-burdened metropolitan areas, as exemplified in Los Angeles, still ranked as the most ozone-polluted city in the nation.

  • Los Angeles experienced its best three-year period since this report began dropping more than one-third of its average number of unhealthy air days since the late 1990s. Four other metropolitan areas reported their fewest high ozone days on average in the 2015 report: Visalia (CA); Bakersfield (CA); Sacramento (CA); and Washington, DC- Baltimore.1

Thirteen of the 25 most ozone-polluted cities had fewer high ozone days on average in 2011-2013 compared with 2010-2012.

  • In addition to the five cities noted above, eight other cities improved: Houston, St. Louis, Chicago, Pittsburgh, Cincinnati, Sheboygan (W), Kansas City, and Memphis.

Twelve cities fared worse, suffering slightly more high ozone days on average while one remained the same.

  • Cities that recorded more high ozone days or days with higher ozone included: Fresno (CA); Dallas-Fort Worth; Modesto-Merced (CA); Las Vegas; Phoenix; New York City; Tulsa (OK); Denver; El Centro (CA); Fort Collins (CO); Grand Rapids (MI); and South Bend (IN). Oklahoma City (OK) recorded the same weighted average for ozone as in the 2014 report.

Changes in ozone likely came from multiple factors, including weather. Ozone forms best in warmer, sunny weather with still winds. The warm, drier summer in 2012 contributed to higher ozone readings and more frequent high ozone days, while the wetter weather in 2013 in the east contributed to a drop in these days. Real success despite extended hot, dry days in California came from concentrated efforts to reduce emissions that contribute to ozone.

The improvements documented in the past sixteen years of this report show the benefit of steps taken to reduce ozone. Even the more polluted cities had significantly fewer unhealthy ozone days than they had a decade ago.

» 25 Most Ozone-Polluted Cities
» 25 Cleanest Cities for Ozone


  1. Complete names for all these metropolitan areas can be found in the tables showing the most polluted and cleanest cities. The full metropolitan areas often include multiple counties, incorporated cities and counties in adjacent states, as the Office of Management and Budget defines them. Note that the full metropolitan areas often include multiple counties, incorporated cities and counties in adjacent states, as the Office of Management and Budget defines them. Not all counties in any metro area will have monitors.
Did You Know?

Did You Know?Cutting air pollution through the Clean Air Act will prevent at least 230,000 deaths and save $2 trillion annually by 2020.

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