Ozone Pollution

More than 4 in 10 people in the United States (44.8%) 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.

More than 140.5 million people live in the 296 counties that received an F for ozone levels. These people live 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.

Ozone worsened in the most polluted metropolitan areas—some substantially—in 2010-2012 compared to 2009-2011. Of the 25 metro areas most polluted by ozone, 22 had worse ozone problems. Weather played a factor. The warmer summers in 2010 and 2012 contributed to higher ozone readings and more frequent high ozone days. Sunlight and heat create conditions that increase the risk of high ozone levels. Fortunately, these cities had significantly fewer unhealthy ozone days than they had a decade ago thanks to steps taken to reduce ozone. Many cities had only slightly more unhealthy days than in the 2013 report.

  • Twenty-two of the 25 most ozone-polluted cities had more high ozone days on average in 2010-2012 compared to 2009-2011. Fortunately, even these places have much better air quality compared to ten years ago (or in our earliest reports). However, rising temperatures create conditions favorable to forming ozone. Communities will need more help to reduce ozone pollution in the warmer temperatures expected from the changing climate.
  • Los Angeles remained the city with the worst ozone pollution problem, and had slightly more days of high ozone in 2010-2012. Measuring more unhealthy days on average were 22 other cities:1 Visalia-Porterfield-Hanford (CA), Bakersfield (CA), Fresno-Madera (CA), Houston, Modesto-Merced (CA), Washington-Baltimore, Las Vegas, Phoenix, New York City, St. Louis, Tulsa (OK), Cincinnati, Philadelphia, Louisville (KY), Oklahoma City, Chicago, Pittsburgh, Fort Collins (CO) Birmingham (AL), Cleveland (OH) and Sheboygan (WI).2
  • Only Dallas-Fort Worth and El Centro (CA) measured fewer unhealthy days among the 25 most ozone-polluted cities. Unfortunately, even with the improvements, people living there are still forced to breathe air that reaches dangerously unhealthy levels. Sacramento (CA) was the only city among the 25 most ozone- polluted to have maintained the same number of unhealthy days.

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

1Complete 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.

2Note 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.

FACT: Particles are smaller than 1/30th the diameter of a human hair. When you inhale them, they are small enough to get past the body’s natural defenses.

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