Ozone Pollution

"More than 119 million people live in counties that were graded F for ozone."
Nearly 4 in 10 people in the United States (38%) live in areas with unhealthful levels of ozone. Counties that were graded F for ozone levels have a combined population of more than 119.3 million. These people live in the 191 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.

"13 of the most polluted cities had their fewest unhealthy ozone days."
The "State of the Air 2013" finds that most cities continued the long-term trend for fewer high ozone days, but not all. Many places recorded slightly more unhealthy days on average than in 2008-2010, which had been the best period to date. Warmer weather may have contributed to the increase. The most common pollutant, ozone has already proven to be one of the hardest to reduce. These findings add to the evidence that the nation needs more aggressive tools to secure continued reductions. 

  • Fifteen of the 27 cities with the most ozone pollution improved their air quality over the past year’s report.1 Thirteen of the country’s most smog-polluted cities experienced their best year yet. Unfortunately, even with the improvements, people living there are still forced to breathe air that reaches dangerously unhealthy levels.
  • 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.
    Los Angeles remained the city with the worst ozone pollution problem, but reported its fewest unhealthy ozone days since the "State of the Air" reports began. Los Angeles has one-third fewer days of unhealthy air than in the first report. Of the 27 metropolitan areas most polluted by ozone, twelve others also reported their fewest unhealthy ozone days: Visalia (CA); Bakersfield (CA); Fresno; Sacramento; San Diego; Merced (CA); Modesto (CA); New York City; Charlotte; Philadelphia; Phoenix; and Pittsburgh. Two others also improved: Hanford (CA); and San Luis Obispo (CA).
  • However, twelve cities suffered more unhealthy days, including three that moved for the first time to the most polluted list: Beaumont-Port Arthur (TX); Oklahoma City; and Tulsa (OK). Other cities reporting more or worse days during 2009-2011 included Houston; Dallas, Washington, DC-Baltimore; El Centro (CA); Cincinnati; Birmingham; Las Vegas; Louisville; and St. Louis.

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

1 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.