What is ozone?
Ozone is a molecule of three oxygen atoms. Ozone attacks lung tissue by reacting chemically with it.
Far more people suffered unhealthy ozone pollution in 2016-2018 than in the last three reports. In 2016-2018, more than 137 million people lived in the 205 counties that earned an F for ozone.
That is significantly higher than in the 2019, 2018 and 2017 reports and is the highest since the 2016 report. This trend shows strong evidence of the impacts on air quality from the warmer years also reported in this period. Of the ten most polluted cities, six did worse than in the 2019 report, including some of the nation's largest metropolitan areas.
All three years in this report mark three of the five warmest years ever recorded.
Why? Increased heat. The three years in this report were three of the five warmest on record in the United States: the year 2016 remains the warmest year on record, while 2017 is now the fourth warmest, and 2018 ranked fifth warmest. Warmer temperatures make ozone more likely to form and harder to clean up.
Changes in where ozone is worst in the U.S. continue a trend seen in the past four reports, where increased oil and gas extraction in the Southwest and cleanup of power plants in the eastern U.S. have shifted the cities that experienced the greatest number of unhealthy air days.
Ozone rankings are all based on unhealthy air days as recorded using the Air Quality Index adopted with the 2015 national air quality standard for ozone. In 2018, EPA officially designated all or parts of the 25 most polluted cities as "nonattainment" areas for that ozone air quality standard. That action requires these areas to take steps to clean up the sources of pollution going forward.
Los Angeles remains at the top of the list of most polluted cities for ozone, as it has for all but one of the 21 reports, despite the metro area's continued fight against ozone. Los Angeles-Long Beach also recorded more unhealthy ozone days in this report, measured by weighted average.
In addition to Los Angeles, 13 others among the 25 cities with the worst ozone pollution each had a higher weighted average of unhealthy days in 2016-2018, including some of the nation's largest metropolitan areas: Phoenix, Las Vegas, Denver, Salt Lake City, Chicago and Milwaukee. Many smaller cities on that list also suffered from more ozone: Visalia, CA; Bakersfield, CA; El Centro, CA; El Paso-Las Cruces, TX-NM; Chico, CA; Fort Collins, CO; and Sheboygan, WI.
Eleven of the 25 cities with the worst ozone pollution had fewer unhealthy ozone days on average in 2016-2018. Those included San Jose-San Francisco-Oakland and Dallas-Fort Worth, each of which reached its fewest unhealthy ozone days ever. Other cities that had fewer high-ozone days included Fresno; Sacramento; San Diego; New York-Newark; Redding-Red Bluff, CA; Houston; Washington-Baltimore; Philadelphia; and Hartford, CT.
Regional Differences. Only seven cities among the worst for ozone are east of the Mississippi River, including the New York City metro area, where Fairfield County, CT, suffers from the highest levels in the eastern U.S. Others in the Northeast and Mid-Atlantic in the 25 most-polluted list are Washington-Baltimore; Philadelphia; and Hartford, CT. The Midwest has three: Chicago; Sheboygan, WI and Milwaukee. For the first time, with Atlanta's improvement, no city in the Southeast has any city on the most-ozone-polluted list.
Cities in the West and the Southwest continue to dominate the most-ozone-polluted list. California retains its historic distinction, as it is home to 10 of the 25 most polluted cities. The Southwest continues to fill most of the remaining slots, with eight of the 25 cities, including three in Texas—Houston, El Paso, and Dallas-Fort Worth. Colorado has two—Denver and Fort Collins. Arizona, Nevada and Utah each have one.
The findings show the continued impact of transported pollution that moves ozone and ozone precursors across state lines. For example, emissions generated in Chicago cross Lake Michigan to reach Sheboygan, WI. Fairfield County, CT, remains the county with the highest ozone in the eastern half of the nation because of the transported ozone and ozone precursors from upwind states.