What Is Ozone?
Ozone is a molecule of three oxygen atoms. Ozone attacks lung tissue by reacting chemically with it. Learn more about ozone.
Of the 25 most ozone-polluted cities, 16 had worse ozone, experiencing more unhealthy air days on average in 2014-2016. Nine cities improved, while five had their fewest days ever.
Increased heat in 2016 likely drove this increase in ozone. Warmer temperatures stimulate the reactions in the atmosphere that cause ozone to form, and 2016 saw the second warmest temperatures on record in the United States.
Los Angeles remains at the top of this list, as it has for all but one of the 19 reports. Los Angeles also recorded more unhealthy air days in this report, measured in the weighted average, a change from last year when it reached its lowest level ever.
In addition to Los Angeles, 15 others among the 25 cities with the worst ozone pollution each had a higher average of unhealthy days than in 2014-2016, including some of the nation’s largest metropolitan areas: New York City; Chicago; Atlanta; Philadelphia; San Diego; San Jose-San Francisco; Washington-Baltimore; and Salt Lake City. Many smaller cities on that list also suffered from more ozone: Bakersfield, CA; Visalia-Porterville-Hanford, CA; Sacramento, CA; Redding-Red Bluff, CA; Hartford, CT; Chico, CA; and Sheboygan, WI.
Increased heat in 2016, the second warmest year on record in the United States, likely drove this increase in ozone.
Fortunately, nine cities had fewer high ozone days, including five that experienced their fewest days since the report began: Modesto-Merced, CA; Las Vegas; Denver; El Centro, CA; and Dallas-Fort Worth. Also improving over last year's report were Fresno-Madera, CA; Phoenix; Houston; and Fort Collins, CO.
These comparisons are all based on the Air Quality Index adopted with the 2015 ozone national air quality standard. Unfortunately, EPA has delayed key steps to formally identify cities that do not meet that standard. In fact, the Lung Association and others had to take legal action to get EPA to announce its long-overdue list of cities that have unhealthy levels of ozone. The court directed EPA to release the final list by the end of April 2018. That crucial step begins the process of cleaning up ozone to meet the current, more protective national air quality standard.
Regional differences. California retains its historic distinction with 11 of the 25 most polluted cities in that state. The Southwest continues to fill most of the remaining slots, with seven of the 25 most ozone-polluted. Texas has two cities in the 25 most-polluted list: Houston and Dallas-Fort Worth. Colorado has two, as well: Denver and Fort Collins. Arizona, Nevada and Utah each have one.
Only six cities of the most polluted are east of the Mississippi River. Three in the Northeast are on the list: New York City, Philadelphia, and Hartford, CT. The Midwest has two: Chicago and Sheboygan, WI. Atlanta is the only southern city to reach the list.
Many of those cities experienced high-ozone days from polluted air blown into their state from upwind sources, as well. Fairfield, CT, part of the New York City metropolitan area, recorded the most high-ozone days on average in the eastern half of the nation, largely driven by ozone blown in from outside the metro area. Hartford, CT, and Sheboygan, WI, both also receive high levels of ozone from upwind sources.
Those rankings reflect trends seen in the past three 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. The impact of climate shows up even in some cities with lowest weighted averages ever. For example, Denver and Las Vegas experienced more high-ozone days in 2016 than in 2015 or 2014, just not as many as in 2013, keeping their 2014-2016 three-year average at its lowest.
- NOAA National Centers for Environmental Information, State of the Climate: National Climate Report for Annual 2016, published online January 2017. Accessed at https://www.ncdc.noaa.gov/sotc/national/201613.
- U.S. District Court for the Northern District of California. Case 4:17-cv-06936-HSG, March 12, 2018. There is one exception to the end of April deadline: The court directed EPA to make its decision on the status of one city, San Antonio, TX, by July 11, 2018.