What is ozone?
Ozone is a molecule of three oxygen atoms. Ozone attacks lung tissue by reacting chemically with it.
Ozone pollution worsened in much of the nation. Of the 25 most-ozone-polluted cities in the U.S, 17 had more high ozone days on weighted average during 2015-2017, than in the 2018 report that covered 2014-2016. Eight of the 25 cities had fewer days, three reaching their fewest days ever.
Increased heat in 2017 likely drove this increase in ozone. Warmer temperatures stimulate the reactions in the atmosphere that cause ozone to form, and 2017 saw the second warmest temperatures on record in the United States. All three years covered in this report rank as the three warmest years ever recorded.1
Los Angeles remains at the top of this list as it has for all but one of the 20 reports. Los Angeles also recorded more unhealthy ozone days in this report, measured by weighted average.
In addition to Los Angeles, 16 others among the 25 cities with the worst ozone pollution each had a higher average of unhealthy days in 2015-2017, including some of the nation's largest metropolitan areas: New York City; Chicago; San Diego; Denver; Phoenix; Houston; Las Vegas; Philadelphia; Washington-Baltimore; and Salt Lake City. Many smaller cities on that list also suffered from more ozone: Visalia, CA; Sacramento, CA; El Centro, CA; Chico, CA; El Paso-Las Cruces, TX-NM; and Sheboygan, WI.
Increased heat in 2017, the third warmest year on record in the United States, likely drove this increase in ozone. All three years in this report mark the warmest years ever recorded.
Eight cities had fewer unhealthy ozone days on average in 2015-2017, including three that reached their fewest unhealthy days ever: Bakersfield, CA; Dallas-Fort Worth; and San Jose-San Francisco (with additional counties in the metro area). Other cities that had fewer high-ozone days included: Fresno-Madera-Hanford, CA; Atlanta, GA; Fort Collins, CO; Redding-Red Bluff, CA; and Hartford, CT.
These rankings are all based on unhealthy air days as recorded using the Air Quality Index adopted with the 2015 ozone national air quality standard. In 2018, EPA officially designated all or parts of the 25 most polluted cities as "nonattainment" for the 2015 ozone national air quality standard. That action requires all to take steps to clean up the sources of pollution going forward.
Regional Differences. Cities in the West and the Southwest continue to dominate the most-ozone-polluted list. California retains its historic distinction with 10 of the 25 most-polluted cities in that state. The Southwest continues to fill most of the remaining slots, with eight of the 25 cities. Texas has three cities in the 25 most-polluted list: Houston, Dallas-Fort Worth and El Paso. Colorado has two: Denver and Fort Collins. Arizona, Nevada and Utah each have one.
Only seven cities are east of the Mississippi River, including the New York City metro area, where Fairfield, CT suffers from the highest levels in the metropolitan area. Others in the Northeast and Mid-Atlantic in the 25 most-polluted list are Philadelphia; Washington, DC-Baltimore, MD; and Hartford, CT. The Midwest has two: Chicago and Sheboygan, WI. Atlanta remains the only southeast city to remain on the list.
The findings show the continued impact of transported pollution that moves ozone and ozone precursors across state lines. Chicago's ozone crosses 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 from other states.
Increased heat played a major role in the higher number of unhealthy air days. This year's report covers the three warmest years on record in the U.S. According to the National Oceanic and Atmospheric Administration, 2017 was the second warmest year on record, following 2016, which was the warmest year, and just ahead of 2015, which was the third warmest. Warmer weather makes ozone more likely to form.
Those changes reflect changes 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.