What Is Ozone?
Ozone is a molecule of three oxygen atoms. Ozone attacks lung tissue by reacting chemically with it. Learn more about ozone.
Twenty of the 25 cities with the worst ozone pollution reduced the number of high ozone days they experienced each year, improving over the previous report. Twelve experienced their best ozone seasons ever during 2013-2015, reaching their lowest weighted average number of days of unhealthy levels each year.
Los Angeles remains at the top of this list as it has for all but one of the 18 reports. Los Angeles also continues its success at cleaning up ozone, dropping its average number of unhealthy days to its lowest level ever.
Also experiencing their fewest high ozone days on average were ten other cities among the 25 most polluted by ozone. They include Bakersfield, CA; Visalia-Porterville-Hanford, CA; Modesto-Merced, CA; Sacramento, CA; New York-Newark; Las Vegas, NV; Dallas-Fort Worth; El Centro, CA; San Jose-San Francisco; Philadelphia; and Chico, CA
Nine others improved, though not reaching their lowest level: Fresno-Madera, CA; Phoenix, Az.; Denver-Aurora, CO; El Centro, CA; Fort Collins, CO; El Paso-Las Cruces, TX-NM; San Antonio; Hartford, CT; and Sheboygan, WI. One city – Redding-Red Bluff, CA – had the same number of unhealthy ozone days on average in this year’s report.
Los Angeles improved over last year and again had its best ozone report in the history of the State of the Air.
Four cities suffered more high ozone days on average than in last year’s report: San Diego; #12 Houston; Salt Lake City; and Baton Rouge, LA.
These comparisons are all based on the Air Quality Index adopted with the 2015 ozone standard. Although EPA has yet to designate any places for cleanup based on that standard, it remains the current official national ambient air quality standard.
Regional differences. Cities in the West and Southwest continue to dominate the most ozone-polluted list. California retains its historic challenge with seven of the ten most polluted metropolitan areas in that state and 11 of the worst 25. California’s weather and geography complicate the strong effort the state continues to make to reduce emissions. The Southwest continues to fill most of the remaining slots, with nine of the 25 most ozone polluted cities. Texas has four cities in the 25 most-polluted list: Houston, Dallas-Fort Worth, El Paso and San Antonio. Colorado has two: Denver and Fort Collins. Arizona, Nevada and Utah each have one. The Northeast also has three metro areas on the list, two of which cover parts of multiple states: New York City, Philadelphia, and Hanford, CT. The Midwest has only Sheboygan, WI in the 25 most-polluted list. The only southern city to remain on the list is Baton Rouge, LA.
Those changes reflect changes seen in the past two reports, where increased oil and gas extraction especially 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.