The American Lung Association says it's getting harder to keep the air we breathe healthy.
Association Vice President for National Policy Janice Nolan says climate change is a big factor.
"As we're getting to a change in climate, as we're getting regular temperatures that are higher and warmer, it's going to make it harder for us to battle ozone," she said.
In its 15th annual State of the Air report, the American Lung Association said 22 of the 25 most ozone-polluted cities in this year's report, including Los Angeles, had more high ozone days on average when compared to last year's report.
On the bright side, 13 of the 25 cities with the worst year-round particle pollution reached their lowest levels yet, including Los Angeles and Bakersfield.
Particle pollution recently was found to cause lung cancer.