Colorado got mixed reviews for its air pollution in the American Lung Association’s 2018 Air Quality Report.

Denver and Fort Collins both made the top 25 list for most polluted cities in terms of ozone, coming in at numbers 14 and 19, respectively.On the flip side, Steamboat Springs, Glenwood Springs and the Pueblo area were recognized for their clean air.

A lot of factors contribute to ozone pollution, like car engines and emissions from power plants. But Janice Nolen, assistance vice president for the American Lung Association, says there’s something special here in Colorado and other western U.S. states.

“Oil and gas extraction often contributes to some of the raw ingredients that add to ozone,” Nolen says. “In the western part of the country we are seeing more places that have higher days of ozone where they have a lot more oil and gas extraction or processing than they did in years past.”

According to the report, which looks at data from 2014-2016, ozone pollution has gotten worse nationwide.

Both Denver and Fort Collins are doing better than they were in last year’s report. However, Nolen says Fort Collins overall is doing worse than when the first report was issued 19 years ago, which she attributes, in part, to climate change.

“The fact that 2016 was the second hottest year on record gave us an increase in ozone -- it made ozone more likely to form,” Nolen says.