Ozone pollution has improved in Denver and Fort Collins, but both cities remain among the 15 worst in the nation, the American Lung Association said.

Denver had the 11th-worst ozone levels and Fort Collins had the 15th-worst, the association said in its 2017 clean air report released Tuesday.

The report, which ranked the 25 most polluted and 25 least polluted cities in the country, did not mention Greeley. The report gave Weld County an F for the number of high ozone days, making Weld one of 10 Colorado counties on the Front Range or in nearby foothills to earn a D or an F.

Denver and the northern Front Range have long struggled to meet federal ozone standards.

Ozone can be harmful to people with respiratory problems and other vulnerable groups, including children and the elderly. People who are active outdoors can also suffer.

The American Lung Association said Colorado’s ozone problem has multiple causes, including vehicle exhaust, the oil and gas industry, trees, coal-burning power plants and weather conditions. Some of Colorado’s ozone drifts in from out of state.

Read more at The Greeley Tribune.