DENVER (AP) -- The American Lung Association says ozone pollution has improved in both Denver and Fort Collins, but both are still among the 15 worst in the nation.
The Association's 2017 clean air report released late Tuesday says Denver had the 11th-worst ozone levels and Fort Collins had the 15th-worst.
The report gives 12 Colorado counties a grade of D or F for the number of high ozone days. Ten of them are on the Front Range or in nearby foothills.
Colorado air quality officials say they're still reviewing the report but they raised questions about some of the American Lung Association's methods.
Ozone can be harmful to people with respiratory problems, children, the elderly and people who are active outdoors.