Cars head north toward downtown Denver on South Broadway on a smoggy day in a file photo. 

 (AP Photo/David Zalubowski)

A new report finds Colorado cities landed high on a list as having among the “most polluted” air in the country.

The American Lung Association placed Denver at eighth and Fort Collins at 10th in the country for poorest air quality regarding levels of ozone,  a colorless, unstable toxic gas.

The group’s Paul Billings says a complex chemistry of sources including vehicles, oil and gas operations, power plants and weather can all contribute to polluted air.

“The problem is not unique to Colorado, but it’s the combination of a large population and these polluting industries that all contribute to too many days of unhealthy air quality which leads to a wide range of health effects," Billings said.

Those can include conditions like asthma.

Billings says air here is actually getting cleaner, but not as rapidly as it is elsewhere.

The Colorado Department of Public Health and Environment says the state is making progress in reducing emissions, including adopting new oil and gas regulations.