Another example is that, over the three-year period from 2015 through 2017 that the Lung Association counted in the latest report, Salt Lake County received an "F" grade for an annual average of 21 days of high ozone.
The report lists "Fresno-Madera-Hanford" as the most polluted for year-round particle pollution.
"Denver residents should be aware that we're breathing unhealthy air, placing our health and lives at risk", JoAnna Strother, regional director of public policy for the Lung Association, said in a statement.
The report also found Denver had fewer days when short-term particle pollution reached unhealthy levels.
Compared to last year, the report says Denver experienced more unhealthy days of high ozone, however Denver saw lower levels of particle pollution throughout the year.
California claimed seven of the top 10 most ozone-polluted cities, with Los Angeles topping the list for the 19th time.
For the second year in a row, Eastern Carolina cities have been ranked among the cleanest in the state when it comes to air quality. Dukes, Plymouth, Suffolk, and Worcester counties all received C's and D's, lower grades than past year.
Climate change is likely to make ozone pollution worse in the future, by making the hot temperatures necessary to create smog more common. This year's report covers the most recent quality-assured data available collected by states, cities, counties, tribes and federal agencies in 2015-2017.
But with the Trump administration's plan to slash the EPA's budget by a third, the American Lung Association worries that things could plateau, or even get worse. "We have much cleaner vehicles, power plants are doing better, but a lot of this is challenged by the current administration's rollbacks and with climate change, because we have such extreme weather patterns of drought and wildfire and that can add to particle pollution", Nolen said.