These 14 Colorado Counties Received an ‘F’ Grade for Air Quality

The American Lung Association just released new data for 2025 that analyzed and crunched the numbers for air pollution in Colorado.

They looked at official government air quality reports and created grades based on the number of days that each county had unhealthy air.

What Causes Colorado’s Smog Problem?

This is shocking. A total of 14 Colorado counties received an F grade for high ozone days. This means that ozone pollution, also known as smog, is a major issue.

Get our free mobile app

Smog, the most obvious sign of air pollution, is caused by sunlight's reaction to nitrogen oxides and other pollutants in the air.

These 14 Colorado Counties Failed the Air Quality Test

These are the 14 Colorado counties that received an 'F' for air quality:

• Adams County
• Arapahoe County
• Boulder County
• Chaffee County
• Clear Creek County
• Denver County
• Douglas County
• El Paso County
• Garfield County
• Gilpin County
• Jefferson County
• Larimer County
• Park County
• Weld County

Only 23 Colorado counties provide data for air quality to the American Lung Association. Colorado has a total of 64 counties.

Archuleta and Mesa Counties Get Top Marks for Clean Air

Archuleta and Mesa counties are the only Colorado counties that received an 'A' rating.

READ MORE: Denver Rated Most Polluted City In U.S. For Consecutive Days

Denver and Boulder counties also received an 'F' for particle pollution. This is when there are solid and liquid particles in the air we breathe.

Denver-Aurora-Greeley Ranked Among Worst in the Nation for Pollution

The American Lung Association also came up with a ranking of the most polluted regions in the country. In the ranking, ALA paired the Denver metro and Northern Colorado. The Denver-Aurora-Greeley region is the sixth most polluted region in the entire country.

You can look at the full report here.