A majority of Coloradans will be under an ozone alert on Friday as public health officials issued an air quality warning that covers most of the Front Range.
Ozone concentrations will make prolonged outdoor exposure unhealthy for people who are sensitive to pollution, including those with asthma or lung disease, according to the Colorado Department of Public Health and Environment. People should avoid prolonged or heavy outdoor exertion between noon and 8 p.m. Friday.
The ozone alert applies to the Interstate 25 corridor from Douglas County to Larimer and Weld counties, including Denver, Boulder, Fort Collins and Greeley.
Ozone at ground level is a pollutant caused by chemical reactions between emissions from cars, power plants and industrial factories, according to the U.S. Environmental Protection Agency. Ozone can trigger a variety of health problems, like coughing, restricted breathing and make the lungs more susceptible to infection.
“During Ozone Action Alerts, avoid rigorous outdoor activity during the heat of the day,” CDPHE officials said in the alert. “Prolonged exposure can cause long-lasting damage to your lungs.”
Smoke from wildfires in the southwest is also expected to cloud the Front Range on Friday and reduce air quality.
Denver’s skyline was already covered in haze at 8:30 a.m. Friday, obscuring views of the Rocky Mountains.
Hazy skies are visible across the Front Range today due to wildfires burning in the Southwest. Smoke aloft is expected to descend on Friday, increasing smoke near the surface & reducing visibility & air quality. Go to https://t.co/DHfbMTLUVB for updated air quality alerts. #cowx pic.twitter.com/HFGdDScfGK
— NWS Boulder (@NWSBoulder) July 31, 2025