Denver is under an Ozone Action Day Alert that began Friday afternoon and remains in effect until at least 4 p.m. Saturday, July 12, 2025. This alert covers the entire Front Range Urban Corridor, including Denver, Boulder, Fort Collins, and Greeley. Recent measurements show that ozone levels have climbed into the Unhealthy for Sensitive Groups category, with an Air Quality Index of 104 recorded Friday evening. This means that while the general public isn’t likely to be affected, people with respiratory conditions, children, older adults, and those who are unusually sensitive to air pollution should limit prolonged or heavy outdoor exertion.

Particulate matter, or PM2.5, has also been measured at moderate levels, with an AQI of 54. For most people, air quality remains acceptable; however, sensitive individuals might experience mild respiratory symptoms and should consider reducing outdoor activities, especially if they have underlying heart or lung disease.

Unsettled weather earlier in the week kept ozone concentrations in the Good to Moderate range, but hot and stagnant conditions are now allowing ozone to build up. Action Days like today are called when air pollution is either already unhealthy or is likely to worsen. This triggers voluntary pollution prevention measures and public health recommendations.

If you see or smell smoke, especially from wildfires in southwestern Colorado, remain indoors and keep windows closed, particularly if you are sensitive to air pollution. The Colorado Department of Public Health and Environment advises everyone, but especially vulnerable groups, to monitor symptoms and avoid unnecessary exposure during periods of higher pollution.

For most Denver residents, outdoor activities are still safe, but it is recommended to stay informed on hourly updates and advisories as conditions can change quickly. Those in sensitive groups should pay particular attention and adjust outdoor plans accordingly to minimize health risks.