Most air quality alerts in Denver are caused by cars and industry. Thursday’s alert is different — and the distinction matters.
The Colorado Department of Public Health and Environment issued an Air Quality Health Advisory for the Denver metro area, Boulder and the northern Front Range not because of smog building in the afternoon heat, but because of a stratospheric ozone intrusion — a phenomenon where naturally occurring ozone from high in the atmosphere descends toward the Earth’s surface.
What’s Different About This Warning: It Came From Above
The National Weather Service in Boulder issued the advisory noting that “an intrusion of ozone from high up in the atmosphere to the surface is expected through at least Thursday afternoon.” The bulletin flagged it as an off-cycle event — meaning it did not follow the standard trajectory of rush-hour emissions baking in the midday sun. Instead, it arrived from above.
At 4 PM, CDPHE escalated the advisory to a full Ozone Action Day Alert covering the Front Range Urban Corridor from Douglas County north to Larimer and Weld counties, including the Denver-Boulder area, Fort Collins and Greeley — extended through at least 4 PM Friday, June 12.
A stratospheric intrusion occurs when ozone moves from high up in the atmosphere down to where we live. Most of the time, the air in the troposphere and the stratosphere do not mix. Under certain conditions — typically during and after cold front passages in spring and early summer — air can move in a way that pulls ozone down to the Earth’s surface. These events are difficult to forecast and typically only last a few hours, though ozone can remain elevated for up to a few days afterward.
Why Colorado Is Especially Vulnerable
“Colorado may experience stratospheric intrusions more than other parts of the country,” the Colorado Department of Public Health and Environment explains.
At 5,280 feet, Denver sits far closer to the stratosphere than coastal cities, so stratospheric air doesn’t have to travel as far to reach the surface. The Rocky Mountain terrain can also pull ozone-rich air downward, amplifying the effect.
This isn’t an isolated occurrence. The Denver metro area has been in EPA non-attainment for the 8-hour ozone standard for years, and June 2026 has seen multiple Ozone Action Days. On June 20, the AQI hit 112 at monitoring stations — firmly in the “Unhealthy for Sensitive Groups” range. On June 22, CDPHE issued another Ozone Action Day Alert through Tuesday June 23, warning of elevated ozone across the full Front Range.
Who Is Affected
The Thursday advisory covers Boulder, Jefferson, Denver, southern Weld, and western Douglas, Arapahoe and Adams counties — including Boulder, Golden, Denver, Castle Rock, Littleton and Brighton. The groups at greatest risk are active children and adults and people with lung disease, such as asthma, who should reduce prolonged or heavy outdoor exertion during the advisory window.
What to Do During an Ozone Action Day
Avoid strenuous outdoor activity between 10 AM and 8 PM. Move exercise and outdoor work to early morning when ozone is at its lowest. During an Ozone Action Day, outdoor burning is prohibited by Colorado law across the non-attainment area — a mandatory restriction.
For real-time ozone levels, check colorado.gov/airquality or the EPA’s AirNow app. The CDPHE advisory hotline is 303-758-4848. Track ongoing alerts at cdphe.colorado.gov.
