But what that actually means isn’t always clear.
Ground-level ozone is different than the ozone layer in the atmosphere that protects Earth. In this case, ozone is a pollutant that forms when other pollutants, such as volatile organic compounds and nitrogen oxide, mix with heat and sun rays. Therefore, summer is the time of year when ozone alerts are common.
The Environmental Protection Agency sets National Ambient Air Quality Standards for a range of pollutants, including ozone. For ozone, the threshold is 70 parts per billion, so when ground-level ozone surpasses that level (or is projected to surpass it the next day), the Colorado Air Pollution Control Division issues an ozone action alert.
Under some conditions, wildlife smoke particles can contribute to the formation of ground-level ozone, but it isn’t the main driver of such alerts. Smoggy days obstructing views of the Rocky Mountains often correlate with ozone alerts, too, but ozone is a colorless gas.
Denver experiences ozone problems because the city is situated in a geographical bowl between the Rockies and the plains. Ozone precursor pollutants get trapped in that bowl, waiting to get cooked into ozone by the sun.
The iPhone weather app shows the air quality in areas, but ozone is only part of that measurement. There are days in Denver when there is an official ozone alert by the state Department of Public Health and Environment, but our phones still show that the air quality is deemed above average. Air Quality Index measures the amount of ozone, particulate pollution, carbon monoxide, sulfur dioxide and nitrogen dioxide in the air, so ozone by itself isn't always enough for the AQI to reach orange or red status — but that doesn't mean the air is pure and clean.
Short-term exposure to excess ozone can cause coughing, chest pain, difficulty breathing, asthma attacks and eye, nose and throat irritation. Long-term exposure to ozone has been linked to lung and cardiovascular disease, and even premature death, according to the CDPHE.
Children, older adults and people who are pregnant or suffering from lung diseases are most at risk from ozone, as well as people spending time outdoors and those lacking in vitamins C and E.
Denver High in Ozone
Denver ranks sixth on the American Lung Association's list of most polluted cities for ozone in 2024.There have already been 42 ozone action alert days in 2024, including every day since July 28. That’s more than the 38 ozone action alert days in 2023.
“One of the major reasons for this year’s uptick is the change in meteorological conditions this year compared to last year,” Air Pollution Control Division spokesman Zachary Aedo says. “Ground-level ozone forms in hot and dry conditions. In 2023, June through mid-July were unusually cool and wet, whereas in 2024, Colorado has experienced above-average temperatures and below-average precipitation.”
As Colorado experiences several wildfires near the Front Range, wildfire smoke has contributed to increased ozone pollution concentrations, Aedo adds. The APCD launched a Colorado Smoke Blog in July for those who want to track smokey conditions in the state.
The division also shares current ozone pollution levels online.
What to Do on Ozone Action Alert Days
The APCD issues daily air quality alerts via email and shares regular updates about ozone on social media from May 31 to August 31. If forecasts show today or tomorrow could exceed the 70 parts per billion standard, messages will notify people that it’s an ozone action day.According to state health officials, less is more on ozone action alert days.
"Although we understand that people cannot always make changes to their daily lives and schedules, these alerts can help Coloradans make informed decisions when planning outdoor activities or considering adjustments to help reduce ozone pollution," Aedo explains.
The APCD suggests limiting time spent outdoors when possible on ozone action days. If it’s not possible to limit those activities, try to schedule them for mornings or late evenings, when ozone pollution is generally lower. People can also swap in less strenuous activities like walking instead of running in order to cut down on the health risks, the division suggests.
Nitrogen oxide is one of the top ozone precursors and is emitted from gas-powered vehicles, so cutting back on automobile travel by carpooling, walking or biking, using public transportation or combining errands can also help on ozone alert days. According to the APCD, keeping gas-powered vehicles properly maintained, using electric vehicles or lawn equipment instead of gas versions, and recycling can help curb ozone, too.
In addition to transportation, oil and gas activities are a major cause of ozone, the Colorado Air Quality Control Commission has a plan to bring Colorado’s Front Range in compliance with federal standards for air pollution. Under the plan, new regulations on oil and gas emissions and reformulated gasoline will be rolled out over the next several years.
On ozone action days, no burning is allowed within areas that don’t meet air quality standards, even if projects have obtained open burn or smoke management permits.
