What is an “Ozone Action Day” and what does it mean for Coloradans?

What exactly are “Ozone Action Day” alerts, and why do they seem to multiply in the heat?

So far in 2024, Colorado health officials have issued 15 Ozone Action Days in June, 22 Action Days in July and five Action Days in August, according to state data. That’s nearly a month and a half of days with poor air quality.

Ozone Action Days are called when the Air Quality Index is forecast to reach unhealthy levels due to a combination of ozone, wildfire smoke and other pollutants, according to the Colorado Department of Public Health and Environment.

The AQI measuring system reports daily levels of air pollution. Generally, AQIs of:

Zero to 50 indicate good air quality;
51 to 100 indicate moderate air quality;
101 to 150 indicate air quality that is unhealthy for sensitive groups;
151 to 200 indicate unhealthy air quality for the general population;
201 to 300 indicate very unhealthy air quality;
301 and above indicate hazardous air quality.

“When ozone occurs naturally in the atmosphere, miles above the Earth’s surface, it protects people from harmful ultraviolet solar radiation,” state officials said. “However, ground-level ozone is air pollution that can harm our health.”

Colorado can experience ozone pollution at any time, but the state most often sees pollution hit unhealthy levels during the summer, according to state officials. The unhealthy ground-level ozone forms when certain air pollutants — including nitrogen oxides and “volatile organic compounds” — bake in the heat and sunlight.

“In general, volatile organic compounds are air pollutants that you can smell,” state officials said. “Vehicles that run on gas, other equipment that runs on gas, and industry operations are all large sources.”

When Ozone Action Days are issued, people should limit the use of gas- and diesel-powered vehicles, lawnmowers and other equipment that can add to the air pollution already impacting an area, according to action day alerts.

Wildfire smoke can also contribute to the formation of ground-level ozone pollution, triggering Ozone Action Day alerts.

State officials said short-term exposure to unhealthy ozone levels can cause coughing; eye, nose and throat irritation; chest pain; difficulty breathing and asthma attacks. Long-term exposure has been linked to a variety of health issues, including lung and cardiovascular disease and premature death.

What should I do when an Ozone Action Day is called?

During action days, avoid rigorous outdoor activity during the heat of the day, state officials said. Prolonged exposure can cause long-lasting damage to your lungs.
- State officials said residents in affected areas should plan any outdoor exercise for before noon or after 8 p.m., when ozone pollution levels will be lower.
Avoid driving gas- or diesel-powered cars, state officials said. Take public transportation or carpool when possible to reduce emissions, and avoid letting your car idle.
- “Letting your car idle for longer than 10 seconds releases more emissions than simply restarting,” air quality officials said. “Avoid heavy traffic times, drive-thru lanes, and any other idling.”
Refuel vehicles after 8 p.m., when the hottest hours of the day have passed, AirNow officials said.
- When you pump gasoline into your car tank or a gas can, some can escape as fumes. On hot summer days, these emissions can react with sunlight and heat to form ground-level ozone.
Conserve electricity and set your air conditioner at a higher temperature, AirNow officials said.
- “Keep your home’s thermostat climate control setting on circulate mode to avoid introducing polluted air into your home,” according to National Jewish Health. “Consider using a High Efficiency Particulate Air filter in air conditioners, air cleaners and vacuums to trap pollutants.”