What is ground-level ozone pollution?

Ground-level ozone pollution is different from the ozone layer of the atmosphere that many learn about in school. When ozone occurs naturally in the atmosphere, miles above the Earth’s surface, it protects people from harmful ultraviolet solar radiation. However, ground-level ozone is air pollution that can harm our health.

How and when does ground-level ozone pollution form?

Colorado can experience higher ozone pollution levels any time, but most frequently during summer months. This is because ground-level ozone pollution forms when certain air pollutants, known as “ozone precursors,” react in heat and sunlight. Ozone precursors include nitrogen oxides and volatile organic compounds. In general, volatile organic compounds are air pollutants that you can smell. Vehicles that run on gas, other equipment that runs on gas, and industry operations are all large sources of ozone precursors in Colorado. Wildfires are also more prevalent during summer, and under certain conditions, wildfire smoke can contribute to the formation of ground-level ozone pollution. This is related to the small particles in wildfire smoke.

What are the potential health effects of ozone pollution?

Whether a person will experience health impacts from ozone pollution depends on how much is in the air and how long they breathe it in. Short-term exposure to unhealthy levels of ozone pollution can cause symptoms, such as:

• Coughing.
• Eye, nose, and throat irritation.
• Chest pains.
• Breathing difficulty.
• More frequent asthma attacks.

Long-term exposure is linked to a variety of poor health outcomes, including lung and cardiovascular disease and premature death.

Who might experience health effects of ozone pollution?

Anyone who spends time outdoors when ozone pollution levels are high may be at risk. People most at risk of health impacts from breathing ozone pollution include:

• People with asthma and other lung diseases.
• Children.
• Older adults.
• Those who are pregnant.
• Adults who are active outdoors, especially outdoor workers.
• People with reduced intake of certain nutrients, such as vitamins C and E.

How can I limit exposure to ozone pollution?

The state health department acknowledges that people cannot always make changes to their daily lives and schedules. When ozone pollution levels are high, you can reduce your exposure by:

• Limiting time spent outdoors.
• Scheduling outdoor activities for mornings or late evenings, when ozone pollution levels are usually lower.
• Substituting less strenuous activities, such as walking instead of running.

How can I stay informed on current ozone pollution levels?

The state health department's Air Pollution Control Division issues daily ozone pollution forecasts for Colorado's north Front Range from May 31 - August 31 each year. If forecasts show that day or the next may exceed National Ambient Air Quality Standards, the email will say an “Ozone Action Day Alert” is in effect. If forecasts do not show elevated ozone pollution levels, the email will say there is no alert in effect. The division sends these daily emails so Coloradans have the necessary information to plan ahead and make informed decisions.

To receive daily email forecasts, subscribe for the ozone.frontrange list. The division forecasts several air pollutants year-round and offers other email alerts as well. During summer, the division emphasizes Ozone Action Days.

For alternative methods to receive alerts, you can:

• Follow the Air Pollution Control Division on Facebook and Twitter.
• Call the advisory hotline at 303-758-4848.
• Subscribe through another accredited organization. For example, the division shares ozone pollution forecasts with the National Weather Service and the Regional Air Quality Council.

The division also shares air quality information online in near real-time, including current Denver metro/northern Front Range air quality advisories and current ozone pollution levels.

How many Ozone Action Days has Colorado experienced this year?

The division tracks Ozone Action Days in a public spreadsheet. The spreadsheet includes Ozone Action Days by date for the current year and tabs for years past.

You may notice that the spreadsheet sometimes shows several Ozone Action Days in a row. This can happen for many reasons. For example, Colorado's topography can trap pollutants for days at a time in some areas along the north Front Range.

Why are there more Ozone Action Days now?

If you have been following Colorado’s Ozone Action Days, you may have noticed that we had more in recent years. This is because in 2015, the U.S. Environmental Protection Agency lowered the standard for concentrations of ground-level ozone pollution from 75 parts per billion to 70 parts per billion. Since then, the division has issued an Ozone Action Day Alert in accordance with the updated National Ambient Air Quality Standard. 

The increased frequency of Ozone Action Days does not mean that the air quality in Colorado is getting worse. It simply means federal air quality standards are becoming more protective. Since the 1990 Clean Air Act Amendments, the EPA has slowly lowered the ozone standard from 85 parts per billion to 70 parts per billion. These updates happened as air quality scientists studied ozone pollution and learned more. The state health department fully supports the EPA adjusting its standards in accordance with new information and the best available science.

Despite challenges associated with increasing population, Colorado has made progress reducing ozone pollution. The state's air quality has slowly improved over time, but not quickly enough to meet the EPA's updated standards.

Can I help reduce ground-level ozone pollution?

The division is working hard to reduce ozone pollution, and you can get involved in developing more measures. The state health department acknowledges that people cannot always make changes to their daily lives and schedules, but Coloradans can also play a role in reducing air pollution. For example, you can:

• Walk or bike to your destination when the air is healthy.
• Combine errands.
• Carpool.
• Use public transportation.
• Make sure your gas-powered vehicle is properly maintained. A well-maintained vehicle generates less air pollution.
• Choose electric vehicles and lawn equipment.
• Use electricity from clean energy sources.
• Recycle. It conserves energy and helps reduce air pollution.

Learn more about how you can help protect air quality through the Regional Air Quality Council's ‘Simple Steps. Better Air.’ program or get tips from the U.S. Environmental Protection Agency's AirNow.

Where can I find more information?

The U.S. Environmental Protection Agency's website includes detailed information on ground-level ozone pollution.

If you are concerned about how ozone pollution may be affecting you, we encourage you to see your doctor and talk to your local public health agency.