Healthy Air

Another ozone season begins in Colorado

Starting May 31 each year in Colorado, Ozone Action Day alerts warn us the air may be unhealthy to breathe.

This image of an Ozone Action Day Alert shows the Denver skyline with smoggy skies in the background.
Colorado Department of Public Health & Environment | Used by permission
Smoggy skies over Colorado too often result in an Ozone Action Day Alert, meaning our air may be unhealthy to breathe.

What is Colorado’s ozone season?

May 31 marks the beginning of our state’s traditional ozone season. This is the time of year when abundant Colorado sunshine and warmer temperatures too often result in harmful levels of ozone pollution in the air. Ozone forms when oil and gas activity, cars and trucks, gas-powered lawn equipment and other sources emit nitrogen oxides (NOx) and volatile organic compounds (VOCs) that combine chemically to form ozone, a main component of what’s more commonly called “smog.”

Why is “ground-level” ozone bad?

When ozone molecules occur high up in our atmosphere — the “ozone layer” — they help form a barrier that protects the Earth from harmful radiation. But down on the Earth’s surface, it’s another story. Exposure to ground-level ozone can cause harmful health effects including lung damage, worsening of existing respiratory conditions such as asthma, and even cardiovascular disease.

In recent years, the Denver Metro/North Front Range region has suffered from some of the highest ozone pollution in the country. These high ozone levels mean the region is failing to meet the national health-based air quality standards for ozone, and in 2022, the EPA reclassified the region from “serious” to “severe” nonattainment of these standards.

What are Ozone Action Day alerts?

Our state’s air quality agencies issue Ozone Action Day alerts every day from May 31 to August 31 that ozone levels are expected to rise to unhealthy levels. Depending on how high the ozone levels will spike, sensitive groups such as elderly people and children will be advised to stay indoors. Sometimes, it gets so bad that the general public is advised the air is unhealthy to breathe. These air quality concerns not only affect our health, but go against our active, outdoor Colorado way of life.

You can find out about the state’s air quality alerts here and follow their Ozone Action Day alert tracking. In addition, the Regional Air Quality Council sends notices on ozone action alert days that are available by email or text. These notices provide information to protect your health as well as ways to reduce your personal impact on ozone pollution. Text “BetterAirCO” to 21000 to sign up for text alerts or visit Simple Steps. Better Air to sign up for emails.

To learn more about where ozone pollution comes from and what you can do to help clean up our air, visit

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