Ozone, unlike the other criteria pollutants, is not emitted directly into the air by any one source. Ground-level ozone is a secondary pollutant. It is formed through chemical reactions of other molecules already in the air, specifically nitrogen oxides (NOx) and volatile organic compounds (VOCs). Ground-level ozone, which exists in the atmosphere close to earth, is not the same as the "ozone layer" in the earth’s outer atmosphere (the stratosphere), where ozone helps to absorb ultraviolet radiation that would otherwise be harmful to organisms on Earth’s surface.

Sources of the NOx and VOCs that contribute to the formation of ground-level ozone include vehicles, lawn and garden equipment, paints and solvents, refueling stations, factories, and other activities that result in the burning of fossil fuels.


Health impacts of ground-level ozone

Ozone can irritate the eyes, nose and throat, and can aggravate asthma and other lung diseases, including bronchitis. Exposure to high levels of ground-level ozone can increase the risk of premature death in individuals already suffering from heart or lung disease. Children, whose lungs are still forming and many of whom spend a large amount of time outdoors, are at particular risk under high ozone concentrations.

For additional information about the health impacts of air pollution in Minnesota, you can check out our most recent air quality report, or visit the Air quality and health webpage. You can also sign up for air quality alerts and forecasts, and check out current air quality.

Air quality standards

The Clean Air Act regulates ozone as a criteria pollutant. The U.S. EPA sets National Ambient Air Quality Standards (NAAQS) for ozone, including both primary standards to protect public health and secondary standards to protect the environment. In 2015, the EPA reviewed the science related to the health and environmental impacts of ozone and revised the NAAQS to reflect the most up-to-date information. The level of both the current primary and secondary eight-hour standards for ozone is 0.070 ppm (70 ppb).

The state of Minnesota is currently in compliance with the national standards for ozone. Ozone is a pollutant of particular concern to Minnesota because levels in the state are relatively close to the national standards. To see the MPCA’s monitoring data for lead and other criteria pollutants, explore our Criteria Pollutant Data Explorer.

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