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Ground Level Ozone - The Harmful Form of Ozone

Ground Level Ozone
(The "Bad" Ozone)

1. What is good ozone
2. The science of ozone depletion
3. The effects of ozone depletion
4. Ozone depletion graphics and images
5. FAQs about ozone depletion
6. Ozone Depletion Publications, References and Resources

Ozone (O₃) is a gas composed of three oxygen atoms. It is not usually emitted directly into the air, but at ground-level is created by a chemical reaction between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight. Ozone has the same chemical structure whether it occurs miles above the earth or at ground-level and can be "good" or "bad," depending on its location in the atmosphere.

In the earth's lower atmosphere, ground-level ozone is considered "bad." Motor vehicle exhaust and industrial emissions, gasoline vapors, and chemical solvents as well as natural sources emit NOx and VOC that help form ozone. Ground-level ozone is the primary constituent of smog. Sunlight and hot weather cause ground-level ozone to form in harmful concentrations in the air. As a result, it is known as a summertime air pollutant. Many urban areas tend to have high levels of "bad" ozone, but even rural areas are also subject to increased ozone levels because wind carries ozone and pollutants that form it hundreds of miles away from their original sources.

Ground-level or "bad" ozone is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight. Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC.

Breathing ozone can trigger a variety of health problems including chest pain, coughing, throat irritation, and congestion. It can worsen bronchitis, emphysema, and asthma. Ground-level ozone also can reduce lung function and inflame the linings of the lungs. Repeated exposure may permanently scar lung tissue.

Ground-level ozone also damages vegetation and ecosystems. In the United States alone, ozone is responsible for an estimated $500 million in reduced crop production each year.

"Good" ozone occurs naturally in the stratosphere approximately 10 to 30 miles above the earth's surface and forms a layer that protects life on earth from the sun's harmful rays.

Learn more about how ozone can be beneficial up high in the stratosphere but harmful at ground level.

For more information about ground-level ozone:

AirNow: Current ground level ozone levels where you live

Health and Environment - Effects of ground-level ozone pollution.

Ozone Standards - Links to technical information related to setting the national air quality standards for ground-level ozone.

Ozone Designations - Regional, state and local information related to ground-level ozone nonattainment.

Ozone Implementation - Contains information related to implementing the NAAQS for ground-level ozone.

Regulatory Actions - Links to proposed and final rules, fact sheets, and other rulemaking documents.

Air Quality Trends - Progress made in reducing ground-level ozone.

Early Action Compacts - Contains information about EPA's voluntary program to reduce ground-level ozone.

Publications - Publications related to ground-level ozone pollution.

Related Links - Other information related to ground-level ozone pollution.