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Ground-level Ozone

Frequently Asked Questions

General

Health and Ecosystems

Designations Process


General

What is ozone?
Ozone is a gas composed of three atoms of oxygen. Ozone occurs both in the Earth's upper atmosphere and at ground level. Ozone can be good or bad, depending on where it is found.

    Good Ozone
    Good ozone occurs naturally in the upper atmosphere, 6 to 30 miles above the Earth's surface, where it forms a protective layer that shields us from the sun's harmful ultraviolet rays. This beneficial ozone is gradually being destroyed by manmade chemicals. When the protective ozone "layer" has been significantly depleted; for example, over the North or South Pole; it is sometimes called a "hole in the ozone."

    Bad Ozone
    Troposheric, or ground level ozone, is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC).  Ozone is likely to reach unhealthy levels on hot sunny days in urban environments. Ozone can also be transported long distances by wind.  For this reason, even rural areas can experience high ozone levels. 

High ozone concentrations have also been observed in cold months, where a few high elevation areas in the Western U.S. with high levels of local VOC and NOx emissions have formed ozone when snow is on the ground and temperatures are near or below freezing. Ozone contributes to what we typically experience as "smog" or haze, which still occurs most frequently in the summertime, but can occur throughout the year in some southern and mountain regions.

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Where does ground level ozone come from?
In the Earth's lower atmosphere, near ground level, ozone is formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources chemically react in the presence of sunlight. Ozone at ground level is a harmful air pollutant.

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Are ozone and smog the same thing?
While the two terms are often used interchangeably for general use, smog is more complex. Smog is primarily made up of ground level ozone combined with other gases and particle pollution.

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What are the ozone levels in my community?
Air quality forecasts are often given with weather forecasts on handheld devices, online or in the paper or television. You can check ozone levels and other daily air quality information by visiting www.airnow.gov and in many areas you can receive air quality notifications through www.enviroflash.info .

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What can I do to reduce ozone?
Air pollution can affect your health and the environment.  There are actions every one of us can take to reduce air pollution and keep the air cleaner and precautionary measures you can take to protect your health.  Visit AIRNow for more information.

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Health and Ecosystems

What are the health effects of ozone?
Ozone in the air we breathe can harm our health. Even relatively low levels of ozone can cause health effects.  Children, people with lung disease, older adults, and people who are active outdoors, including outdoor workers, may be particularly sensitive to ozone. 

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.
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Who is most at risk?
Children are at greatest risk from exposure to ozone because their lungs are still developing and they are more likely to be active outdoors when ozone levels are high, which increases their exposure.  Children are also more likely than adults to have asthma.
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What are the ecological effects of ozone?
Ozone also affects sensitive vegetation and ecosystems, including forests, parks, wildlife refuges and wilderness areas.  In particular, ozone harms sensitive vegetation, including forest trees and plants during the growing season. Visit the ecosystems page for more information.

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Designations Process

What does designation mean?
After working with the states and tribes and considering the information from air quality monitors, EPA "designates" an area as attainment or nonattainment with national ambient air quality standards. If an area is designated as nonattainment states must develop and implement control plans to reduce ozone-forming pollution.

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What does nonattainment mean?
The Clean Air Act identifies six common air pollutants that are found all over the United States. These pollutants can injure health, harm the environment and cause property damage. EPA calls these pollutants criteria air pollutants because the agency has developed health-based criteria (science-based guidelines) as the basis for setting permissible levels in the outdoor air.

Ozone is a criteria pollutant. There are national ambient air quality standards (NAAQS) for each of the criteria pollutants. These standards apply to the concentration of a pollutant in outdoor air. If the air quality in a geographic area meets or does better than the national standard, it is called an attainment area; areas that don't meet the national standard are called nonattainment areas.

In order to improve air quality, states must draft a plan known as a state implementation plan (SIP) to improve the air quality in nonattainment areas. The plan outlines the measures that the state will take in order to improve air quality. Once a nonattainment area meets the standards, EPA will designate the area to attainment as a "maintenance area."

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How long has ozone been a problem in my area?
Ozone levels can vary from one area to the next and they can also vary over time or from one season to the next. Ozone is typically a summertime problem, but can be a year-round issue for some areas. Some areas have experienced problems with ozone for years, while other areas have not. To learn how long ozone has been a problem where you live, visit EPA’s Air Trends web site.

For more questions and answers related to Ozone, visit AIRNow

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