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Ground-level Ozone

Basic Information

Ozone is found in two regions of the Earth's atmosphere – at ground level and in the upper regions of the atmosphere.  Both types of ozone have the same chemical composition (O3). While upper atmospheric ozone protects the earth from the sun's harmful rays, ground level ozone is the main component of smog. 

Tropospheric, 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.

Ground level ozone- what we breathe- can harm our health. Even relatively low levels of ozone can cause health effects.  People with lung disease, children, older adults, and people who are active outdoors may be particularly sensitive to ozone. 

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.

Ozone also affects sensitive vegetation and ecosystems, including forests, parks, wildlife refuges and wilderness areas.  In particular, ozone harms sensitive vegetation, including trees and plants during the growing season. 

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.

Under the Clean Air Act, EPA has established health and environmentally protective standards for ozone in the air we breathe. EPA and others have instituted a variety of multi-faceted programs to meet these standards. Learn more about EPA's ozone standards and regulatory actions.

Throughout the country, additional programs are being put into place to cut NOx and VOC emissions from vehicles, industrial facilities, and electric utilities. Programs are also aimed at reducing pollution by reformulating fuels and consumer/commercial products, such as paints and chemical solvents that contain VOC.

Voluntary and innovative programs encourage communities to adopt practices, such as carpooling, to reduce harmful emissions.

Learn more about how you can reduce ozone in and around your home.

The AIRNow Web site at www.airnow.gov provides daily air quality reports for many areas. These reports use the Air Quality Index (or AQI) to tell you how clean or polluted the air is.

EnviroFlash, a free service, can alert you via email when your local air quality is a concern. Sign up at www.enviroflash.info.

Learn more about Stratospheric, or "good" ozone.

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