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Ozone (O3)


Note: EPA no longer updates this information, but it may be useful as a reference or resource.

Please see www.epa.gov/airtrends for the latest information on Air Quality Trends.


Nature and Sources of the Pollutant: Ground-level ozone (the primary constituent of smog) is the most complex, difficult to control, and pervasive of the six principal air pollutants. Unlike other pollutants, ozone is not emitted directly into the air by specific sources. Ozone is created by sunlight acting on NOx and VOC in the air. There are thousands of types of sources of these gases. Some of the common sources include gasoline vapors, chemical solvents, combustion products of fuels, and consumer products. Emissions of NOx and VOC from motor vehicles and stationary sources can be carried hundreds of miles from their origins, and result in high ozone concentrations over very large regions.

Health and Environmental Effects: Scientific evidence indicates that ground-level ozone not only affects people with impaired respiratory systems (such as asthmatics), but healthy adults and children as well. Exposure to ozone for 6 to 7 hours, even at relatively low concentrations, significantly reduces lung function and induces respiratory inflammation in normal, healthy people during periods of moderate exercise. It can be accompanied by symptoms such as chest pain, coughing, nausea, and pulmonary congestion. Recent studies provide evidence of an association between elevated ozone levels and increases in hospital admissions for respiratory problems in several U.S. cities. Results from animal studies indicate that repeated exposure to high levels of ozone for several months or more can produce permanent structural damage in the lungs. EPA's health-based national air quality standard for ozone is currently set at 0.12 ppm (measured as the second daily 1-hour maximum concentration). Ozone is responsible for approximately 1 to 2 billion dollars of agricultural crop yield loss in the U.S. each year. Ozone also damages forest ecosystems in California and the eastern U.S. New scientific studies indicate that ozone causes adverse health and environmental effects at lower concentrations and longer periods of exposure than the current standards. As a result, EPA is reviewing whether revisions to the current ozone standard are warranted.

Ozone Concentrations
VOC Emissions

Trends in Ozone Levels: Trends in ozone concentrations are influenced by year-to-year changes in meteorological conditions as well as changes in emissions. National ozone concentrations in 1995 were 6 percent lower than those in 1986. However, between 1994 and 1995, national ozone concentrations increased 4 percent. Because of the hot, dry summer, meteorological conditions in 1995 were conducive to ozone formation, especially from the Midwest and Gulf States to the eastern U.S. Emissions of VOC (which contribute to ozone formation) decreased 9 percent between 1986 and 1995 and 2 percent between 1994 and 1995. Based on air quality monitoring data, over 70 million people lived in counties with air quality levels above EPA's health-based national air quality standard for ozone in 1995.

In 1994, EPA established a new monitoring network to gather further data on causes of ozone air pollution. This network of monitors, called Photochemical Assessment Monitoring Stations (PAMS), is located in ozone nonattainment areas of the U.S. which are classified as "serious" "severe," or "extreme." Concentration data were collected in 22 areas for ozone, NOx , and a variety of VOC (including several toxic air pollutants) that form ozone. The majority of the PAMS sites showed decreases in the monitored concentrations of toxic air pollutants and ozone-forming VOC.

This document is provided for historical purposes only. The most recent version can be found at AIRTrends


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