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) has remained a pervasive pollution problem throughout many areas of the U.S. Ozone is not emitted directly into the air but is formed by the reaction of VOCs and NOx in the presence of heat and sunlight. Ground-level ozone forms readily in the atmosphere, usually during hot summer weather. VOCs are emitted from a variety of sources, including motor vehicles, chemical plants, refineries, factories, consumer and commercial products, and other industrial sources. Nitrogen oxides are emitted from motor vehicles, power plants, and other sources of combustion. Changing weather patterns contribute to yearly differences in ozone concentrations from city to city. Ozone and the precursor pollutants that cause ozone also can be transported into an area from pollution sources found hundreds of miles upwind.
Health and Environmental Effects: Ozone occurs naturally in the stratosphere and provides a protective layer high above the earth. At ground-level, however, it is the prime ingredient of smog. Short-term exposures (1 to 3 hours) to ambient ozone concentrations have been linked to increased hospital admissions and emergency room visits for respiratory causes. Repeated exposures to ozone can make people more susceptible to respiratory infection and lung inflammation, and can aggravate preexisting respiratory diseases such as asthma. Other health effects attributed to short-term exposures to ozone, generally while individuals are engaged in moderate or heavy exertion, include significant decreases in lung function and increased respiratory symptoms such as chest pain and cough. Children active outdoors during the summer when ozone levels are at their highest are most at risk of experiencing such effects. Other at-risk groups include outdoor workers, individuals with preexisting respiratory disease such as asthma and chronic obstructive lung disease, and individuals who are unusually responsive to ozone. Recent studies have attributed these same health effects to prolonged exposures (6 to 8 hours) to relatively low ozone levels during periods of moderate exertion. In addition, long-term exposures to ozone present the possibility of irreversible changes in the lungs which could lead to premature aging of the lungs and/or chronic respiratory illnesses.
The recently completed review of the ozone standard also highlighted concerns associated with ozone effects on vegetation for which the 1-hour ozone standard did not provide adequate protection. These effects include reduction in agricultural and commercial forest yields, reduced growth and decreased survivability of tree seedlings, increased tree and plant susceptibility to disease, pests, and other environmental stresses, and potential long-term effects on forests and ecosystems. Because ground-level ozone interferes with the ability of the plant to produce and store food, plants become more susceptible to disease, insect attack, harsh weather and other environmental stresses. In long-lived species, these effects may only become evident after several years or even decades. Ozone also damages the foliage of trees and other plants, decreasing the natural beauty of our national parks and recreation areas, and reducing the quality of the habitat for wildlife, including endangered species.
The New Ozone Standard: In 1997, EPA revised the national ambient air quality standards for ozone. After a lengthy scientific review process EPA issued a rule that replaced the 1-hour ozone 0.12 ppm standard with a new 8-hour 0.08 ppm standard to better protect public health and the environment. Although areas that do not meet the new 8-hour standard will not be designated "nonattainment" until the year 2000, EPA can begin to track trends in 8-hour levels of ozone. Nationally, 8-hour levels of ozone have decreased 11 percent over the past ten years.
Trends in Ozone Levels: Ambient ozone trends are influenced by year-to-year changes in meteorological conditions, population growth, loadings of VOC and NOx in the atmosphere, and by changes in emissions from ongoing measures. Between 1987 and 1996, ambient ozone concentrations decreased 15 percent, and the estimated number of accedences of the 1-hour ozone standards decreased 73 percent. Between 1996 and 1996, ambient ozone concentrations decreased 6 percent, while the estimated number of accedences of the 1-hour ozone standards decreased 37 percent.
In order to address ozone pollution, EPA has traditionally focused its control strategies on reducing emissions of VOC in nonattainment areas. However, EPA and the states have recognized a need for an aggressive program to reduce regional emissions of NOx. In 1998 EPA expects to issue a rule that will significantly reduce emissions of NOx in 22 eastern states, and, in turn, reduce the regional transport of ozone. National trends in emissions of NOx and VOC underscore the importance of this new approach. Volatile organic compound emissions decreased 7 percent between 1996 and 1996, while NOx emissions decreased 2 percent. Between 1987 and 1996, emissions of VOCs have decreased 18 percent whereas emissions of NOx have increased 3 percent. Further, between 1970 and 1996 emissions of VOCs have decreased 38 percent whereas emissions of NOx have increased 8 percent and NOx emissions from coal-fired power plants has increased 42%.
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 were and are classified under the Clean Air Act as "serious" "severe," or "extreme" for the 1-hour ozone standard. 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 between 1994 and 1996.