Ozone air pollution has reduced growth and damaged foliage of the tulip poplar.

Airborne nitrogen compounds, especially nitrogen oxides, contribute to the formation of air pollutants like smog, which can restrict visibility and affect human health. Ground-level ozone is the main component of smog. Ground-level ozone is not emitted directly into the air, but is created by chemical reactions between airborne nitrogen compounds and other compounds in the presence of sunlight. Sunlight and hot weather cause ground-level ozone to form in harmful amounts in the air. As a result, it is known as a summertime air pollutant. Many urban areas tend to have high levels of ground-level ozone, but increased ozone affects rural areas too, because wind can carry ozone and the pollutants from hundreds of miles away. People with lung disease, children, older adults, and people who are active can be affected when ozone levels are unhealthy. Breathing ozone can trigger a variety of health problems including chest pain, coughing, throat irritation, and congestion. It can worsen bronchitis, emphysema, and asthma. Under the Clean Air Act, EPA has set protective health-based standards for ozone in the air we breathe. EPA and states have implemented a variety of programs to meet these health-based standards.

Case Study: Reducing Nitrogen Oxide Emissions and Ozone Pollution Across the Eastern United States

Wind and weather can carry ground-level ozone hundreds of miles from urban areas to rural areas, where it can impact human health and vital ecosystems. Ground-level ozone can also strongly impact the health of many plants, including a variety of commercial and ecologically important forest tree species. Effects can include reduced growth and productivity, damaged foliage, and increased susceptibility to diseases, insects, pests, and other stresses.

To address the problem of ground-level ozone, EPA took steps to dramatically reduce air pollution across state boundaries and issued the Clean Air Interstate rule (CAIR) in 2005. CAIR addresses the problem of interstate transport of ozone and fine particle pollution or soot. CAIR requires certain states in the Eastern U.S. to limit annual emissions of nitrogen oxide and sulfur dioxide from power plants and some industrial facilities, which contribute to the formation of ground-level ozone and soot. Between 2005 and 2010, annual nitrogen oxide emissions from CAIR and acid rain program sources dropped 42 percent from the 2005 levels. During this same period, ozone season nitrogen oxide emissions from CAIR sources alone decreased by approximately 27 percent. This has translated into significant human health and environmental benefits. nitrogen oxide emissions reductions attributable to CAIR are expected to have between $305 million and $1.5 billion annually. A majority of this monetized benefit comes from premature deaths avoided — 10,000 to 26,000 annually due to reduction in fine particle pollution and 39 to 180 annually due to reduction in ozone.

In addition, since implementation of the nitrogen oxide budget program and CAIR, the number of forested areas studied with significant reduced growth due to ozone exposure was 16 percent for the period of 2007 to 2009, down from 37 percent for the period of 2000 to 2002 for seven tree species in the eastern United States that are sensitive to ground-level ozone: black cherry, yellow-poplar, sugar maple, eastern white pine, Virginia pine, red maple and quaking aspen. The results illustrate how EPA’s programs to reduce emissions of nitrogen oxide and other major pollutants from power plants and some industrial facilities are resulting in broad, regional-scale benefits to human health and the environment.