Nitrogen Dioxide (NO2)
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: Nitrogen dioxide is a light brown gas that can become an important component of urban haze. Nitrogen oxides (NOx) usually enter the air as the result of high-temperature combustion processes, such as those occurring in automobiles and power plants. Nitrogen dioxide plays an important role in the atmospheric reactions that generate ozone. Home heaters and gas stoves also produce substantial amounts of NO2.
Health and Environmental Effects: Nitrogen dioxide can irritate the lungs and lower resistance to respiratory infections such as influenza. The effects of short-term exposure are still unclear, but continued or frequent exposure to concentrations higher than those normally found in the ambient air may cause increased incidences of acute respiratory disease in children. Nitrogen oxides are an important precursor to both ozone and acid rain and can affect both terrestrial and aquatic ecosystems. The regional transport and deposition of nitrogenous compounds arising from emissions of NOx are a potentially significant contributor to such environmental effects as the growth of algae and subsequent unhealthy or toxic conditions for fish in the Chesapeake Bay and other estuaries. In some parts of the western U.S., NOx have a significant impact on particulate matter.
Trends in NO2 Levels: Over the past ten years, ambient NO2 concentrations decreased 10 percent. However, total emissions of NOx increased 3 percent, due primarily to increased emissions from coal-fired electric utilities. Between 1996 and 1996, national average annual mean NO2 ambient concentrations remain unchanged, while total NOx emissions decreased 2 percent. Between 1987 and 1996, NOx emissions from highway vehicles decreased 6 percent.