Jump to main content.


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 belongs to a family of highly reactive gases called nitrogen oxides (NOx). These gases form when fuel is burned at high temperatures, and come principally from motor vehicle exhaust and stationary sources such as electric utilities and industrial boilers. A suffocating, brownish gas, nitrogen dioxide is a strong oxidizing agent that reacts in the air to form corrosive nitric acid, as well as toxic organic nitrates. It also plays a major role in the atmospheric reactions that produce ground-level ozone (or smog).

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 that are typically much higher than those normally found in the ambient air may cause increased incidence of acute respiratory illness in children. EPA's health-based national air quality standard for NO2 is 0.053 ppm (measured as an annual arithmetic mean concentration). Nitrogen oxides contribute to ozone formation and can have adverse effects on both terrestrial and aquatic ecosystems. Nitrogen oxides in the air can significantly contribute to a number of environmental effects such as acid rain and eutrophication in coastal waters like the Chesapeake Bay. Eutrophication occurs when a body of water suffers an increase in nutrients that leads to a reduction in the amount of oxygen in the water, producing an environment that is destructive to fish and other animal life.

Trends in Nitrogen Dioxide Levels: Nationally, annual NO2 concentrations remained relatively constant throughout the 1980's, followed by decreasing concentrations in the 1990's. Average NO2 concentrations in 1995 were 14 percent lower than the average concentrations recorded in 1986. The two primary sources of the NOx emissions in 1995 were fuel combustion (46 percent) and transportation (49 percent). Between 1986 and 1995, emissions from fuel combustion decreased 6 percent, and emissions from highway vehicles decreased 2 percent. Overall, national total NOx emissions decreased 3 percent. Additionally, 1995 is the fourth year in a row that all monitoring locations across the nation, including Los Angeles, met the Federal NO2 air quality standard.

Nitrogen Dioxide Concentrations

Nitrogen Dioxide Emissions

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


Local Navigation


Jump to main content.