Nitrogen dioxide (NO2) is a highly reactive gas, and is representative of a group of other highly reactive gases known collectively as nitrogen oxides (NOx). These gases, especially nitrogen dioxide, are products of vehicle, power plant, and off-road equipment emissions caused by fuels burning at high heat.

Nitrogen dioxide, when prevalent in the air, appears as a reddish-brown haze. Nitrogen dioxide and other nitrogen oxides react with other chemicals in the air to form other pollutants, known as secondary pollutants. These secondary pollutants include ozone, particulate matter, acid rain and other toxic chemicals.


Nitrogen dioxide is a gas composed of one nitrogen atom and two oxygen atoms (NO2) in each molecule. It is one of a group of gases known as oxides of nitrogen (NOx). Other oxides of nitrogen include nitric oxide (NO), nitrous oxide, nitrous acid, and nitric acid. NO is primarily produced in fuel combustion processes from nitrogen and oxygen in the atmosphere. NO is then converted to NO2 either within the exhaust system of the combustion device or in the atmosphere. It is emitted from cars, trucks, other vehicles or machinery that burn fuel, and from power plants and other equipment involving fuel combustion.

Health Effects of Nitrogen Dioxide

Breathing air with a high concentration of NO2 can irritate airways in the human respiratory system. Such exposures over short periods can aggravate respiratory diseases, particularly asthma, leading to respiratory symptoms (such as coughing, wheezing or difficulty breathing). Longer exposures to elevated concentrations of NO2 may contribute to the development of asthma and potentially increase susceptibility to respiratory infections. People with asthma, as well as children and the elderly are generally at greater risk for the health effects of NO2.

Also, NO2 and other oxides of nitrogen react with other chemicals, including volatile organic compounds, in the air to form both particulate matter and ozone. Both of these are also harmful when inhaled due to effects on the respiratory system.

Air Quality Standards and Monitoring

The primary National Ambient Air Quality Standards (NAAQS), based on health effects, for NO2 are 100 parts per billion (ppb) averaged over one hour (98th percentile of 1-hour daily maximum concentrations, averaged over 3 years) and 53 ppb averaged over one year.

Missouri monitors NO2 concentrations in the air at locations across the state. Three of these sites are near roadways with heavy traffic, two in the St. Louis area and one in Kansas City. Click on the Monitoring tab to learn more about these air monitoring sites. Click on the Data tab to learn more about the data collected from this sites.

More information is available on EPA's Nitrogen Dioxide webpage.


Nitrogen Dioxide

Nitrogen dioxide is a foul-smelling gas. It comes primarily from the burning of fossil fuels — coal, oil and gas. NO2combines with volatile organic compounds (VOCs) in the presence of sunlight and heat to create ozone. (VOCs are compounds with carbon that easily become vapors or gases.)

In 2010, EPA revised the one-hour NO2 standard to 100 parts per billion. An area is in compliance if the design value is at or below the standard. The one-hour standard is based on a three-year design value, which is calculated by taking the 98th percentile of the daily high one-hour average concentrations recorded each year, for three years, and averaging the three years together. The annual standard of 0.053 parts per million is based on the annual arithmetic mean and is not to be exceeded.

Departmental information about nitrogen dioxide (NO2)

  • 2010 standard and related documents

Nitrogen Dioxide Monitoring Network