Nitrogen dioxide (NO2) is one of a group of highly reactive gases containing nitrogen and oxygen in varying amounts (known collectively as oxides of nitrogen, or NOx). Many of these gases are colorless and odorless. But one, nitrogen dioxide (NO2), often is seen along with particle pollution as a reddish-brown layer in the air over urban areas.


NOx 2005 Anthropogenic Emissions (649 TPSD)


Nitrogen oxide gases are produced by high-temperature fuel combustion. Primary sources of NOx emissions include motor vehicles, electric utilities and other industrial, commercial and residential sources that burn fuels. Nitrogen oxides react with other pollutants to form ground-level ozone.

Health Effects of Nitrogen Dioxide & Who is Most at Risk

Nitrogen dioxide irritates the nose and throat, especially in people with asthma, and appears to increase susceptibility to respiratory infections.

Ground-level ozone, which is formed when NO2 and other gases react in the presence of sunlight, also can:

  • Irritate the respiratory system and cause coughing, throat irritation and uncomfortable sensations in the chest.
  • Reduce lung function, leading to more rapid and shallow breathing that may limit a person's ability to engage in vigorous activities.
  • Heighten sensitivity to allergens such as pet dander, pollen and dust mites that commonly trigger asthma attacks, leading to more doctor and emergency room visits and greater use of medication.
  • Inflame the lung lining. Typically, damaged cells are shed and replaced much like skin peels after sunburn, but studies suggest that repeated inflammation over long periods of time can result in permanent scarring and loss of lung function.

When nitrogen dioxide and/or ground-level ozone reach unhealthy levels, children and people with respiratory disease are most at risk.

Environmental Effects of Nitrogen Dioxide

As a contributor to the formation of ground-level ozone, nitrogen dioxide can have adverse effects on both land and water ecosystems. Along with sulfur dioxide, nitrogen oxides in the air can significantly contribute to acid rain. NO2 and other NOx gases also can enter bodies of water and lead to eutrophication, a process whereby excess nitrogen causes explosive overgrowth of algae and weeds in lakes, estuaries and slow-moving streams. The "bloom" and subsequent decomposition of plants reduce water quality and dissolved oxygen in the water to stress or kill fish and other wildlife.

Nitrogen Dioxide Standards

The health-based primary National Ambient Air Quality Standard (NAAQS) for nitrogen dioxide in the air, established by the U.S. Environmental Protection Agency (EPA), is .053 parts per million averaged over the course of an entire year.

Long-Term Nitrogen Dioxide Trends

Based on decades of monitoring, Massachusetts has long met the National Ambient Air Quality Standard (NAAQS) for nitrogen dioxide. Levels of NO2 in our air are currently, and for many years have been, well below the health-based standard established by the U.S. Environmental Protection Agency (EPA).


NO2 Trends 1985-2007 Annual Arithmetic Means Standard = 0.05 ppm