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.

Learn more on EPA’s website.

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

There are two health-based primary National Ambient Air Quality Standards (NAAQS) for nitrogen dioxide:  100 parts per billion (ppb) as a 1-hour daily maximum and 53 ppb averaged over the course of a year.   These were established by the U.S. Environmental Protection Agency (EPA) in 2010 and 1971 respectively.  The secondary welfare-based standard is the same as the annual primary standard (53 ppb).  Monitored levels of NO2 in Massachusetts meet the NO2 standards and have been steadily decreasing.

For more information on nitrogen dioxide monitoring, standards, and trends see MassDEP’s Annual Air Quality Reports.