What Is Nitrogen Dioxide?

Nitrogen dioxide, or NO2, is a gaseous air pollutant composed of nitrogen and oxygen and is one of a group of related gases called nitrogen oxides, or NOx. NO2 forms when fossil fuels such as coal, oil, gas or diesel are burned at high temperatures. NO2 and other nitrogen oxides in the outdoor air contribute to particle pollution and to the chemical reactions that make ozone. It is one of six widespread air pollutants that have national air quality standards to limit them in the outdoor air. NO2 can also form indoors when fossil fuels like wood or natural gas are burned.

What Are the Health Effects?

Nitrogen dioxide causes a range of harmful effects on the lungs, including:

  • Increased inflammation of the airways;
  • Worsened cough and wheezing;
  • Reduced lung function;
  • Increased asthma attacks; and
  • Greater likelihood of emergency department and hospital admissions.1

New research warns that NO2 is likely to be a cause of asthma in children.2

A large new study found evidence that people with lung cancer faced greater risk from NO2, ozone, and other outdoor air pollutants. The 2016 study tracked the air pollution levels from 1988 to 2011 experienced by more than 350,000 cancer patients in California. The researchers found that exposure to these air pollutants shortened their survival.3

Looking beyond the lungs, newer research has linked NO2 to cardiovascular harm, lower birth weight in newborns and increased risk of premature death.4

What Are the Sources of Nitrogen Dioxide Emissions?

Cars, trucks, and buses are the largest sources of emissions, followed by power plants, diesel-powered heavy construction equipment and other movable engines, and industrial boilers. Man-made sources in the U.S. emitted 14 million metric tons of nitrogen oxides, mainly from burning fuels, in 2011.5 Emissions of nitrogen dioxide will decline as cleanup of many of these sources continue in future years.

Where Do High NO2 Concentrations Occur?

Monitors show the highest concentrations of outdoor NO2 in large urban regions such as the Northeast corridor, Chicago and Los Angeles.6 Levels are higher on or near heavily traveled roadways.

NO2 can be a problem indoors, as well. Kerosene or gas space heaters and gas stoves also produce substantial amounts of nitrogen dioxide. If those heaters or stoves are not vented fully to the outside, levels of NO2 can build up indoors.

  1. U.S. Environmental Protection Agency. Integrated Science Assessment for Oxides of Nitrogen -- Health Criteria. EPA/600/R-15/068. January 2016. Available at: https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=310879

  2. U.S. EPA, 2016.

  3. Eckel SP, Cockburn M, Shu Y-H, et al. F. Air pollution affects lung cancer survival. Thorax. 2016: 71: 891-898.

  4. U.S. EPA, 2016.

  5. U.S. EPA. Air Emissions Sources: Nitrogen Oxides, National Summary of Nitrogen Oxides Emissions, 2011. Available at https://www.epa.gov/air-emissions-inventories/2011-national-emissions-inventory-nei-data

  6. U.S. EPA. Risk and Exposure Assessment to Support the Review of the NO2 Primary National Ambient Air Quality Standard. EPA-452/R-08-008a, November 2008. Available at: http://www.epa.gov/ttn/naaqs/standards/nox/data/20081121_NO2_REA_final.pdf

Page last updated: February 12, 2020

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