Nitrogen Dioxide

What is Nitrogen Dioxide?

Nitrogen dioxide , or NO2, is a gaseous air pollutant composed of nitrogen and oxygen.  NO2 forms when fossil fuels such as coal, oil, gas or diesel are burned at high temperatures.  NO2 also mixes in the outdoor air to form particle pollution and 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:

  • Increased inflammation of the airways
  • Worsened cough and wheezing
  • Reduced lung function
  • Increased asthma attacks
  • Greater likelihood of emergency department and hospital admissions
  • Increased susceptibility to respiratory infection, such as influenza [1]

What are the sources of nitrogen dioxide emissions?

Each year manmade sources in the U.S. emit 19.4 million metric tons of nitrogen oxides from sources, mainly from burning fuels.  On-road sources like 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.  Emissions of nitrogen dioxide are slated to decline as regulations on many of these sources are phased 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. [2]  Levels are highest on or near heavily travelled 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,  higher levels  of NO2 can build up indoors than outdoors. 

EPA Proposes  Short-Term National Outdoor Air Quality Standards for Nitrogen Dioxide and Roadside Monitoring Program

EPA must set national limits, called ambient (outdoor) air quality standards for nitrogen dioxide. Standards are the official limit on the amount of this pollutant considered safe to breathe. Recent research shows that the current annual average standard set in 1971 fails to protect the health of children, older adults and people with asthma and of people who live and work near major highways. [2]  In 2009, EPA proposed adding a new one-hour standard to the existing annual standard. EPA also has proposed establishing a roadside monitoring network to track expected higher levels of NO2 along major highways.  EPA must make a final decision on the standards by January 22, 2010.[3]

Fact sheet on the proposed standard

This downloadable fact sheet explains the background behind EPA's proposed tighter standard for nitrogen dioxides outdoors.

American Lung Association Comments on the EPA proposed nitrogen dioxide standard and monitoring proposal

The Lung Association filed these extensive comments on the proposed standard on September 14, 2009 with several environmental partners. 

Public Hearing Testimony

Mary Partridge, American Lung Association National Board Chair

Gwen Young, American Lung Association National Assembly Vice Chair

Janice E. Nolen, American Lung Association Staff


[1] U.S. Environmental Protection Agency. Integrated Science Assessment for Oxides of Nitrogen -- Health Criteria. EPA/600/R-08/071. July 2008. Available at:

[2] 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:

[3] U.S.EPA. Proposed Revisions for National Ambient Air Quality Standards for NO2. July 15, 2009. Available at