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. Nitrogen dioxide forms when fossil fuels such as coal, oil, methane gas (natural 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 for which there are national air quality standards to limit their levels in the outdoor air. NO2 can also form indoors when fuels like wood or gas are burned.

What Are the Health Effects of Nitrogen Dioxide Pollution?

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.

Scientific evidence suggests that exposure to NO2 could likely cause asthma in children.

A 2022 review of multiple studies found that elevated levels of NO2, as well as elevated particulate matter and sulfur dioxide, were strongly associated with heart and lung harm, affected pregnancy and birth outcomes, and were likely associated with increased risk of kidney and neurological harm, autoimmune disorders and cancer.

What Are the Sources of Nitrogen Dioxide Emissions?

As of 2020, human-made sources in the U.S. emit 7.64 million short tons of nitrogen oxides per year (down from 15 million short tons per year in 2011) mainly from burning fuels. Trucks, buses, and cars are the largest sources of NO2 emissions, followed by diesel-powered non-road equipment, industrial processes such as oil and gas production, industrial boilers and other movable engines, and coal-fired power plants. 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. Levels are higher on or near heavily traveled roadways.

It is important to note that NO2 and other nitrogen oxides are also produced from burning natural gas (methane), both outdoors and indoors. Outdoors, this can include gas-fired power plants and from facilities that extract, process or transport oil and gas if they burn it in flares or to power equipment. Indoors, appliances such as stoves, dryers and space heaters that burn natural gas, liquified petroleum gas (or LPG, which includes propane and butane) and kerosene can produce substantial amounts of nitrogen dioxide. If those appliances are not fully vented to the outside, levels of NO2 can build up to unhealthy levels indoors.

Who Is at Risk?

While everyone is at risk from health impacts of nitrogen dioxide pollution, those that live near the emission sources are at higher risk. Other vulnerable subpopulations at higher risk from nitrogen dioxide exposure include:

  • Individuals who are pregnant;
  • Infants, children and teens;
  • Older adults (>65 years of age);
  • People with pre-existing medical conditions such as asthma, chronic obstructive pulmonary disease (COPD), cardiovascular disease, diabetes, lung cancer
  • Current or former smokers;
  • People with low socioeconomic status; and
  • People of color.

What Can We Do about It?

The good news is that for much of the nation, the outdoor air has much lower levels of nitrogen dioxide now than in previous decades. Under the federal Clean Air Act, more protective standards nationwide have helped drive down nitrogen dioxide emissions. Power plants, industrial sites and on-road vehicles are cleaner than they used to be, which has driven nationwide improvement in air quality. However, far too many people still breathe in unhealthy levels of nitrogen dioxide pollution.

Individuals can take steps to protect themselves on days with unhealthy levels of air pollutants and also ask policymakers at all levels of government to continue to require cleanup of air pollution.

Page last updated: October 26, 2023

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