What is nitrogen dioxide (NO₂)?

Nitrogen dioxide (NO₂) is a major contributor to the formation of smog and a precursor to many harmful secondary pollutants, including ozone and particulate matter. It’s highly reactive with other chemicals and is a strong oxidizing agent.

Nitrogen dioxide is a deep red-orange gas. When released into the air, it is seen as a reddish-brown haze. NO₂ has a pungent, acrid odor.¹

Where does nitrogen dioxide come from?

Nitrogen dioxide can be formed from both natural and human activity. Natural sources include:²

• lightning strikes
• volcanoes
• oceans
• biological decay

Combustion creates oxides of nitrogen, a major portion of which is nitrogen dioxide. When vehicles emit oxides of nitrogen, 90 to 95 percent of the emissions are nitric oxide (NO).

However, nitric oxide quickly oxidizes in outdoor air when reacting to oxygen, ozone, and volatile organic compounds (VOCs) to form nitrogen dioxide. The oxidization process occurs indoors, but a slower rate.³

Is nitrogen dioxide a primary or secondary pollutant?

Though nitrogen dioxide can be a primary pollutant, it’s mostly a concern as a secondary pollutant. As a primary pollutant, NO₂ is emitted in limited amounts through vehicles into the air.

Nitrogen dioxide is also a secondary pollutant because it can be formed through oxidization.Nitrogen dioxide further oxidizes into nitric acid (HNO₃), which can enter the environment through the ground as droplets or nitrate-containing particles.⁴

How does nitrogen dioxide affect our health?

Children, people with asthma, and adults with heart and respiratory disorders are most affected by nitrogen dioxide levels.⁵
High concentration levels of nitrogen dioxide can cause:

• coughing and wheezing
• lung irritation
• reduced lung function
• increased asthma attacks
• cardiovascular damage
• lower birth weight
• risk of premature death

A 1998 study published by the National Research Council Committee on Toxicology investigated incidents of accidental exposure to NO₂. Workers in agriculture, mining explosions, space exploration, and military activities have been accidentally exposed to high concentrations of NO₂, resulting in a wide range of severe medical ailments, including:⁶

• breathing difficulty
• fever
• bronchial pneumonia
• acute bronchitis
• death

How does nitrogen dioxide affect children’s health?

Children are more susceptible to the effects of nitrogen dioxide. For example, there are increased chances of respiratory infection for children through nitrogen dioxide inhalation.
A 2011 study published in the European Respiratory Journal found that indoor exposure to nitrogen dioxide was connected to increased asthma symptoms and reduced lung function in children.⁷

What are nitrogen dioxide’s environmental impacts?

When reacting to water, nitrogen dioxide can chemically transform into nitric acid and contribute to lake acidification and acid rain.⁸ Acid rain has been responsible for denuded forests across Europe and lake “deaths” in Canada and the United States as well as crop and health concerns in Asia. Acidity has even damaged buildings and limestone gravestones.⁹

Is nitrogen dioxide a greenhouse gas?

By itself, nitrogen dioxide is not a direct greenhouse gas. However, when combined with sunlight and VOCs, nitrogen dioxide can produce tropospheric or ground-level ozone. The presence of ground-level ozone can, in turn, result in atmospheric warming through radiation forcing, or absorbing more sunlight than is reflected.¹⁰

Ozone pollution can harm respiratory systems. Ground-level ozone’s health impacts include:¹¹

• throat irritation
• congestion
• chest pain
• coughing
• reduced lung function
• potential lung scar tissue
• more intense bronchitis, asthma, and emphysema

What makes nitrogen dioxide a significant pollutant?

Nitrogen dioxide is one of several oxides of nitrogen, including pollutants like nitric oxide (also known as nitrogen monoxide) (NO).¹² Nitrogen dioxide’s health effects are better understood than the other forms.

Because it’s closely tied to other oxides of nitrogen, nitrogen dioxide’s reduction will impact others as well.

Regulations have improved air quality in many parts of the world, but cities around the globe still struggle with the human and financial consequences of polluted air. See how our Cost of Air Pollution counter illustrates how clean air can save lives while lowering the economic burden of air pollution.

What can be done about nitrogen dioxide emissions?

Nitrogen dioxide can’t be removed from combustible fuels like coal, oil, or natural gas. Removal is only possible after combustion. Uncontrolled oil and coal produce more nitrogen than natural gas does, but all three do contain nitrogen.¹³

There are several methods to control nitrogen dioxide emissions, such as:

• using low NO_x burners in boilers
• setting policies that reduce the use of diesel transportation
• switching diesel fuel-burning vehicles to electric vehicles
• encouraging public transport, biking, and walking
• using an air purifier for gas filtration

The Joint Research Centre of the European Commission’s science and knowledge service estimates that reducing NOX-emissions from passenger diesel cars, trucks, and vans could improve NO_x emissions in urban centers by 40 percent on average.¹⁴ European cities whose annual NO_x emissions largely result from traffic emissions include:

• Athens
• London
• Madrid
• Marseille
• Milan
• Paris
• Prague
• Rome

Wherever you go, track air pollutants to monitor air quality data when NO₂ information is supplied.