What Is Nitrogen Oxide Pollution?

NOx pollution contributes to unhealthy smog over cities.
NOx pollution contributes to unhealthy smog over cities. Cultura RM/Justin Borucki/Getty Images

NOx pollution occurs when nitrogen oxides are released as a gas into the atmosphere during high-temperature combustion of fossil fuels. Nitrogen oxides consist mainly of two molecules, nitric oxide (NO) and nitrogen dioxide (NO2). Other nitrogen based molecules are also considered NOx, but occur in much lower concentrations. A closely related molecule, nitrous oxide (N2O), is a significant greenhouse gas playing a role in global climate change.

What Are the Environmental Concerns Associated with NOx?

NOx gases play an important role in the formation of smog, producing the brown haze often observed over cities, particularly during the summer. When exposed to the UV rays in sunlight, NOx molecules break apart and form ozone (O3). The problem is made worse by the presence in the atmosphere of volatile organic compounds (VOC), which also interact with NOx to form dangerous molecules. Ozone at the ground level is a serious pollutant, unlike the protective ozone layer much higher up in the stratosphere.

Nitrogen oxides, nitric acid, and ozone can all readily enter the lungs, where they create serious damage to the delicate lung tissue. Even short-term exposure can irritate the lungs of healthy people. For those with medical conditions like asthma, a short time spent breathing these pollutants has been shown to increase the risks of an emergency room visit or a hospital stay.

Approximately 16% of houses and apartments in the United States are within 300 feet of a major road, increasing exposure to hazardous NOx and their derivatives. For these residents, and in particular the very young and elderly, this air pollution can lead to respiratory diseases such as emphysema and bronchitis.

NOx pollution can also worsen asthma and heart disease, and is tied to elevated risks of premature death.

More environmental problems are caused by NOx pollution. In the presence of rain, nitrogen oxides form nitric acid, contributing to the acid rain problem. Additionally, NOx deposition in the oceans provides phytoplankton with nutrients, worsening the problem of red tides and other harmful algae blooms.

Where Does NOx Pollution Come From?

Nitrogen oxides form when oxygen and nitrogen from the air interact during a high temperature combustion event. These conditions occur in car engines and fossil fuel-powered electricity plants.

Diesel engines, in particular, produce large amounts of nitrogen oxides. This is due to the combustion features characteristic of this type of engine, including their high operating pressures and temperatures compared to gasoline engines. In addition, diesel engines allow excess oxygen to exit the cylinders, diminishing the effectiveness of catalytic converters, which in gasoline engines prevent the release of most NOx gases.

What Role Does NOx Pollution Play in the Volkswagen Diesel Scandal?

Volkswagen has for a long time marketed diesel engines for most vehicles in their fleet.

These small diesel engines provide ample power and impressive fuel economy. Concerns over their nitrogen oxide emissions were appeased as the little Volkswagen diesel engines met the stringent requirements policed by the U.S. Environmental Protection Agency and the California Air Resources Board. Somehow, few other car companies seemed to be able to design and produce their own powerful, but thrifty and clean diesel engines. It soon became clear why, when in September 2015 the EPA revealed that VW had been cheating the emissions tests. The automaker had programmed its engines to recognize testing conditions and react by automatically operating under parameters that produce very low amounts of nitrogen oxides. When normally driven, however, these cars produce 10 to 40 times the maximum allowable limit.


EPA. Nitrogen Dioxide – Health.

EPA. Nitrogen Dioxide (NOx) – Why and How They Are Controlled.

This article was written with assistance from Geoffrey Bowers, Professor of Chemistry at Alfred University, and author of the book Understanding Chemistry Through Cars (CRC Press).