Nitrogen oxides are a group of gaseous chemicals. The two most relevant nitrogen oxides are nitrogen dioxide and nitric oxide, which are often collectively referred to as NO_x. NO_x are brown gases that are formed in a variety of processes. They are released from the combustion of fossil fuels, such as the burning of coal, oil, natural gas, and the use of motor vehicles. They can also be formed during metalworking activities like welding and electroplating. NO_x can also be intentionally formed for use in producing other chemicals, dyes, and explosives.

Once released into the air, NO_x can travel long distances from their original source, impacting human health and the environment across large geographic areas. Nitrogen dioxide can react with other chemicals in the environment to produce nitric acid, which is one of the main components of acid rain. NO_x can also react with other chemicals in the environment to produce ozone, which contributes to the greenhouse effect and planetary warming.

Breathing NO_x can have adverse health effects. People who work in or live near power plants and highways are the most likely to be exposed to outdoor air contaminated with NO_x. Using wood-burning fireplaces, kerosene heaters, or gas stoves can also expose people to NO_x in the air in their homes. NO_x can also be found in tobacco smoke. Breathing low levels of NO_x can cause irritation of the eyes, nose, throat, and lungs. This can lead to shortness of breath, fatigue, nausea, and buildup of fluid in the lungs. Exposure can worsen asthma and might cause asthma if the exposure lasts for a long time. Breathing high levels of NO_x can cause muscle spasms, tissue swelling, reduced flow of oxygen in the body, and even death. There is some evidence that exposure is also associated with heart disease and diabetes. In studies of laboratory animals, NO_x exposure during pregnancy caused defects in development of the fetus. The variety of short- and long-term health effects of NO_x exposure, as well as the serious environmental impacts, makes clear that NO_x pose significant danger to people and the planet.