NOx is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. [such as nitric oxide (NO) and nitrogen dioxide (NO2)].

Many of the nitrogen oxides are colorless and odorless. However, one common pollutant, nitrogen dioxide (NO2) along with particles in the air can often be seen as a reddish-brown layer over many urban areas.

Nitrogen oxides form when fuel is burned [...]. The primary sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels.

In the atmosphere, nitrogen oxides can contribute to formation of photochemical ozone (smog) and have health consequences. They also lead to acid rain and contribute to global warming.