Nitrogen oxides are a class of compounds made of nitrogen and oxygen in varying percentages.
Nitrogen dioxide, one of six pollutants tracked nationally, is included in the family of nitrogen oxides. NOx are emitted from high temperature combustion processes. Sources include motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels. In 1997, over 23 million tons of nitrogen oxides were emitted into the air in the United States. An important characteristic of NOx emissions is that they can be transported long distances and cause problems far from the original emissions source.