The chemical compound nitrogen dioxide (NO₂) is a red or orange/brown gas with a characteristic sharp, biting odor. It is considered an insidious deadly poison by inhalation.

Nitric oxide (NO) spontaneously produces the dioxide when exposed to air:

2 NO + O₂ → 2NO₂

It is also generated by various nitric acid reactions, e.g. when metals, such as copper or silver, are exposed to nitric acid.

It reacts readily with water, even at normal temperature and pressure, where it disproportionates to nitric and nitrous acid. That reaction is used in the Ostwald process, which in turn allows the industrial production of fertilizers and explosives.

Red fuming nitric acid has a certain percentage of dissolved nitrogen dioxide, hence its red color.

Nitrogen dioxide is in constant equilibrium with Nitrogen tetroxide

2NO₂ <-> N₂O₄

This reaction is pushed towards Nitrogen tetroxide as temperatures go down. Nitrogen dioxide rarely exists as a liquid or solid because almost all of it will be converted to Nitrogen tetroxide.

Nitrogen dioxide is one of the most prominent air pollutants. Long-term exposure to concentration levels above 40-100 microgram/m³ causes adverse health effects (http://www.euro.who.int/document/e79097.pdf). From a health point of view, the most important source of nitrogen dioxide is road traffic, emitting nitrogen oxides very near people and causing levels of concern in streets and cities, including exceedances of European limit values. The picture below, depicting results of satelite measurements, illustrates nitrogen dioxide as large scale pollutant, with rural background ground level concentrations in some areas around 30 microgram/m³, not far below unhealthy levels. Nitrogen dioxide plays a crucial role in atmospheric chemistry, including the formation of tropospheric ozone.