Nitrogen dioxide is the chemical compound NO₂. It is one of the several nitrogen oxides. This reddish-brown gas has a characteristic sharp, biting odor. NO₂ is one of the most prominent air pollutants and a poison by inhalation.

Safety and pollution considerations

Nitrogen dioxide is toxic by inhalation. Symptoms of poisoning (lung edema) tend to appear several hours after one has inhaled a low but potentially fatal dose. Also, low concentrations (4 ppm) will anesthetize the nose, thus creating a potential for overexposure.

Long-term exposure to NO₂ at concentrations above 40–100 µg/m³ causes adverse health effects [1]. The most important source of NO₂ is internal combustion engines, which emit nitrogen oxides near people. A major industrial source is pulp mills.

The map shown below, depicting results of satellite measurements over Europe, illustrates nitrogen dioxide as large scale pollutant, with rural background ground level concentrations in some areas around 30 µg/m³, not far below unhealthful levels. Nitrogen dioxide plays a role in atmospheric chemistry, including the formation of tropospheric ozone. A recent study by researchers at the University of California, San Diego, suggests a link between NO₂ levels and Sudden Infant Death Syndrome [2].  

See also

• Nitryl
• Nitrous oxide or N₂O, "laughing gas", a linear molecule, isoelectronic with CO₂ but with a nonsymmetric arrangement of atoms (NNO)
• Nitric oxide or NO, a problematic pollutant, related to CO but with one additional electron.
• NO_x = all of the above in unspecified proportions but tending toward NO₂.

More esoteric nitrogen oxides include N₂O₅ and the blue species N₂O₃.

Oxidized (cationic) and reduced (anionic) derivatives of many of these oxides exist: nitrite (NO₂⁻), nitrate (NO₃⁻), nitronium or NO₂⁺, and nitrosonium or NO⁺. NO₂ is intermediate between nitrite and nitronium:

NO₂⁺ + e⁻ → NO₂

NO₂ + e⁻ → NO₂⁻

• Birkeland-Eyde process

References