Nitrogen dioxide is the chemical compound with the formula NO2. It exists as a radical in nature. One of several nitrogen oxides, NO2 is an intermediate in the industrial synthesis of nitric acid, of which millions of tons are produced each year.
This reddish-brown toxic gas has a characteristic sharp, biting odor and is a prominent air pollutant. Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air.
Nitrogen dioxide is toxic by inhalation, but this could be avoided as the material is acrid and easily detected by our sense of smell. One potential source of exposure is fuming nitric acid, which spontaneously produces NO2 above 0 °C. Symptoms of poisoning (lung edema) tend to appear several hours after one has inhaled a low but potentially fatal dose.