what is nitrogen dioxide?

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

A recent study by researchers at the University of California, San Diego, suggests a link between NO2 levels and Sudden Infant Death Syndrome

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.

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. The most important source of NO2 is internal combustion engines, which emit nitrogen oxides near people.

A kind of pollutant that pollute our atmosphere.
It is produced by manufacturing, burning rubbish by incinerators etc.