Nitrogen dioxide (NO2) is a chemical compound with the formula NO2. It is a reddish-brown gas with a pungent, acrid odor above 21.2 °C (70.2 °F; 294.3 K) and becomes a yellowish-brown liquid below 21.2 °C (70.2 °F) . NO2 is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year for use primarily in the production of fertilizers. NO2 is favored at higher temperatures, while at lower temperatures, N2O4 predominates. NO2 is paramagnetic due to its unpaired electron, while N2O4 is diamagnetic. NO2 is a good oxidizer due to the weakness of the N–O bond, and it will combust, sometimes explosively, in the presence of hydrocarbons. NO2 reacts with water to give nitric acid and nitrous acid.

NO2 is used as an intermediate in the manufacturing of nitric acid, as a nitrating agent in the manufacturing of chemical explosives, as a polymerization inhibitor for acrylates, as a flour bleaching agent, and as a room temperature sterilization agent. It is also used as an oxidizer in rocket fuel, for example in red fuming nitric acid. NO2 is a highly poisonous gas that can cause inflammation of the airways at high concentrations. Breathing air with a high concentration of NO2 can irritate airways in the human respiratory system, aggravate respiratory diseases, particularly asthma, leading to respiratory symptoms (such as coughing, wheezing or difficulty breathing), hospital admissions and visits to emergency rooms. Longer exposures to elevated concentrations of NO2 may contribute to the development of asthma and potentially increase susceptibility to respiratory infections. NO2 also contributes to the production of small particles (PM) and ozone at ground level.

In summary, NO2 is a chemical compound that is used in the manufacturing of nitric acid, chemical explosives, and rocket fuel. It is also a gaseous air pollutant that can cause inflammation of the airways and aggravate respiratory diseases, particularly asthma.