Nitrogen oxides, including NO and NO2, are produced as a result of combustion in a car engine. Pure combustion takes hydrocarbons (gasoline) and pure oxygen (O2) and produces CO2 and H2O. But pure combustion only occurs under highly controlled conditions. In reality, Earth's atmosphere is 80% nitrogen (N2). So the air going into a car engine has mostly nitrogen, therefore, nitrogen oxides are going to be produced as a by product of combustion. Nitrogen oxides are air pollutants and are one of the components of smog in major cities. They are polluting in that they are strong oxidizers and can also cause oxygen radicals to form in the air. Between acting as oxidants and radical initiators, nitrogen oxides are bad to breath and also cause other produce other pollutants through chemical reactions. Catalytic converters in car exhaust systems help reduce the amount of nitrogen oxides emitted in a car's exhaust.