Air that’s drawn from the atmosphere into an engine contains almost 80% Nitrogen. Under the high temperatures and pressure of combustion, this nitrogen combines with oxygen to produce oxides of nitrogen. Almost all internal combustion engine exhaust gases contains these chemicals. They are more likely to be produced when high peak temperatures occur during combustion.
If a lean mixture is used, formation of hydrocarbons and carbon monoxide is reduced, but for oxides of nitrogen, it is increased. This is due to the high temperature, and the increase in available oxygen.
Oxides of nitrogen are claimed to be major contributors to photo-chemical smog.
Compression-ignition engines can produce high levels of oxides of nitrogen.
Oxides of nitrogen irritate the eyes, nose and throat. In extreme cases, coughing and lung damage can occur.