The term ‘oxides of nitrogen’ (also known as NOx) refers to a group of compounds each containing one or more atoms of oxygen and one of more atoms of nitrogen.
They are unique amongst exhaust gas pollutants in that they do not derive from the fuel supplied to the engine. Instead they are formed as oxygen and nitrogen (which are both present in the intake air) react with each other in the very high temperature and pressure environment of the combustion chamber.
For this reason formation is greater under high-load conditions, and diesel engines are bigger producers than spark-ignition engines because of the higher compression ratios they employ.
By far the most common of the NOx group is nitric oxide (NO) which is fortunately one of the least toxic.
More of a concern is nitrogen dioxide (NO2) which affects the respiratory tract and lungs, and can have especially pronounced effects on asthmatics.
As far as the wider environment is concerned, nitrogen dioxide reacts with hydrocarbons in the presence of sunlight to create ozone and photochemical smog, and is also involved in the formation of acid rain.
Unfortunately nitric oxide can be converted to nitrogen dioxide by some emission control systems and this is a problem which needs to be taken into account when selecting a system.