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Nitrogen oxide (NOx)

Nitrogen oxide (NOx) is a collective term for gaseous compounds, the most important of which are nitrogen monoxide (NO) and nitrogen dioxide (NO2). Together with hydrocarbons, nitrogen oxides are responsible for the formation of ozone. They also contribute to the formation of particulate matter.

Nitrogen oxides are formed during the combustion of coal, oil, gas, wood and waste. This means that wherever fossil fuels are used to generate energy for propulsion or heating, large quantities of nitrogen oxide are produced and released into the air. In cities, however, it is vehicles that are responsible for around half to two-thirds of the pollution. Of these, 80 per cent come from diesel engines. This also includes diesel engines from ships. In port cities like Hamburg, the port is responsible for about one third of the nitrogen oxide. 

The nitrogen damages humans, especially the respiratory tract, but also plants such as street trees, as they age prematurely and the leaves turn yellow, which then function less well as air filters. Nitrogen oxide also eventually gets into the soil. This leads to too much fertiliser and acidification of the soil.

Since the problem has been known for a long time, attempts have been made for years to reduce the amount of this pollutant. With success: between 1990 and 2015, nitrogen oxide emissions were reduced by 56 percent across Europe.

In Europe, a nitrogen dioxide limit value of 40 micrograms per cubic metre of air applies as an annual average. The WHO advocates lowering this value to only 10 micrograms. If the EU adopts this, it is only a matter of time before there are further diesel driving bans in many European cities.