Nitrogen oxides (NOx)

Other names

Nitric oxides


  • include, among others, nitrogen monoxide (NO) and nitrogen dioxide (NO2)
  • total emissions as nitrogen dioxide-equivalents
  • nitrogen monoxide: colourless gas
  • nitrogen monoxide: turns into nitrogen dioxide in the atmosphere
  • nitrogen dioxide: reddish in higher concentrations

Main sources

  • motorised road transport (combustion processes, at high temperatures)
  • boilers

Thresholds for the reporting requirement of facilities in accordance with Annex 2 PRTRO

(Ordinance on the Register relating to Pollutant Release and the Transfer of Waste and of Pollutants in Waste Water)

  • Air               100,000 kg/year
  • Water           -
  • Soil               -


  • respiratory illnesses
  • damage plants and sensitive ecosystems in multiple ways through the combined impacts of a number of pollutants
  • over-fertilisation of ecosystems
  • nitrogen dioxide: irritates eyes, respiratory system and skin
  • nitric oxides: major precursor in the formation of acid precipitation and secondary aerosols (particulate matter)
  • nitric oxides with volatile organic compounds: important precursors to the formation of photochemical oxidants such as ozone

Ambient limit values of nitrogen dioxide

30 µg/m3 annual mean (arithmetic mean)
100 µg/m3 95% of the 1/2-h-means of a year of less than or equal to 100 µg/m3
80 µg/m³ 24-hour mean; may be exceeded once per year at most

Status and changes

Nitric oxides are major air pollutants. Nitrogen dioxide pollution in the air has decreased significantly since 1990. The ambient limit values of the annual mean of nitrogen dioxide are sometimes greatly exceeded in large cities and along heavily travelled roads. In suburban areas, pollution levels are around the limit value or less. In rural areas beyond roads, pollution levels are well below the limit value. A potential cause of the smaller decrease in nitrogen dioxide concentrations is the increase in the proportion of passenger vehicles with diesel engines. They emit more nitrogen dioxide than automobiles with petrol engines. The measures taken to date are insufficient to reach the air pollution targets (ambient limit values of ozone).

    Further information

    Last modification 04.08.2020

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