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Home > The ABC of air > Pollutants
Nitrogen oxides (NOx)

The sources

The combination of nitrogen and oxygen in the air leads to chemical compounds of various forms grouped under the term NOx. Regularly measured, nitric oxide (NO) and nitrogen dioxide (NO2) are emitted during combustion. NO2 is derived from the oxidation of NO. The main sources are transportation (road, sea and river transport), industry, agriculture and power production.NO2 also occurs inside premises where gas appliances such as gas cookers, water heaters, etc. operate.

The origins of the majority of the NO2 are from major industrial complexes (Gonfreville, Harfleur, Oudalle, Gravenchon, Lillebonne, Grand-Quevilly, Petit-Couronne, Grand-Couronne, …) and from major cities (Caen, Le Havre, Rouen, Evreux).

The majority of the NO2 comes from the industrial sector (45 %) and transportation (44 %). The emissions from sea transport represent 8 % of transportation.

Health effects

NO2 is an irritant gas irritant for the airways. In asthmatics, it increases the frequency and severity of crises. In children, it makes lung infections more likely. 

Effects on the environment

NO2 participates in the phenomena of acid rain, the formation of ground level ozone where it is one of the precursors, attacks on the stratospheric ozone layer, and participates in the greenhouse effect.

The reference standards

Information and recommendation threshold for sensitive individuals Alert warning threshold for the general population
200 µg/m3 hourly average 400 µg/m3 hourly average
Limit values for the protection of human health
hourly average annual average
200 µg/m3 hourly average not to be exceeded more than 18 hours per year 40 µg/m3 
Limits for the protection of vegetation
30 µg/m3 annual average for the sum of NO and NO2