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Evolution of the nitrogen dioxide concentrations in the air in the Brussels Region

Forty percent of the NO2 concentrations measured in the air in Brussels is believed to originate from outside the Region and 47% is reportedly caused by traffic. The proximity of the principal sources of emission, such as traffic, consequently affects the average readings. In approximately a third of the Brussels' monitoring sites it is therefore impossible to comply with the European limit value for the annual average.


Nitrogen dioxide harms the environment (it contributes to the formation of ozone and secondary particles, and to acidification) and affects human health (impact on the airways). The concentration in the atmosphere is linked to the nitrogen dioxide emission from the combustion processes in vehicles and in the heating systems of buildings.

European limit value

The European directive 2008/50/EC, which aims to protect the public health, determines that the annual average NO2 concentrations must not exceed 40 µg/m3 as of 2010 (red curve on the graph); this value also corresponds with the target recommended by the World Health Organisation.

NO2 concentration in the air

NO2 is permanently monitored in the Brussels Region at 11 monitoring sites of the telemetric air quality network. Our indicator uses the readings of the monitoring site in Molenbeek-Saint-Jean (code 41R001) as it is representative of an urban environment that is strongly influenced by road traffic.

Comparison of the annual NO2 averages with the European limit value - monitoring site Molenbeek-Saint-Jean (1986-2012)
Source: Environnement Bruxelles-Leefmilieu Brussel, Laboratory for Environmental Research (air)
Comparison of the annual NO2 averages with the European limit value - monitoring site Molenbeek-Saint-Jean (1986-2012)

Since the end of the 1990s, the average NO2 concentrations at this monitoring site have remained relatively stable and have been for most of the years above the limit value. In 2012, the average NO2 concentration at the monitoring site of Molenbeek-Saint-Jean amounted to 41 µg/m³.

In  the other sites of the telemetric network, the annual average NO2 concentration was between 25 and 48 µg/m³, depending on the proximity of the sources of nitrogen dioxide emissions, such as traffic. Approximately a third of the Brussels' monitoring sites does not comply with the limit value imposed.


The concentrations recorded in the monitoring sites result from aggregate contributions by different sources: background contamination (as recorded for instance in the Ardennes), contributions from outside the region (brought to the BCR via airflows), urban background contamination, the urban contributions that are mainly linked to traffic, and additional contributions from traffic as recorded in areas with a high traffic density.
On an annual basis, an average 40% of the NO2 concentration measured originates from outside the Brussels Region (sum of background contamination and contributions from outside the region). 13% is caused by urban background contamination and 47% is related to traffic.
Also worth mentioning is the fact that as opposed to the decreased levels of NOX since the 1990s (see indicator with regard to NOX), the proportion of NO2 in the NOX emissions from road traffic has been increasing since a few years. Explanatory factors for this observation include:

  • the dieselisation of the vehicle stock (diesel emits relatively more NO2);
  • the oxidation catalytic converters imposed by the EURO 3 standard (which increase the proportion of NO2 compared to NO in the emission);
  • the particulate filters for trucks (which indirectly increase NO2 emissions).



Datum van de update: 29/05/2020