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Air quality: nitrogen dioxide concentrations

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 in 2014. The standard related to hourly concentrations was well respected.


Nitrogen dioxide affects human health (impact on the respiratory tract) and harms the environment (it contributes to the formation of ozone and secondary particles, and to acidification). 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 10 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 evolution of the annual NO2 averages with the European limit value - monitoring site Molenbeek-Saint-Jean (1986-2014)
Source : Brussels Environment, Laboratory for Environmental Research (air)

Since the end of the 1990s, average concentrations of NO2 at the Molenbeek-Saint-Jeanstation have remained at a relatively constant level, only respecting the limit value, as an annual average, on a few occasions. In 2014, the average concentration of NO2 was 39 µg/m³.

In other measuring stations of the network, the average concentration of NO2 was between 22 and 61 µg/m³ (still for 2014), depending on their proximity to nitrogen oxide emitters, such as road traffic. Approximately one third of the measuring stations in Brussels do not adhere to the imposed limit value (although this represents a steady improvement over the last few years). However in April 2014, the European Commission requested clarification from the Brussels Region with regards to their non-compliance with the limit value for NO2 concentrations as an annual average.

It should be noted however that the standard related to the hourly threshold of 200 µg/m3 is adhered to at all of the measuring stations.


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.

Furthermore, it should also be noted that, unlike the reduction of emissions of NOx observed since the 1990s (see the specific indicator), the proportion of NO2 in the emissions of NOX from road transport increased until 2009 (and has since stabilised). 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).

This phenomenon has been observed in all Belgian agglomerations, as well as in Germany, the Netherlands and in London. This partially explains the non-compliance with the 40 μg/m³ standard for NO2 at the measuring stations which are influenced by road traffic.

Date de mise à jour: 26/10/2018