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Air quality: concentration of fine particles (pm10)

The particles that are suspended in the air (PM) are very different in terms of origin as well as chemical and physical characteristics. The PM concentrations and emissions in the air have been regulated by Europe as these particles have a major impact on human health, specifically on respiration and blood circulation. The annual average concentrations of PM10 across all of the Region's air quality monitoring sites comply with the European limit value. The daily limit value, which was a problem for many years, was adhered to in 2014. Various sources should be taken into account: the trans-regional import of PM, road traffic and the massive formation of secondary aerosols in certain meteorological conditions.


All particles smaller than 10 micrometres in diameter are denoted by the acronym "PM10", regardless of their composition or physical nature. Particles which are suspended in the ambient air come from a variety of sources: "primary" particles are emitted directly through a natural process (for instance soil erosion or particles from the Sahara), or caused by human activities (incineration, road surface wear, construction and demolition works, …), while "secondary" particles occur in the atmosphere through chemical reactions between other molecules that are already present (nitrates, sulphates, ammonium, nucleation of gaseous substances, …).

European limit values

With a view to protecting public health, the European directive 2008/50/EC provides for the obligation to adhere to two limit values for the PM10 concentration in the ambient air. These have been in force already since 1 January 2005:

  • 50 µg/m3 as the daily average, with a maximum of 35 exceedance days per year.
  • 40 µg/m3 as the annual average: this limit value has not been exceeded at any site of the Brussels' monitoring network since 2004.

PM10 concentration in the air

In the Brussels Region, PM10 is permanently monitored at 6 monitoring sites of the telemetric air quality network. We base our indicator on the data of monitoring station Écluse 11 in Molenbeek-Saint-Jean (code 41R001) as it is representative of an urban area with a strong influence of road traffic. The Brussels' PM10 indicator relates only to the daily averages.

PM 10 : Evolution at monitoring site Molenbeek-Saint-Jean of the number of days in exceedance of the 50 µg/m3 daily limit value (1997 to 2014)
Source : Brussels Environment, Laboratory for Environmental Research (air)

Up to 2009, the monitoring site of Molenbeek-Saint-Jean systematically indicated more exceedance days than the allowed35 days/year. An improvement has nonetheless been observed since 2012.

However, the measuring station at Avant-Port (Haren), situated on the banks of the Canal, was in exceedance up to 2013 included. The Region was therefore in violation up to this date, as a limit value is considered to be infringed from the moment an exceedance is observed at one of the measuring stations of the Region monitoring network.

On the opposite in 2014, the limit was respected. The recent improvement observed at the Molenbeek-Saint-Jean measuring station seems to be applicable for all stations.

Sources of PM10

Air masses can transport PM10 across great distances because the particles are so small. This means that the concentrations recorded in Brussels are not the sole result of local emissions: the PM10 concentrations are also associated with background contamination (as measured in the Ardennes for instance), contributions from outside the region (brought into the BCR via airflows), urban background contamination (resulting from the emission by heating and traffic, as recorded at the monitoring sites of Uccle and Berchem-Sainte-Agathe), local urban contributions primarily related to traffic (as is the case of a more densely populated area such as Molenbeek-Saint-Jean), and additional intermittent contributions of traffic found in areas with a high ratio of vehicles (such as at monitoring station Arts-Loi).

It is estimated that, under normal meteorological conditions, traffic (through direct emission) contributes to 10% of the PM10 recorded. Furthermore, traffic is responsible for re-suspending particles with a diameter between 2.5 and 10 µm, as a result of the motion of the vehicles: these indirect emissions from traffic represent on average an additional 10% of the concentrations.

The analysis of the data from the different stations within the monitoring network teaches us that the urban background contamination and/or the transport of PM through the air masses between the regions alone are at the base of a considerable number of exceedances (more than 20 days of the allowable 35 days in the case of the stations of Uccle and Berchem). The surplus exceedances observed at the monitoring sites of Molenbeek-Saint-Jean and Avant-Port results eitherfrom the resupension of particles between 2 and 10 µm, or from a direct emission through an activity in the immediate surroundings.

Date de mise à jour: 26/10/2018