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Air quality: tropospheric ozone concentrations

Tropospheric ozone is formed in the atmosphere from other substances, such as nitrogen dioxide NO2. The photochemical reaction mainly occurs between mid-June and mid-August, potentially causing ozone peaks in the event of a heatwave combined with the presence of certain other catalysts, such as nitrogen dioxide. If ozone occurs at abnormally high quantities, it can cause serious health issues. In the Brussels Region, the European target for the protection of the public health has been observed since 2005.


Ozone is a secondary pollutant. This means that ozone is not emited directly in the ambient air by human activities, but formed by photochemistry. The phenomenon occurs from mid-June to mid-August as a result of the irradiation of primary pollutants (such as NO2) by ultra-violet radiation (UV) in the presence of oxygen. Please refer to the methodological sheet which gives a detailed description of the ozone formation and depletion processes.

Ozone tops the list of air quality indicators, due to its impact on human health (impaired respiratory function) and on the environment. The toxicity of ozone varies according to its concentration.

European target values

In order to prevent harmful long-term effects on human health and/or on the entire environment, European ambient air quality directive 2008/50/EC sets the following target value for ozone, legally binding since 2010:

  • 120 µg/m³ as the maximum daily moving 8-hour mean,
  • maximum 25 exceedance days per year, averaged over 3 consecutive years.

Evolution of O3 concentration in the air

The Region's telemetric network comprises 7 monitoring sites that continuously monitor the tropospheric ozone. Our indicator is based on the data recorded in Uccle. As the monitoring site in Uccle is located in a residential area with only light traffic and at a reasonable distance from the major trunk roads, the ozone formation processes prevail over the depletion processes that occur when NO is present (gas emitted by traffic for instance).

Evolution of annual average and median concentrations of tropospheric ozone at the monitoring site of Uccle (1986-2014)
Source : Brussels Environment, Laboratory for Environmental Research (air)

In 2014, the annual average and median value of tropospheric ozone at the Uccle station was 48 µg/m³. The evolution of concentrations over time appears to show an upward trend in the median concentration in the 1990s, which is much less pronounced in the 2000s. The annual median value appears to be stable.
The upward trend in the 1990s can be explained in particular by a general decrease in levels of NO (ozone-depleting pollutant).

The monitoring locations in Berchem-Sainte-Agathe and at the European Parliament, albeit to a lesser extent for the latter, reveal high annual average concentrations. By contrast, in the city centre and in the vicinity of trunk roads (monitoring sites of Woluwe-Saint Lambert and the Quai aux Briques) primary nitrogen monoxide emissions from traffic contribute directly to ozone depletion, explaining the reduced ozone concentrations.

Compliance with the European target values

Number of exceedance days recorded at the monitoring site of Uccle for the target of 120 µg/m³ applicable to the 8-hour mean ozone concentrations (1986-2014)
Source : Brussels Environment, Laboratory for Environmental Research (air)

In the Brussels Region, the European target value for health protection has been adhered to since the period 2005-2007. During the 3-year period 2012-2014, the average number of exceedance days, which depends on the characteristics of the surrounding area, ranged from 6 (monitoring site Sainte-Catherine and Avant-Port) to 10 (monitoring site Berchem Sainte Agathe). This number is well below the allowed average of 25 exceedance days, averaged over 3 years.

It is striking that, invariably, for all the years in which the months July and August were sunny and warm, more exceedance days were recorded for that particular year. This occurred for instance in the years 2003, 2006, 2010 and 2013. However, this characteristic would no longer appear to be the reason for an exceedance of the European target value these last few years.

Datum van de update: 26/10/2018