Air quality in Brussels / 24 08 2022


Last autumn, the World Health Organization announced new threshold limits of air pollutants present in the lower layers of the atmosphere. Over the past years, more and more evidence has pointed to the direct effects of poor air quality on people's health and well-being. Thus, the new limits published in 2021 have been sharply tightened. You can get acquainted with the old (published in 2006) and new (September 2021) threshold limits’ in the previous material prepared by our team. Among many air pollutants, WHO has focused attention on three components: particulate matter (PM2.5 & PM10), nitrogen dioxide (NO2) and ozone (O3) classifying them as primary air pollutants. Therefore, in this article Airscan prepared an overview of air pollution levels in the Brussels Capital Region for the last 4 years. All the data presented and discussed in this article were obtained from an open-source database Irceline (Belgian Interregional Environment Agency)

Particulate matter (PM2.5)

Airborne particulate matter (PM) is not a single pollutant, but rather is a mixture of many chemical species. It is a complex mixture of solids and aerosols composed of small droplets of liquid, dry solid fragments, and solid cores with liquid coatings. Particles vary widely in size, shape and chemical composition, and may contain inorganic ions, metallic compounds, elemental carbon, organic compounds, and compounds from the earth’s crust. Particles are defined by their diameter for air quality regulatory purposes. Those with a diameter of 10 microns or less (PM10) are inhalable into the lungs and can induce adverse health effects. Fine particulate matter is defined as particles that are 2.5 microns or less in diameter (PM2.5). Today scientists classify two main sources of PM present in the atmosphere: anthropogenic and natural. Heating, combustion engines, construction sites, and agricultural and industrial processes are considered natural PM dust sources. While transboundary PM transportation and complex reactions of chemicals such as sulfur dioxide and nitrogen oxides in the atmosphere contribute to natural PM pollution levels. Therefore, PM pollution levels in Brussels are not only affected by the human activities and industrial processes running in the city but also by the volume of dust particles transferred from other regions (Sahara as an example) and by chemical processes taking place in the atmosphere.

Particulate matter measurement datasets from the 1st of January 2019 showed that the average PM pollution levels in Brussels (10.7 µg/m3 in 2019, 9.1 µg/m3 in 2020, 9.7 µg/m3 in 2021 and 9.2 µg/m3 during the first 8 months of 2022) are above the most recent WHO limit (5µg/m3). On the other hand, the measured values comply with the threshold value (10µg/m3) recommended in 2006. Looking more into details, it is important to mention that the strict COVID-19 lockdown in the spring of 2020 did not have a great positive impact on ambient PM levels as was observed initially. For example, from figure 1 it is clear that PM2.5 levels were increasing 3 months in a row during the lockdown period (February – March – April 2020). Moreover, the average background PM2.5 concentration in Brussels in April and May 2021 and 2022 (limited COVID restrictions) was lower (Figure 1) compared to the concentration measured in 2020 (strict pandemic restrictions). This observation confirms that the impact of transboundary pollution on PM levels is sometimes underestimated and should be included in the reporting. At the same time, the average PM2.5 concentration was fluctuating around 10 µg/m3. This indicates that the environmental project, mobility programs, and recent technology improvements positively impact air quality in Brussels.


Particulate Matter levels in Brussels

Nitrogen Dioxide

Nitrogen Dioxide (NO2) is a pungent gas that, along with fine airborne particulate matter, contributes to the reddish-brown haze characteristic of smoggy air in California. NO2 is comprised of one atom of nitrogen and two atoms of oxygen and is a gas at ambient temperatures. It has a pungent smell and is brownish red in colour. NO2 is a member of a family of chemicals comprised of nitrogen and oxygen that are collectively known as nitrogen oxides. The two most prevalent nitrogen oxides are NO2 and nitric oxide (NO), and the combination is often referred to as NOX. Although NO2 can be directly emitted from combustion sources, much of the NO2 in the ambient air is formed in the atmosphere through reactions between nitric oxide (NO) and other air pollutants that require the presence of sunlight (photochemical reactions). NO2 contributes to the formation of several other air pollutants, including ozone (O3), nitric acid (HNO3), and nitrate (NO3-) -containing particles that also form through photochemical reactions. NO2 levels in air vary with direct emission levels, as well as with changing atmospheric conditions, particularly the amount of sunlight. NO2 can also form indoors when fossil fuels like wood or natural gas are burned.

The average nitrogen dioxide gas concentration in Brussels reduced starting from 2019 till nowadays. For instance, in 2019 the average annual NO2 concentration was 30 µg/m3, in 2020 the average value decreased to 23 µg/m3 (-23%). In 2021 the average NO2 concentration (25 µg/m3) slightly increased (+8%) compared to 2020. Finally, the average NO2 concentration during the first 8 months of 2022 (24 µg/m3) remained at the same level as in 2021. A comparison of the measured values in Brussels with the most recent WHO threshold limit (10 µg/m3) shows that the average background NO2 concentration in the city was more than 2 times higher than the limit during the last 4 years. On the other hand, the measured values remain under the limit reported in 2005 (40 µg/m3). At the same time, it is important to mention that the average NO2 concentration in Brussels remains almost at the same level during the last 3 years. A comparison of the average nitrogen dioxide concentration in Brussels per month per year is given in figure 2.


Nitrogen Dioxide levels in Brussels


Ozone, an important component of smog, is a highly reactive and unstable gas capable of damaging living cells, such as those present in the linings of the human lungs. This pollutant forms in the atmosphere through complex reactions between chemicals directly emitted from vehicles, industrial plants, consumer products and many other sources. Ozone is a powerful oxidant – its actions can be compared to household bleach, which can kill living cells (such as germs or human skin cells) upon contact. It forms in greater quantities on hot, sunny, calm days. During the summer heat waves, ozone concentrations frequently exceed existing health-protective standards and might remain high for several days. Ozone is formed in the atmosphere through chemical reactions between pollutants emitted from vehicles, factories and other industrial sources, fossil fuels, combustion, consumer products, evaporation of paints, and many other sources. Hydrocarbons and nitrogen oxide gases react in the presence of sunlight to form ozone. Hot, sunny, and calm weather promotes ozone formation. Ozone has a very characteristic pungent odour, and it can sometimes be detected after lightning strikes or during electrical discharges. Individual humans vary in their ability to smell ozone; some people can smell it at levels as low as 0.05 ppm. The average ozone concentration in Brussels was 42 µg/m3 in 2019, 44 µg/m3 in 2020, 40 µg/m3 in 2021 and 54 µg/m3 during the first 8 months of 2022. It is important to mention that the average ozone concentration measured in 2022 is higher compared to the first 3 years because the value was not balanced by autumn and December values (Figure 4).


Ozone levels during the last 4 years in Brussels

On the other hand, it is important to mention that the Ozone levels in the spring-summer period of 2020 and 2022 are higher compared to the measurements from 2019 and 2021. This can be explained by the number of sunny days and the consequently higher average temperature in the region. Indeed, looking at the temperature vs Ozone levels graph it is clear that the average temperature in spring- summer 2020 and 2022 was higher compared to the values recorded in 2019 and 2021(Figure 5).


Temperature vs Ozone levels in Brussels

In conclusion, the overall air quality conditions in Brussels do not meet the threshold limits from the WHO organisation published in 2021. On the other hand, the limit values are low and it is hard to achieve the most recent version of the guidelines. In the next material, our team will compare pollution levels in Brussels with other capitals of the EU.