Air quality

1. Key messages

2. Particulate matter

Particulate matter (PM) refers to all of the fine microscopic particles suspended in the air. Generally, a distinction is made between PM with a diameter smaller than 10 micrometers (PM₁₀) and with a diameter smaller than 2.5 micrometers (PM_2.5). As PM_2.5 only contains the smaller particles of PM, its composition and health effects are different compared to PM₁₀.

Household heating is the most important direct source of fine particulate matter

PM can be emitted directly, but it can also be formed indirectly from precursors through chemical reactions. According to the most recent Belgian figures (2019), the most important direct source of PM_2.5 is heating by households, where the use of wood disproportionally contributes to emissions. Other major sources of direct PM emissions are transport, where road traffic is most important, and heavy industry. The most important precursors of indirect PM are ammonia, nitrogen oxides, and sulphur dioxide. Ammonia is primarily emitted by the agricultural sector, especially by livestock. Sulphur dioxide, itself an air pollutant, originates mostly from heavy industry [1].

Exposure to particulate matter can lead to cardiovascular and respiratory diseases and lung cancer

The World Health Organization (WHO) guidelines recommend a maximum exposure of 5 µg m^-3 mean concentration annually and 15 µg m^-3 daily average for PM_2.5, and 15 µg m^-3 annually and 45 µg m^-3 daily for PM₁₀. The health effects of acute exposure to particulate matter include increased mortality due to cardiovascular and respiratory diseases and strokes [2], and for PM_2.5 increased asthma attacks [3]. Chronic exposure to PM is associated with ischemic heart disease, respiratory diseases, and lung cancer [4]. 

Particulate matter concentrations are higher in the Flemish Region compared to the Walloon Region

To detect spatial patterns in the occurrence of air pollutants, concentration levels can be mapped across the Belgian territory. The maps below visualise the 2020 yearly average PM_2.5 and PM₁₀ concentrations for each statistical sector, the smallest administrative unit in Belgium. The values are shown relative to the AQG for the pollutant, as an indication of the sector inhabitant’s exposure as compared to the WHO-advised value. 

A clear north-south gradient is visible on the map of the AQG-relative concentration for PM_2.5 in Belgium. Concentrations are generally high in the north of the Flemish Region, intermediate in central Belgium, including the Brussels Capital Region, and low in the Walloon Region, especially in the Ardennes. The same pattern appears for PM₁₀, with the difference that relative pollution levels are generally lower compared to PM2.5. A possible explanation for this variation is that Flanders, being the more densely-populated region, has more PM sources (residential heating, transport, agriculture including intensive livestock) and fewer sinks (removal by vegetation) compared to Wallonia. While about half of the country has pollution levels above the annual WHO guideline value for PM₁₀, more than two-thirds of the area has levels above the AGQ for PM_2.5.

Population exposure to particulate matter is high but decreasing in Belgium

The Belgian regions can be compared in terms of exposure to PM_2.5 and PM₁₀, using the population-weighted concentration. This metric expresses the average pollutant concentration taking into account where people live, and as such is used as a measure for exposure. When considering the most recent figures for the year 2020, a similar pattern for both types of PM emerges: the exposure in the Brussels Region is comparable to the Belgian average, and exposure is substantially higher in the Flemish Region and lower in the Walloon Region. Two-thirds of the Belgian population is exposed to pollution levels exceeding the long-term WHO guideline value for PM₁₀, and more than 90% of the people are exposed to levels above the AGQ for PM_2.5.

The time series of the population-weighted concentration give an indication of whether exposure to PM_2.5 and PM₁₀ in the Belgian population is generally increasing or decreasing. The trends for both PM types are alike, and similar for all regions. Population-weighted concentrations peak in 2018, and then decrease to a minimum in 2020. The fall in concentration is less pronounced for Flanders compared to the other regions, and its value is – with one exception for PM_2.5  – the highest compared to the rest of the country. Exposure in Brussels in 2017 was higher than average, with a decrease that puts it close to average by 2020. Exposure to PM is by far lower in the Walloon region.

The overall decrease in exposure is mainly the result of an overall reduction in PM concentrations (not shown). Based on the trends visible in the graphs for PM_2.5 and PM₁₀, air quality in Belgium is improving since 2017. This finding is consistent with the most recent air quality report published by IRCEL-CELINE (Belgian Interregional Environment Agency) which started measuring in 1997 and shows a decreasing trend in both PM concentrations and exposure [5]. 

Belgium has the fourth highest exposure to particulate matter compared to similar EU countries

Internationally, Belgium has the fourth highest population-weighted concentration for both PM2.5 and PM10, compared to the other EU-14 countries, well above the European average concentration [8].

3. Nitrogen dioxide

Nitrogen oxides (NO_x) including nitrogen dioxide (NO₂) and nitrogen monoxide (NO) are formed and emitted together as a result of combustion, as occurs in car engines and power plants. NO is a colourless gas that is harmless at the concentrations present in the atmosphere, while ambient NO₂ constitutes a serious health hazard.

Road traffic is by far the leading source of nitrogen dioxide emissions

Transport is the leading source of NOx emissions, where road traffic is by far the most important contributor, followed by shipping (both maritime and inland) and air traffic [1]. The NO₂ exposure as a result of road traffic is further increased due to emissions occurring near surface level and often in densely inhabited areas [9], and because of street canyons, where the pollutant gets trapped in narrow roads with tall adjacent buildings [5]. Other, less significant sources are industry and agriculture.

Exposure to nitrogen dioxide can lead to chronic respiratory diseases and infections

The WHO recommends a maximum average yearly exposure to NO₂ of 10 µg m^-3 and 25 µg m-³ daily. Long-term exposure to NO₂ has been linked to mortality due to chronic obstructive pulmonary disease and acute lower respiratory infection [4]. 

Higher nitrogen dioxide concentrations in cities compared to the countryside

Considering the pollution map for NO₂, concentrations are generally higher in the Flemish Region and the north of the Walloon Region, including the Sambre and Meuse basins, as compared to the Ardennes. Road traffic being the principal source of NO_x, areas that include large cities and major highways are easily recognized on the map in red. The Brussels Capital Region, a highly urbanised area, exhibits major pollution, except for the forested part in the southeast. About 40% of Belgium’s territory has pollution values above the annual WHO guideline value for NO₂.

Relative concentration of NO₂ per statistical sector in Belgium, 2020
Source: Own calculations based on air pollution data provided by IRCEL-CELINE [6]

Population exposure to nitrogen dioxide is decreasing in Belgium

Applying the same population-weighted concentration metric as above, NO₂ exposure is highest in the Brussels Region, followed by Flemish Region, and much lower in the Walloon Region which has fewer urban centers. In the whole of Belgium, 20% of the population is exposed to long-term NO₂ levels above the WHO’s guideline value.

For all regions, concentrations have been steadily declining. Brussels is consistently the highest, but it has also seen the most significant fall in exposure, leading to convergence with the other regions. The declining trend in population-weighted concentration is similar for the other regions, although somewhat smaller for the Walloon region.

The overall decrease in exposure is mainly the result of an overall reduction in NO₂ concentrations (not shown). As with PM, these observations correspond to a finding in the most recent air quality report published by IRCEL-CELINE. Data collection starting in 1997 confirm this decreasing trend in NO₂ concentrations [5]. 

Belgium has the fifth highest exposure to nitrogen dioxide compared to similar EU countries

Internationally, Belgium has the fifth highest population-weighted concentration for NO₂ compared to the other EU-14 countries, well above the average European concentration [8].

4. Ozone

Ozone (O₃) is a highly reactive gas, which makes it harmful to both humans and ecosystems. Although its presence in the stratosphere, tens of kilometers high up in the atmosphere, protects lifeforms against the most damaging solar radiation, ozone in the lower troposphere poses a serious health hazard.

There is more ozone in summer, especially on sunny days

O₃ is not emitted directly but formed in the atmosphere by chemical reactions under the influence of sunlight. O₃ is formed from so-called precursors, including nitrogen oxides, methane, and volatile organic compounds. As a consequence, O₃ concentration is strongly dependent on weather, season, the time of the day, and emissions of precursors. On a short time scale, there will be more O₃ during daytime, and on sunny days. Over the year, O₃ concentrations are higher in the summer, with the peak ozone season ranging from April to September in Belgium. In the atmosphere, a chemical equilibrium exists between O₃ and NO on the one hand, and O₂ (oxygen) and NO₂ on the other. The most significant consequence of this is that NO, such as emitted by road traffic, breaks down ozone to form NO₂ [5].

Ozone peaks cause respiratory problems and premature mortality 

Because of its seasonal and diurnal character, the WHO guidelines for O₃ are based on daily maximum 8-hour mean concentration, and only peak season values are taken into account. The daily AQG is 100 µg m^-3, the annual limit is 60 µg m^-3 for the 8-hour maximum values averaged over the peak ozone season. The acute effects include mortality in adults, days with mildly reduced activity, hospitalizations for respiratory problems, adult use of bronchodilators, days with cough, and lower respiratory problems in children. There is still great uncertainty concerning the effects of chronic O₃ exposure. Some studies find a weak relationship between long-term exposure to O₃ and all-cause and respiratory mortality [4]. 

The countryside has higher ozone concentrations compared to cities

Considering the pollution map for O₃, concentrations are generally higher in the Walloon Region compared to the Flemish Region and the Brussels Capital Region. The spatial pattern of O₃ appears to a large extent to be the inverse of the pattern of NO₂. The likely explanation is that the NO emitted together with NO₂ by cars and other vehicles breaks down the ozone formed locally by chemical reactions. The result is that urban centers and highways, confronted with busy traffic, experience lower concentrations compared to the countryside, making exposure to ozone primarily a rural problem.

Relative concentration of O₃ per statistical sector in Belgium, 2020
Source: Own calculations based on air pollution data provided by IRCEL-CELINE [6]

Exposure to ozone has been stable over the last few years

In terms of population-weighted concentration of O₃ in 2020,  exposure in the Flemish Region is comparable to the Belgian average, while it is slightly higher in the Walloon Region and substantially lower in the Brussels Region. In Belgium, the entire population is exposed to O₃ concentrations above the annual WHO AQG.

There is no clear trend in terms of concentrations over the last few years. However, considering the longer measurement series in IRCEL-CELINE’s air quality report, a slightly increasing trend in O₃ concentrations and exposure is observable over the previous three decades [5]. This is in contrast to the other considered air pollutants, which show a steady decline.

Belgium has the sixth lowest exposure to ozone compared to similar EU countries

The European comparison of ozone exposure is not based on average concentration, as is the case for the AQGs, but on a metric called SOMO35: the sum of means (daily maximum 8-hour) over 35 ppb. As this is a cumulative figure, the values can become high as compared to metrics based on averages.

Belgium has the sixth lowest population-weighted SOMO35 for O₃ compared to the other EU-14 countries, well below the average European concentration [8].

5. Read more

View the metadata for this indicator

Background

Poor air quality constitutes the single biggest environmental health risk, responsible for millions of premature deaths and healthy life years lost worldwide. Exposure to air pollution has been associated with respiratory disease, cardiovascular disorders, and lung cancer. It disproportionally affects vulnerable groups, including young children, the elderly, and people with lung diseases and asthma. Examples of ambient air pollutants include particulate matter, nitrogen dioxide, and ozone [4].

To improve air quality and public health, the WHO publishes the Air Quality Guidelines (AQGs), which are a set of recommended limit values for specific air pollutants. The AQGs were last updated with recent scientific evidence in 2021, and contain recommendations for daily concentrations as well as long-term averages [4]. Aside from the WHO’s guidelines, the European Union enforces legally binding air quality standards. The EU standards are less stringent than the corresponding WHO guidelines, as these are the result of political negotiations, and consider health as well as economic feasibility [9].

The air pollutants addressed in this report are particulate matter with aerodynamic diameters <2.5 µm and <10 µm (PM_2.5 and PM₁₀), nitrogen dioxide (NO₂), and ozone (O₃). Air quality assessment is based on pollutant data provided by IRCEL-CELINE, in the form of high-resolution maps depicting yearly average concentration for the years 2017 to 2020. The pollution maps are the result of high-resolution models, calibrated against actual measurements but still subject to a degree of uncertainty [6]. Exposure to air pollution is approached using the population-weighted average concentration, with population data provided by Statbel [7].

Definitions

Concentration, sources, and sinks

Air quality can be quantified by the concentration of known air pollutants. Air pollution concentration is commonly expressed in the form of mass concentration, giving you the mass of a polluting substance present in a volume of air. As this mass is usually very small compared to the space it occupies, a common unit is microgram per cubic meter (µg m^-3; a microgram is equal to 1 millionth of a gram). 

The concentration of air pollutants is dependent on sources (direct or indirect) and sinks, which are factors or processes that emit or remove the pollutant, respectively. Emissions and removals are commonly expressed as rates, for instance as kilogram per hour or tonne per year.

Population-weighted average concentration 

The population-weighted average concentration is used as an indication of population exposure to air pollution. It is used to aggregate concentration values into a larger region. Instead of calculating a simple ‘spatial average’ concentration of the area, the population at each location is taken into account as the weight for the corresponding concentration level. As it incorporates information on air quality as well as where people live, it can serve as a measure of exposure to air pollutants.

References

1. Air pollutant emissions – European Environmental Agency, 2022. https://www.eea.europa.eu/data-and-maps/dashboards/national-air-pollutant-emissions-data
2. Orellano, P., Reynoso, J., Quaranta, N., Bardach, A., & Ciapponi, A. (2020). Short-term exposure to particulate matter (PM10 and PM2.5), nitrogen dioxide (NO2), and ozone (O3) and all-cause and cause-specific mortality: Systematic review and meta-analysis. https://doi.org/10.1016/j.envint.2020.105876
3. Orellano, P., Quaranta, N., Reynoso, J., Balbi, B., & Vasquez, J. (2017). Effect of outdoor air pollution on asthma exacerbations in children and adults: Systematic review and multilevel meta-analysis. https://doi.org/10.1371/journal.pone.0174050 
4. WHO Global Air Quality Guidelines. World Health Organisation, 2021. https://www.who.int/news-room/questions-and-answers/item/who-global-air-quality-guidelines
5. Jaarrapport luchtkwaliteit in België 2020 – IRCEL-CELINE. https://irceline.be/nl/documentatie/publicaties/jaarrapporten/jaarrapport-luchtkwaliteit-in-belgie-2020/view
6. ATMO-Street. IRCEL-CELINE. https://www.irceline.be/en/documentation/models/atmo-street?set_language=en
7. Structure of the Population. Statbel, 2022. https://statbel.fgov.be/en/themes/population/structure-population
8. Air Quality Health Risk Assessments (NUTS3). European Environment Agency. https://www.eea.europa.eu/data-and-maps/data/air-quality-health-risk-assessments-nuts3/air-quality-health-risk-assessments-nuts3
9. EU air quality standards. European Commission. https://environment.ec.europa.eu/topics/air/air-quality/eu-air-quality-standards_en

Please cite this page as: Sciensano. Determinants of Health: Air quality, Health Status Report, 22 Nov 2022, Brussels, Belgium, https://www.healthybelgium.be/en/health-status/determinants-of-health/air-quality