Air Quality in 2025: assessment based on preliminary data

Summary

Air quality in 2025 deteriorated slightly compared with 2024. Both the annual average concentrations of nitrogen dioxide and fine particulate matter were higher than in 2024. The number of exceedances of (current and future) daily limit values was also higher in 2025 than in 2024. The number of ozone days (2) was the same in 2025 as in 2024. Over the long term, however, air quality has shown significant improvement.

From 2030 onwards, new stricter European limit values will apply. In 2025 these future limit values were not yet met at all monitoring locations. If this situation persists in 2026 and subsequent years through 2029, and if projections indicate that the current policy will still result in exceedances in 2030, then “roadmaps” will need to be drawn up in which the regions indicate which (additional) emission-reducing measures will ensure compliance by 2030. Under certain conditions, time extensions may also be requested from the European Commission.

Achieving the World Health Organization (WHO) health guideline values everywhere in Belgium remains far out of reach.

Introduction

This analysis provides a first preliminary estimate of air quality in 2025 based on the trends of three pollutants — nitrogen dioxide (NO₂), fine particulate matter (PM₂.₅), and ozone (O₃) — at urban background monitoring sites. Urban background sites are locations where air quality is determined not by very local sources but by all sources in the wider surrounding area. Measured concentrations at these sites provide a good indication of the average exposure of the urban population to air pollution.

At “hotspot” locations, where air quality is strongly affected by local sources, concentrations are higher than at nearby background sites. 

Nitrogen Dioxide (NO₂)

Nitrogen dioxide is a traffic-related pollutant. At locations with heavy traffic, there is a direct link between NO₂ concentrations and local (diesel) traffic.
Annual average NO₂ concentrations have generally declined since 2014, but over the last three years (2023–2025) there has been a stagnation, with concentrations in 2025 slightly higher than in 2024. This recent increase is mainly attributable to less favourable meteorological conditions in early 2025. Despite this small increase, concentrations in 2025 remained below those measured in the first year of the COVID-19 pandemic (2020). 

Factors contributing to the long-term decline include:

• A decrease in nitrogen oxide (NOₓ) emissions across all sectors and in all EU Member States.

• For road traffic specifically: stricter emission standards and the reduction of diesel vehicles in the passenger fleet. Diesel cars still emit more nitrogen oxides on average than gasoline cars.[1]

• The latest generation diesel vehicles (from Euro 6d) emit significantly less NOₓ due to the implementation of “real-driving emissions” tests, which assess vehicles under realistic driving conditions rather than only under laboratory conditions.

• The introduction of low-emission zones (LEZ) in a number of cities, which has resulted in the removal of the oldest and most polluting vehicles. In cities with an LEZ, NO₂ concentrations have declined somewhat more strongly [2]

The slight increase in NO₂ concentrations in 2025 compared with 2024 is mainly attributed to less favourable meteorological conditions. In 2024, conditions were exceptionally favourable for air quality (including record precipitation), resulting in the lowest NO₂ concentrations ever measured. In the spring of 2025 (February, March, and April), meteorological conditions were unfavourable for air quality — these months were on average drier and with little wind. From May onwards, monthly average concentrations in 2025 were lower than in 2024. As a result, the annual average in 2025 was only slightly higher than in 2024 (+6%).

The current European limit value of 40 µg/m³ was not exceeded in any automatic monitoring station in 2025. In Antwerp, Ghent, Brussels, Namur, and Charleroi, passive samplers measure two-weekly average NO₂ concentrations at a number of traffic-busy locations. Full results for 2025 are not yet available for passive measurements. Since no exceedances were observed with passive samplers in 2023 and 2024, it is unlikely that they occurred in 2025. Air quality is also assessed using high-resolution models for locations without measurements. Model results for 2025 will be available in summer or autumn 2026.

The stricter WHO annual guideline value (10 µg/m³) is met only at the most rural stations in Flanders and south of the Sambre–Meuse Valley.

In October 2024 the European Parliament and the Council of the European Union adopted a revised air quality directive with stricter limit values by 2030. The NO₂ annual limit value will be lowered from 40 to 20 µg/m³. If this future limit value is not met in 2026, Member States must draw up a “roadmap” by 2028 indicating which measures will ensure compliance with the 2030 limit.

In 2025 the future 2030 limit was slightly exceeded at urban background stations in Antwerp and Brussels, but it appears achievable by 2030 at these sites. At some of the busiest traffic locations, concentrations between 30 and 40 µg/m³ are still measured with passive samplers. Additional efforts and emission reductions will be needed to achieve the 20 µg/m³ limit everywhere by 2030.

daily limit 

In addition to the annual limit, the new EU directive also includes a daily NO₂ limit: 50 µg/m³, which may be exceeded no more than 18 days per year. This (less strict than the annual) limit is respected at nearly all monitoring stations. The number of exceedances of this future daily limit has fallen sharply since 2010. The very strict WHO daily guideline value of 25 µg/m³, which may be exceeded only three to four times per year, is achieved only at rural stations in the Ardennes and at the Belgian coast.

Fine Particulate Matter (PM₁₀ and PM₂.₅)

The current European annual limit values for PM₁₀ (40 µg/m³) and PM₂.₅ (25 µg/m³) continue to be met at all monitoring sites. PM₁₀ refers to particulate matter with a diameter smaller than 10 micrometers; PM₂.₅ refers to the finer fraction with a diameter smaller than 2.5 micrometers. 

Over a longer period, annual average concentrations have declined. After stagnation between 2020 and 2022, concentrations fell again in 2023 and 2024, but increased once more in 2025. This trend is illustrated by measurements at four urban background sites (Ghent, Antwerp, Brussels, and Charleroi) between 2010 and 2025.

The favourable long-term trend stems from reductions in emissions of both primary particulate matter and precursor pollutants (NOₓ, ammonia, and sulfur dioxide) at the European level. However, less favourable weather conditions in the spring of 2025 (dry and low wind) and exceptionally favourable conditions in 2024 explain the higher 2025 concentrations compared with 2024.

During January 2025, and especially in February, March, and April, PM₂.₅ concentrations were significantly higher than in the same months in 2024. On several days in these months, daily average concentrations exceeded the smog information threshold of 35 µg/m³. The smog information phase was activated six times in 2025, compared with none in 2024. From May onwards, monthly average concentrations in 2025 were lower than or equal to those in 2024. Due to higher concentrations in spring, the annual average in 2025 was about 11 % higher than in 2024.

The current EU annual limit of 25 µg/m³ for PM₂.₅ is respected everywhere. The future EU limit of 10 µg/m³ (2030) was exceeded at several monitoring sites (15 sites in Flanders) in 2025, compared with only one site in 2024. In 2024, ten sites measured exactly 10 µg/m³, narrowly avoiding exceedance of the future limit.

The WHO annual guideline value for PM₂.₅ is 5 µg/m³, this is reached only at rural sites in Wallonia (south of the Sambre–Meuse Valley).

daily limit

The current European daily limit for fine particulate matter (PM₁₀) is 50 µg/m³ (daily average), not to be exceeded more than 35 days per year. In 2025 this daily limit was not exceeded at any monitoring site. The highest number of daily limit exceedances measured was 24 days (Sint-Kruiswinkel, Ghent). Most stations recorded more exceedances in 2025 than in 2024, due mainly to less favourable weather conditions in spring 2025 compared with 2024.

The 2008 EU directive did not include a daily limit for PM₂.₅. The new directive includes a PM₂.₅ daily limit of 25 µg/m³, which may be exceeded up to 18 days per year. In 2025 this future daily limit was exceeded at 27 monitoring sites (24 in Flanders and 3 in Wallonia), whereas in 2024 it was respected everywhere. Over the long term, the number of days with peak concentrations has declined significantly.

The very strict WHO daily guideline for PM₂.₅ (15 µg/m³), which may be exceeded only three to four times per year, is not met anywhere in Belgium.

Although fine particulate matter concentrations have declined significantly over the past 15 years, local sources such as wood stoves can still cause short-term particulate peaks. A study by TNO [3] found that emissions from wood burning can contribute to exceedances of the WHO daily guideline and also cause nuisance from odour. Wood combustion remains the main source of (primary) fine particulate emissions.

Ozone (O₃)

The summer of 2025 was drier, warmer, and sunnier than normal, and there were two heatwaves. Despite weather conditions that typically favour ozone formation, there were only two ozone days in 2025: one in June (13th) and one in July (2nd). An ozone day is defined as a day on which the European information threshold of 180 µg/m³ is exceeded at at least one monitoring site in Belgium. In August there were no ozone days. Overall, 2025 was a year with few ozone days.

The reduction in emissions of nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) in recent decades across Europe has led to fewer and less intense ozone peaks. As a result, the number of ozone days and the intensity of ozone peaks have decreased even under similar or less favourable meteorological conditions.

Under the EU Air Quality Directive (2008/50), the ozone target value for human health protection is 120 µg/m³ as the highest 8-hour average in a day. The three-year average for days exceeding this value (2023–2025) remained below the limit at all monitoring sites. The WHO guideline value is 100 µg/m³. This guideline value was not met anywhere in Belgium.

Impact on health

Despite improvements in air quality over recent decades, the impact on health remains significant. Air pollution increases the risk of both acute and chronic conditions. Exposure to pollutants such as particulate matter, nitrogen dioxide, and ozone can lead to respiratory diseases (as e.g. , reduced lung function) and cardiovascular disease. In the long term, air pollution also contributes to increased mortality and hence a reduced life expectancy. This is particularly the case among vulnerable groups such as children, the elderly, and people with pre-existing health conditions.

To quantify this impact, several methods are used, including (among others) the calculation of years of life lost or the number of premature deaths. The most recent available figures on the number of premature deaths attributable to air pollution in Belgium can be consulted here:
https://www.irceline.be/nl/documentatie/publicaties/wetenschappelijke-rapporten/impact-luchtkwaliteit-2024_finaal.pdf (NL)
https://www.irceline.be/fr/documentation/publications/rapports-scientifiques/impact-luchtkwaliteit-2024_finaal_fr.pdf (FR)
(figures for 2024). Data on the impact in 2025 will become available during the course of 2026.

More information

This overview is based on preliminary measured concentrations of fine particulate matter, nitrogen dioxide, and ozone up to 26 December 2025. A comprehensive discussion of air quality in 2025 will be published in the course of 2026. The three Belgian regions will also publish reports and/or indicators with more information, including additional pollutants.

For more information on air pollution trends in Belgium, see the 2024 annual report on air quality in Belgium: 

NL: https://www.irceline.be/nl/documentatie/publicaties/jaarrapporten/jaarrapport2024_nl.pdf/view
FR: https://www.irceline.be/fr/documentation/publications/annual-reports/jaarrapport2024_fr.pdf 

Air Quality in Flanders (VMM): https://www.vmm.be/lucht/evolutie-luchtkwaliteit/hoe-evolueert-de-luchtkwaliteit-in-vlaanderen

Air Quality in Wallonia (AWAC/ISSeP): https://www.wallonair.be/fr/publications/12-qualite-de-l-air-ambiant/4-wallonie-rapports-annuels.html

Air Quality in Flanders (Brussels): https://document.environnement.brussels/opac_css/index.php?lvl=notice_display&id=13025