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Brussels, the capital of Belgium and the administrative heart of the European Union, occupies a distinctive geographic position in the heart of Western Europe. Situated in the central plateau region of Belgium, the city lies within the Senne River valley, with an average elevation of approximately 15 meters above sea level, though it features gentle hills that rise to about 100 meters in its southeastern suburbs. This low-lying terrain, combined with its location in the densely populated and heavily industrialized Flanders region, creates a basin-like effect that can trap pollutants. Brussels is approximately 60 kilometers from the North Sea coast, a proximity that typically brings maritime air masses that help disperse pollution, but this effect is often counteracted by the city's position within a major European transport corridor. The urban area is surrounded by a mix of suburban development, agricultural land, and remnants of the Sonian Forest to the southeast, creating an urban–rural gradient that influences local air circulation patterns. The city's extensive road network, including the congested Brussels Ring motorway that encircles the metropolitan area, intersects with major European highways, making vehicle emissions a persistent concern. Additionally, the Port of Brussels, though smaller than Antwerp's major port to the north, contributes to local emissions through river traffic and associated industrial activity. The city's compact urban core, characterized by historic architecture and narrow streets, further exacerbates pollution buildup by limiting air flow, while its role as a political and economic hub ensures continuous human activity that drives emission sources year-round.

Brussels experiences distinct seasonal variations in air quality, driven by its marine west coast climate and urban emission patterns. During winter, particularly from November through February, pollution peaks sharply due to a combination of factors: increased residential heating emissions, heightened vehicle use in colder weather, and frequent temperature inversions that trap pollutants near the ground. These inversions are often exacerbated by calm winds and persistent fog, creating stagnant air conditions that can last for days. Sensitive groups, such as those with respiratory conditions, should limit prolonged outdoor exertion during these months, especially on days with low wind speeds. Spring brings gradual improvement as temperatures rise and Atlantic winds become more consistent, dispersing pollutants more effectively; however, occasional Saharan dust intrusions can temporarily degrade air quality. Summer offers the cleanest air for outdoor activities, with longer daylight hours reducing heating needs and prevailing westerly winds from the North Sea providing natural ventilation, though heatwaves in July and August can occasionally lead to ozone formation from vehicle emissions. Autumn sees a return to deteriorating conditions as temperatures drop and atmospheric stability increases, with October often marking the transition toward winter pollution buildup. Throughout the year, rainfall helps cleanse the air, but Brussels' moderate precipitation means this effect is intermittent. For optimal air quality, visitors and residents should favor outdoor activities in late spring to early autumn, while remaining vigilant during winter stagnation episodes.