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Liège, nestled in the Walloon region of Belgium, occupies a strategically significant, yet geographically complex, position. The city is situated at the confluence of the Meuse and Ourthe rivers, within a valley carved by glacial activity during past ice ages. This valley setting, while providing picturesque landscapes, also creates a natural funnel for air masses, potentially trapping pollutants. The terrain is undulating, rising towards the Ardennes hills to the south and east, and gently sloping towards the Campine plains to the north. Historically, Liège’s growth was heavily influenced by its river access, facilitating trade and, subsequently, industrial development. The surrounding landscape is a mosaic of agricultural land – particularly arable farming in the lower-lying areas – and forested hills. A notable industrial belt stretches along the Meuse valley, both within and outside the city limits, contributing to localised emissions. The urban-rural gradient is relatively steep, with densely populated areas concentrated around the city centre, transitioning quickly to more rural landscapes within a few kilometres. This proximity to both industrial sources and natural features significantly influences Liège’s air quality profile, making it susceptible to both local and transboundary pollution.

Liège experiences a temperate maritime climate, resulting in distinct seasonal air quality patterns. Winter, from November to February, often sees the poorest air quality. Low temperatures and frequent temperature inversions trap pollutants near the ground, exacerbated by increased residential heating using wood and fossil fuels. Calm, stable weather conditions limit dispersion. Spring (March-May) brings gradual improvement as temperatures rise and winds become more frequent, though pollen counts can become a concern for allergy sufferers. Summer (June-August) generally offers the cleanest air, with strong winds and convective activity effectively dispersing pollutants. However, occasional heatwaves can lead to increased ozone formation. Autumn (September-October) sees a return to poorer conditions as heating demand increases and atmospheric stability returns. Fog, common in the Meuse valley during autumn and winter, can further concentrate particulate matter. Sensitive individuals – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during winter months and periods of fog. Monitoring forecasts and avoiding peak traffic hours are advisable year-round, but particularly during seasonal transitions.