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Prilly, nestled in the canton of Vaud, Switzerland, occupies a gently sloping terrain at the foot of the Jura Mountains, overlooking the northern shore of Lake Geneva (Lac Léman). Its coordinates (46.5333, 6.6000) place it within a relatively sheltered microclimate, influenced by both the lake’s moderating effect and the surrounding hills. The city is effectively a suburb of Lausanne, forming part of a continuous urban agglomeration along the lakefront. This proximity to Lausanne, a regional economic hub, introduces a degree of urban-generated pollution. The landscape immediately surrounding Prilly is a mix of vineyards, residential areas, and patches of woodland, transitioning to more rugged terrain as one ascends into the Jura. The urban-rural gradient is quite sharp, with agricultural land and forested slopes readily accessible. While Prilly itself lacks heavy industry, the broader Vaud canton hosts some industrial activity further inland, and prevailing westerly winds can transport pollutants towards the lake basin. The lake’s thermal stratification can also contribute to localised air stagnation, trapping pollutants during certain periods. The elevation, around 400m, influences atmospheric circulation and pollutant dispersion, making Prilly’s air quality sensitive to both local and regional factors.

Prilly’s air quality follows a distinct seasonal pattern. Winter, from December to February, often sees the poorest conditions. Reduced sunlight hours and temperature inversions, common in the lake basin, trap pollutants near the ground, particularly particulate matter from wood burning for heating. Calm, stable air masses exacerbate this. Spring (March-May) brings gradual improvement as temperatures rise and winds become more frequent, dispersing pollutants. However, agricultural activities, including fertiliser application, can contribute to ammonia levels. Summer (June-August) generally offers the cleanest air, with strong convective currents and prevailing westerly winds flushing out pollutants. However, increased ozone formation due to high temperatures and sunlight can be a concern. Autumn (September-November) sees a return to more stagnant conditions as the weather transitions, and the resumption of wood burning contributes to particulate matter. Fog, frequent in autumn and winter, further reduces visibility and traps pollutants. Sensitive individuals – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during winter inversions and periods of high ozone in summer. Monitoring pollen levels is also crucial during spring and summer.