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Renens, nestled in the canton of Vaud, Switzerland, occupies a strategically important position within the Lausanne urban area, approximately 3 kilometers southeast of the city center. Its coordinates (46.5333, 6.5833) place it on a gently sloping plateau, part of the larger Swiss Plateau, at an elevation of around 465 meters above sea level. The surrounding landscape is a blend of urban development and agricultural land, characteristic of the Vaudois countryside. To the north, the urban sprawl of Lausanne dominates, while southwards, the terrain gradually descends towards the fertile plains of the Orbe valley. The proximity to Lake Geneva (Lac Léman) plays a crucial role in Renens’ microclimate and, consequently, its air quality. The lake’s influence moderates temperatures and can induce localized breezes, though its impact is less pronounced than in coastal cities. Renens sits within a region historically significant for agriculture, with vineyards and orchards dotting the landscape, contributing to a rural-urban gradient. While Renens itself is primarily residential, it benefits from the economic activity of Lausanne, and is connected by efficient public transport. The absence of heavy industry within Renens itself is a positive factor for air quality, but it remains susceptible to pollutants originating from the broader Lausanne metropolitan area and occasional transboundary transport from the north. The terrain, while generally favorable for ventilation, can occasionally trap pollutants during periods of stable atmospheric conditions, particularly in the colder months.

Renens experiences a temperate climate with distinct seasonal variations that significantly impact air quality. During the warmer months (June-August), prevailing westerly winds, influenced by Lake Geneva, generally provide good ventilation, dispersing pollutants and resulting in relatively clean air. However, periods of prolonged sunshine and high temperatures can lead to photochemical smog formation, particularly in the afternoons, as sunlight triggers reactions between nitrogen oxides and volatile organic compounds. Autumn (September-November) often brings calmer conditions and increased inversions, where a layer of warm air traps cooler air near the ground, hindering pollutant dispersion. This is a period when air quality can decline, especially during still days. Winter (December-February) presents the greatest challenge. Cold, stable air masses frequently settle over the region, creating temperature inversions that trap pollutants close to the ground. Fog, common during this season, further exacerbates the problem by reducing visibility and hindering dispersion. Spring (March-May) sees a gradual improvement as temperatures rise and wind patterns become more variable. Sensitive groups, such as children, the elderly, and individuals with respiratory conditions, should be particularly cautious during winter and autumn months, limiting outdoor activity on days with stagnant air or visible fog. During summer, while generally good, avoiding strenuous activity during peak afternoon hours can be beneficial. The lake’s influence helps to mitigate extreme pollution events, but localized variations can still occur depending on weather patterns and regional emissions.