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Saint-Jean, nestled in the Occitanie region of southwestern France, presents a unique geographic setting influencing its air quality. Located at 43.6664° N, 1.5058° E, the city sits within the Garonne River valley, a relatively flat alluvial plain. This valley floor, though fertile and supporting agricultural activity, can be prone to temperature inversions, trapping pollutants during stable atmospheric conditions. The surrounding landscape is characterised by the foothills of the Pyrenees mountains to the south, providing a natural barrier but also influencing wind patterns. To the north and west, the terrain gradually rises into rolling hills and agricultural lands, primarily dedicated to cereal crops and vineyards. Saint-Jean’s proximity to Toulouse, a significant industrial and aerospace hub approximately 30km to the east, introduces a potential source of long-range pollutant transport. The urban-rural gradient is relatively sharp, with the city centre transitioning quickly to open countryside. The Garonne River itself, while a vital resource, can contribute to humidity and potentially exacerbate particulate matter issues, especially during periods of low flow. The city’s elevation is low, around 150m, further contributing to the potential for pollutant accumulation. Understanding these geographical factors is crucial for assessing and mitigating air quality challenges in Saint-Jean.

Air quality in Saint-Jean follows a distinct seasonal pattern. Winter, from December to February, often sees the poorest air quality due to increased residential heating using wood and fossil fuels, coupled with frequent temperature inversions trapping pollutants in the Garonne valley. Calm, cold conditions limit atmospheric dispersion. Spring (March-May) brings improvement as temperatures rise and winds become more frequent, diluting concentrations. However, agricultural activities, including fertilizer application, can contribute to ammonia emissions. Summer (June-August) generally offers the cleanest air, with strong convective currents and prevailing winds flushing out pollutants. However, prolonged heatwaves can trigger ozone formation, a secondary pollutant. Autumn (September-November) is a transitional period. The return to heating systems begins to degrade air quality, and damp conditions can favour the formation of secondary organic aerosols. October and November are often marked by increased fog, which can trap particulate matter. Sensitive individuals – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during winter mornings and periods of stagnant air. Monitoring pollen levels is also important during spring. Avoiding peak hours near roads and industrial areas is advisable year-round.