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Gisors, nestled in the heart of Normandy, France, occupies a gently undulating landscape within the Vexin Français regional natural park. Situated at coordinates 49.2811° N, 1.7772° E, the town benefits from a relatively elevated position, approximately 100-150 meters above sea level, contributing to good natural ventilation. The Epte valley runs nearby, providing a natural corridor and influencing local wind patterns. Historically, Gisors’ strategic location on ancient trade routes, including the route between Paris and Rouen, fostered its development. Today, the surrounding area is predominantly agricultural, characterised by arable farming and pastureland, with pockets of woodland. While not heavily industrialised, proximity to larger industrial zones around Rouen and Paris means long-range transport of pollutants is a factor. The urban-rural gradient is quite sharp; the town itself is compact, quickly giving way to open countryside. This proximity to agricultural land introduces potential sources of ammonia and particulate matter from farming activities, while the prevailing westerly winds can carry pollutants from the more densely populated and industrialised areas to the east. The town’s topography, while generally open, features minor depressions that can occasionally trap pollutants during stable atmospheric conditions, impacting localised air quality.

Air quality in Gisors exhibits a distinct seasonal pattern. Winter, from December to February, often sees poorer air quality due to increased residential heating with wood and fossil fuels, coupled with temperature inversions that trap pollutants near the ground. Calm, damp conditions exacerbate this, reducing dispersion. Spring (March-May) brings improvement as temperatures rise and wind speeds increase, diluting concentrations. However, agricultural activities intensify, potentially increasing ammonia levels. Summer (June-August) generally offers the best air quality, with strong winds and ample sunshine promoting dispersion. However, heatwaves can trigger ozone formation, a secondary pollutant. Autumn (September-November) sees a return to more stable atmospheric conditions and increased heating demand, leading to a gradual decline in air quality. October and November are often the most challenging months. 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 crucial in spring and summer. Fog, common in autumn and winter, can trap pollutants and reduce visibility, further impacting respiratory health. Avoiding peak traffic hours can also minimise exposure.