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Yerres, nestled within the Île-de-France region of France, occupies a geographically significant position southeast of Paris. Its coordinates (48.7171, 2.4881) place it on the southern edge of the densely populated urban sprawl, transitioning towards a more rural landscape. The city’s terrain is characterized by gentle slopes and valleys, sculpted by the Yerres River, which flows through its heart, contributing to a relatively humid microclimate. Elevation varies subtly, generally ranging from 50 to 80 meters above sea level. This proximity to the Yerres River, while providing aesthetic and recreational benefits, can also influence local air quality, particularly during stagnant weather conditions where pollutants can accumulate near the water's surface. Yerres sits within the broader Parisian Basin, a sedimentary geological formation that tends to trap atmospheric pollutants. The surrounding landscape is a mix of agricultural fields – primarily used for cereal crops and pasture – and patches of woodland, offering some natural air filtration. However, the city’s location within the commuter belt of Paris means it experiences considerable traffic volume, a primary source of air pollution. The urban–rural gradient is gradual; Yerres isn't sharply delineated from the countryside, but rather blends into it, resulting in a complex interplay between urban emissions and rural air quality influences. The presence of industrial zones further out in the Île-de-France region, while not directly within Yerres, contributes to regional background pollution levels impacting the city’s air quality.

Yerres’ air quality exhibits a distinct seasonal pattern dictated by the interplay of meteorological conditions and regional pollution sources. Winter months (December-February) often present the most challenging period. Temperature inversions, a common occurrence during this time, trap cold air near the ground, preventing the vertical dispersion of pollutants released from vehicles and nearby industrial areas. Fog, frequently occurring in winter due to the river’s humidity, exacerbates this issue, further concentrating particulate matter. Spring (March-May) brings a gradual improvement as temperatures rise and wind speeds increase, facilitating pollutant dispersal. However, pollen levels also surge, impacting respiratory health. Summer (June-August) generally sees the best air quality, with consistent winds and higher temperatures promoting efficient ventilation. Occasional heatwaves can, however, lead to ozone formation, a secondary pollutant. Autumn (September-November) marks a transitional period. While wind patterns remain relatively favourable initially, the return of cooler temperatures and increased rainfall can lead to periods of stagnant air and elevated particulate matter, particularly in late autumn. Sensitive groups, including children, the elderly, and individuals with pre-existing respiratory conditions, should be particularly cautious during winter and early spring, limiting outdoor activity during periods of low visibility or reported poor air quality. During summer, monitor ozone levels and avoid strenuous outdoor exercise during peak sunlight hours. Regular monitoring of local air quality reports is recommended throughout the year to inform personal decisions regarding outdoor exposure.