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Legnano, nestled in the heart of Lombardy, Italy, occupies a geographically significant position within the Po Valley. The city sits at an elevation of approximately 178 meters (584 feet) above sea level, characterized by a relatively flat terrain typical of the alluvial plain formed by the Ticino and Olona rivers. This flatness, while conducive to urban development, contributes to the potential for stagnant air masses and pollutant accumulation. Surrounding Legnano is a landscape transitioning from intensive agriculture – particularly rice paddies – to the south and east, and increasingly towards industrial and commercial zones closer to Milan, located just 20km to the southeast. The urban-rural gradient is therefore quite sharp, with a noticeable increase in industrial activity and associated emissions as one moves towards the metropolitan area. The Olona River flows through Legnano, offering some limited dispersion potential, but its flow is often insufficient to fully mitigate pollution build-up, especially during periods of low precipitation. The Po Valley’s natural bowl-like shape, combined with the Apennine and Alpine mountain ranges restricting air circulation, exacerbates these conditions, making Legnano particularly vulnerable to air quality issues. The proximity to major transportation corridors, including highways and rail lines, further contributes to local emission sources.

Legnano’s air quality follows a distinct seasonal pattern. Winter, from December to February, typically sees the poorest air quality. Low temperatures create stable atmospheric conditions, often accompanied by temperature inversions trapping pollutants near the ground. Reduced sunlight hours limit photochemical reactions that would otherwise break down some pollutants. Heating demands surge, increasing particulate matter emissions from wood and fossil fuel combustion. Spring (March-May) brings gradual improvement as temperatures rise and rainfall increases, washing away accumulated pollutants. However, agricultural activities, including fertilizer application, can contribute to ammonia emissions. Summer (June-August) generally offers the cleanest air, aided by higher temperatures, stronger winds, and increased dispersion. However, ozone formation can become a concern during prolonged heatwaves. Autumn (September-November) sees a deterioration as heating systems are reactivated and agricultural burning may occur. Fog, common in the Po Valley during autumn and winter, further exacerbates pollution by trapping pollutants. Sensitive individuals – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during winter and autumn months. Monitoring forecasts and avoiding peak pollution periods is crucial for protecting health.