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Sehwan, nestled in the Sindh province of Pakistan at coordinates 26.4193° N, 67.8594° E, occupies a geographically significant, yet challenging, position. The city is situated on the eastern bank of the Indus River, a vital lifeline but also a potential conduit for pollutants. The surrounding landscape is predominantly flat alluvial plain, characteristic of the lower Indus basin, with a gradual rise towards the Kirthar Mountains to the west. This terrain limits natural air dispersion. Sehwan’s elevation is relatively low, increasing susceptibility to temperature inversions, a common phenomenon trapping pollutants near the ground. The urban area of approximately 75,167 inhabitants displays a typical South Asian city structure – a dense core gradually transitioning to agricultural lands dominated by cotton, wheat, and sugarcane cultivation. While not a major industrial hub itself, Sehwan serves as a regional trade centre, experiencing increased vehicular traffic and associated emissions. The proximity to agricultural zones introduces seasonal biomass burning, contributing to particulate matter. The Indus River, while providing water resources, can also carry industrial and agricultural runoff, impacting local air quality through volatilisation and aerosol formation. The urban-rural gradient is relatively sharp, with limited green spaces within the city itself exacerbating the heat island effect and reducing air purification capacity.

Sehwan’s air quality follows a distinct seasonal pattern dictated by the region’s subtropical climate. The summer months (April-June) bring intense heat and dust storms originating from the west, significantly elevating particulate matter concentrations. Hot, dry winds exacerbate the issue, carrying dust and sand across the landscape. Monsoon season (July-September) offers temporary relief through rainfall, washing away some pollutants, but increased humidity can also foster fungal growth and associated allergens. However, stagnant air during breaks in the monsoon can lead to localised pollution build-up. Autumn (October-November) sees a return to drier conditions and a rise in agricultural burning as farmers prepare fields for winter crops, resulting in a peak in PM2.5 and PM10. This is arguably the worst period for respiratory health. Winter (December-February) is characterised by cooler temperatures and frequent fog, creating strong temperature inversions that trap pollutants close to the ground. Reduced sunlight also limits photochemical reactions that break down some pollutants. Sensitive groups – children, the elderly, and those with pre-existing respiratory conditions – should limit outdoor activity during autumn and winter. Avoiding peak traffic hours and using masks can offer some protection. Months of July-August, despite humidity, generally offer the best air quality.