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Sanwal, nestled in the Lumbini Province of Nepal (27.6300° N, 83.6500° E), occupies a geographically significant position within the Terai plains, a low-lying, fertile region bordering India. The city’s terrain is predominantly flat, characteristic of the Terai, with gentle undulations reflecting alluvial deposits from the surrounding hills. Elevation averages around 275 meters above sea level, contributing to a generally warm and humid climate. Sanwal’s location is crucial to understanding its air quality challenges. It lies within a transitional zone between the Himalayan foothills and the expansive Terai landscape, influencing regional wind patterns and atmospheric stability. The surrounding landscape is a mosaic of agricultural fields – primarily rice and wheat cultivation – interspersed with small villages and scattered pockets of sal forest. This agricultural activity, particularly the burning of crop residue after harvest, is a significant contributor to seasonal particulate matter. To the south, Sanwal borders India, potentially exposing it to transboundary pollution originating from industrial belts and urban centers across the border. The urban–rural gradient is relatively gradual, with Sanwal exhibiting a mix of traditional rural settlements and emerging urban infrastructure. The proximity to the Rapti River, though providing a water source, can also contribute to localized humidity and fog formation, impacting pollutant dispersion. The flat topography limits vertical mixing, often trapping pollutants close to the ground, especially during periods of atmospheric stability.

Sanwal’s air quality follows a distinct seasonal pattern dictated by Nepal’s monsoon climate. The pre-monsoon season (March-May) typically witnesses the worst air quality, driven by agricultural practices. Farmers burn crop residue to clear fields for the next planting season, releasing substantial amounts of particulate matter and other pollutants. This is exacerbated by increasing temperatures and decreasing rainfall, creating stable atmospheric conditions that trap pollutants near the surface. The monsoon season (June-September) brings relief, with frequent rainfall washing away pollutants and strong winds dispersing them. However, heavy rainfall can also lead to increased humidity and localized flooding, potentially impacting respiratory health. The post-monsoon season (October-November) sees a gradual return to drier conditions, and a resurgence of agricultural burning, though often less intense than in the pre-monsoon period. Winter (December-February) is generally characterized by cooler temperatures and frequent fog, particularly in the early morning hours. This fog, combined with temperature inversions – where a layer of warm air sits above cooler air – further restricts vertical mixing and traps pollutants, leading to periods of reduced visibility and potentially elevated pollution levels. Sensitive groups, including children, the elderly, and individuals with pre-existing respiratory conditions, should exercise caution during the pre-monsoon and winter months, limiting outdoor activity during peak pollution periods and wearing appropriate masks. Maintaining indoor air quality through ventilation and air purifiers is also advisable.