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Sonhauli, Bihar, situated at approximately 25.51°N, 86.47°E, occupies a geographically significant position within the fertile Gangetic plains of India. The town’s location, with an elevation of roughly 62 meters (203 feet) above sea level, places it within a relatively flat terrain characteristic of the Indo-Gangetic Plain. This flatness, while conducive to agriculture, also contributes to air quality challenges. Sonhauli lies near the confluence of the Gandak River and the Sone River, although the immediate proximity to these waterways doesn't significantly influence local air quality due to the prevailing wind patterns and distance. The surrounding landscape is predominantly agricultural, with extensive fields of rice, wheat, and maize dominating the rural areas. This agricultural activity, particularly the burning of crop residue after harvest, is a key contributor to seasonal pollution spikes. To the west and north, the terrain gradually rises, though not dramatically, towards the Chota Nagpur Plateau. The urban–rural gradient around Sonhauli is relatively sharp; the town itself is small, transitioning quickly to agricultural land. The region's proximity to industrial belts in Bihar and Uttar Pradesh, while not immediately adjacent, means that pollutants can be transported via prevailing winds. The lack of significant topographical barriers exacerbates this, allowing pollutants to spread across the plains. The flat, expansive nature of the region, combined with agricultural practices and regional industrial influence, creates a vulnerability to poor air quality events.

Sonhauli’s air quality follows a distinct seasonal pattern dictated by the region’s monsoon climate. The dry season, typically spanning October to May, presents the most significant air quality concerns. Following the monsoon withdrawal in October, a period of stagnant air often sets in, frequently accompanied by dense fog, particularly during November and December. This fog traps pollutants near the ground, exacerbating existing issues. Agricultural burning, a widespread practice after the harvest of Kharif crops (rice, maize), intensifies during this period, releasing substantial particulate matter into the atmosphere. January and February often experience temperature inversions, where a layer of warm air sits above cooler air, preventing pollutants from dispersing vertically. The monsoon season (June to September) brings relief, with rainfall washing away pollutants and strong winds dispersing them. However, even during the monsoon, localized pollution can occur due to construction activities and vehicular emissions. The pre-monsoon period (March to May) sees a gradual increase in temperatures and a decrease in rainfall, leading to a build-up of pollutants. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should exercise caution during the dry season, particularly during periods of fog and temperature inversions. Outdoor activities are best avoided during peak pollution episodes, and the use of air purifiers and masks is recommended. The transition months of October and May require careful monitoring as conditions shift between the dry and wet seasons.