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Kumharsan, nestled in the state of Bihar, India, occupies a geographically significant position within the fertile Gangetic plains. Located at approximately 25.6421° N, 86.1971° E, the town’s terrain is predominantly flat, characteristic of the Indo-Gangetic alluvial plain, resulting in minimal elevation changes. This flatness, while conducive to agriculture, presents challenges for air quality dispersion. Kumharsan lies within a region heavily reliant on agriculture, with surrounding landscapes dominated by rice paddies and other cultivated fields. The proximity to the Ganges River, though providing a vital water source, can also contribute to humidity and stagnant air conditions, particularly during certain times of the year, hindering pollutant dispersal. The urban–rural gradient around Kumharsan is gradual, with smaller villages and agricultural land seamlessly blending into the town's periphery. Further afield, the Chota Nagpur Plateau lies to the south, influencing regional weather patterns and potentially impacting wind direction and pollutant transport. Industrial activity in the broader Bihar region, particularly in areas further west and south, can contribute to transboundary pollution events. The lack of significant topographical features means that pollutants released within Kumharsan and its immediate surroundings tend to accumulate, especially during periods of low wind speed and stable atmospheric conditions. The flat terrain also limits natural ventilation, exacerbating the potential for localized air quality issues. The soil composition, rich in alluvial sediments, can also influence dust generation, a significant contributor to particulate matter pollution.

Kumharsan’s air quality experiences a distinct seasonal cycle heavily influenced by the monsoon and regional weather patterns. The dry season, typically spanning from October to May, presents the most significant air quality challenges. During these months, lower humidity and reduced rainfall lead to increased dust generation from agricultural activities and unpaved roads. Temperature inversions, common during the colder months (December-February), trap pollutants near the ground, resulting in periods of stagnant air and elevated concentrations. The burning of crop residue after the harvest season (November-December) is a major contributor to particulate matter pollution, significantly impacting air quality. The monsoon season (June-September) brings relief, with frequent rainfall washing away pollutants and strong winds dispersing them. However, the monsoon also introduces its own challenges; increased humidity can promote the formation of secondary pollutants, and heavy rainfall can lead to flooding, which can stir up dust and contaminants. The transitional periods (March-May and September-November) are characterized by variable conditions, with dust storms and occasional rainfall. Individuals with respiratory conditions, children, and the elderly should exercise caution during the dry season, particularly during periods of crop residue burning. Outdoor activities are generally more favourable during the monsoon months, although vigilance regarding potential flooding is advised. Maintaining indoor air quality through ventilation and air purification can be beneficial throughout the year, especially during peak pollution periods. Awareness of regional fire events and associated smoke plumes is crucial for minimizing exposure.