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Purnea, situated in the northeastern plains of Bihar, India, occupies a strategic position in the fertile Gangetic alluvial basin, approximately 40 kilometres south of the Nepal border. The city lies at a modest elevation of around 40 metres above sea level, characterised by flat to gently undulating terrain typical of the Indo-Gangetic Plain, which significantly influences its air quality dynamics. Purnea is enveloped by extensive agricultural zones, primarily dedicated to rice, wheat, and maize cultivation, with the Kosi River flowing to the north and the Mahananda River to the east, though neither directly abuts the urban core. This agricultural hinterland contributes to seasonal biomass burning, particularly post-harvest stubble burning, which exacerbates particulate pollution. The urban-rural gradient is pronounced, with Purnea serving as a commercial and administrative hub for surrounding rural districts, leading to a mix of vehicular emissions, small-scale industries like rice mills and brick kilns, and domestic fuel use. Its location in the Gangetic basin, prone to temperature inversions, traps pollutants close to the ground, especially during cooler months. The absence of significant topographical barriers allows pollutants to accumulate, while proximity to Nepal can occasionally bring transboundary haze from agricultural fires, though local sources dominate. The city's growth, marked by unplanned expansion and inadequate green cover, further intensifies its air quality challenges, making it a microcosm of urban pollution in agrarian regions.

Purnea's air quality follows a distinct seasonal rhythm shaped by its monsoon-influenced humid subtropical climate. In winter (December to February), pollution peaks due to a combination of factors: low temperatures foster ground-level temperature inversions that trap pollutants, while calm winds and high humidity lead to dense fog and smog, particularly in January. This period sees heightened emissions from biomass burning for heating and post-harvest agricultural residue burning, making it the worst time for outdoor activities, especially for sensitive groups like children, the elderly, and those with respiratory conditions who should limit exposure and use masks. Spring (March to May) brings gradual improvement as temperatures rise and winds increase, dispersing pollutants somewhat, though sporadic dust storms from dry fields can spike particulate levels. The monsoon season (June to September) offers the cleanest air, as heavy rainfall scrubs the atmosphere, and strong southwesterly winds aid dispersion, making this the best time for outdoor pursuits. Post-monsoon (October to November) sees a rapid deterioration as agricultural burning resumes and weather conditions become stagnant, with pollution building up towards the winter peak. Throughout the year, indoor air quality can also be compromised by cooking with solid fuels, advising year-round ventilation strategies and air purifiers for vulnerable residents.