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Hājīpur, situated in the Vaishali district of Bihar, India, occupies a central position within the fertile Gangetic plains. Located at 25.6925°N latitude and 85.2084°E longitude, the city’s terrain is predominantly flat alluvial land, a characteristic feature of this region. The Burhi Gandak river flows nearby, influencing local humidity and potentially contributing to localised air stagnation during calm weather. Hājīpur’s urban character is rapidly evolving, transitioning from a primarily agricultural market town to a more diversified urban centre, though agriculture remains a dominant economic activity in the surrounding areas. This agricultural landscape, while vital for food security, contributes to seasonal biomass burning, a significant source of particulate matter. The city’s proximity to Patna, a major industrial and commercial hub, also means it experiences some degree of regional pollution transport. The urban-rural gradient is relatively sharp, with densely populated areas concentrated around the city centre, gradually giving way to agricultural fields and smaller villages. This gradient impacts air quality, with higher concentrations of pollutants typically found in the urban core due to vehicular emissions and localised industrial activity. The flat topography limits natural dispersion of pollutants, exacerbating air quality challenges, particularly during winter months when temperature inversions are common.

Hājīpur experiences a distinct seasonal air quality pattern dictated by the monsoon and post-monsoon periods. The monsoon season (June-September) generally brings improved air quality due to rainfall washing away pollutants and increased wind speeds dispersing emissions. However, high humidity can also foster mould growth, impacting respiratory health. October-November sees a deterioration in air quality as post-monsoon conditions prevail – calmer winds, cooler temperatures, and the widespread practice of agricultural residue burning after the harvest contribute to increased particulate matter. This is often the most challenging period for those with respiratory sensitivities. December-February are characterised by temperature inversions, trapping pollutants near the ground and leading to consistently poor air quality. Fog further exacerbates the situation, reducing visibility and increasing pollutant concentrations. March-May sees a gradual improvement as temperatures rise and winds pick up, though dust storms are common, increasing particulate matter levels. Sensitive groups – children, the elderly, and those with pre-existing respiratory or cardiovascular conditions – should limit outdoor activity during the post-monsoon and winter months. Monitoring local conditions and using appropriate respiratory protection is advisable during periods of poor air quality.