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Ōiso, nestled in Kanagawa Prefecture, Japan, occupies a geographically significant position between the sprawling urban expanse of Tokyo and the mountainous interior of Honshu. Located at approximately 35.3086° N, 139.3128° E, the city’s terrain is characterized by a gentle slope rising from the Sagami Bay coastline, transitioning into rolling hills and eventually the foothills of the Tanzawa Mountains. This coastal proximity moderates temperatures and introduces a maritime influence on local weather patterns. The surrounding landscape is a blend of agricultural fields – primarily rice paddies and vegetable farms – and forested areas, creating a distinct urban-rural gradient. Ōiso’s elevation ranges from sea level near the bay to approximately 300 meters at its highest points, influencing local air circulation. While not directly within a major industrial belt, Ōiso benefits from its proximity to the greater Tokyo-Yokohama metropolitan area, which indirectly impacts air quality through transported pollutants. The Sagami Bay acts as a natural barrier, partially shielding the city from prevailing westerly winds, but also creating conditions conducive to fog formation, which can trap pollutants. The city’s urban character is primarily residential, with pockets of commercial activity, contributing to localized emissions from vehicles and small businesses. The interplay of coastal breezes, mountainous terrain, and agricultural activity shapes the city’s microclimate and, consequently, its air quality profile. The gradual transition from coastal plains to higher elevations also influences the dispersion of pollutants, with valleys potentially experiencing greater concentrations during periods of stagnant air.

Ōiso’s air quality experiences a distinct seasonal cycle heavily influenced by Japan’s monsoon climate. Spring (March-May) generally sees improved air quality as winds increase and rainfall washes away accumulated pollutants from winter. However, pollen season can significantly impact respiratory health, creating a separate, albeit non-chemical, air quality concern. Summer (June-August) is typically the cleanest period, with frequent rainfall and strong sea breezes dispersing any potential pollutants. The humid conditions, however, can exacerbate respiratory issues for some individuals. Autumn (September-November) marks a transition period, with decreasing rainfall and a gradual increase in temperature inversions, particularly in October and November. These inversions trap cooler air near the ground, preventing the vertical dispersion of pollutants and leading to periods of poorer air quality. Winter (December-February) often presents the most challenging conditions. Cold, stable air masses combined with infrequent rainfall and occasional fog create ideal conditions for pollutant accumulation. Temperature inversions are common, and the lack of strong winds further exacerbates the problem. Sensitive groups, such as children, the elderly, and individuals with pre-existing respiratory conditions, should limit outdoor activity during these periods, especially on days with dense fog. While Ōiso benefits from its coastal location, periods of stagnant air, particularly during autumn and winter, can lead to localized air quality degradation. Maintaining awareness of weather forecasts and air quality reports is crucial for minimizing exposure and protecting public health.