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Mihara City, nestled in the southeastern part of Hiroshima Prefecture, Japan, presents a fascinating interplay of geography and urban development impacting its air quality. Situated at coordinates 34.3975°N, 133.0786°E, the city occupies a coastal plain bordered by the Seto Inland Sea and the rugged mountains of the Chugoku region. This terrain significantly influences local wind patterns and pollutant dispersion. Mihara’s elevation varies, rising from sea level along the coast to hilly areas inland, creating microclimates and affecting the concentration of pollutants in valleys. The city’s position within a broader industrial belt, historically focused on shipbuilding and steel production, contributes to potential emission sources. While much of the immediate coastline is urbanised, the surrounding landscape transitions rapidly to agricultural zones – particularly citrus orchards – and forested hills. This urban-rural gradient plays a role in air purification, but also in the transport of agricultural pollutants. The Seto Inland Sea provides a moderating influence on temperature, but can also contribute to humidity and fog formation, trapping pollutants during stable atmospheric conditions. Mihara’s location makes it susceptible to air masses originating from both the Asian continent and the Pacific Ocean, carrying pollutants from distant sources.

Mihara’s air quality follows a distinct seasonal pattern. Spring (March-May) often sees increased particulate matter from agricultural burning and dust storms originating from the Asian continent, exacerbated by dry winds. Pollen from the abundant citrus groves also contributes to respiratory irritation. Summer (June-August) brings the humid subtropical climate, with monsoon rains offering periods of cleansing, but also high humidity that can trap pollutants. Typhoons, common during this season, can temporarily improve air quality through strong winds, but also disrupt infrastructure and potentially release pollutants. Autumn (September-November) is generally the clearest period, with stable high-pressure systems and cooler temperatures promoting dispersion. However, temperature inversions can occur, trapping pollutants in the valleys. Winter (December-February) experiences the lowest temperatures and frequent stable atmospheric conditions, leading to pollutant accumulation, particularly from heating sources. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during spring and winter. Monitoring wind direction is crucial; onshore winds tend to bring pollutants from the industrial areas, while offshore winds offer cleaner air. Avoiding peak traffic hours also minimizes exposure.