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Miyoshi, Hiroshima Prefecture, occupies a geographically significant position within the Chūgoku region of Japan. Situated at approximately 34.8°N, 132.85°E, the city lies nestled within a valley carved by the Miyoshi River, a tributary of the larger Shinji River. This riverine setting contributes to a relatively humid climate and influences local weather patterns. The surrounding landscape is characterized by a blend of rolling hills and low mountains, part of the broader Chugoku Mountains range, providing a natural buffer against some windborne pollutants but also creating conditions conducive to temperature inversions, particularly during the colder months. Miyoshi’s elevation averages around 150 meters, which, while not exceptionally high, can still trap pollutants within the valley. The urban fabric of Miyoshi is a mix of residential areas, small-scale commercial zones, and limited industrial activity, primarily focused on food processing and light manufacturing. The urban–rural gradient is relatively sharp; the city transitions quickly into agricultural land, including rice paddies and orchards, which can contribute to seasonal particulate matter from agricultural practices like burning crop residue (though increasingly regulated). Its proximity to Hiroshima City, a major industrial and commercial hub, means Miyoshi is indirectly affected by regional pollution trends, although the distance mitigates the direct impact. The valley’s topography and the surrounding mountains significantly shape the city’s microclimate and influence the dispersion of air pollutants, making localized monitoring crucial.

Miyoshi’s air quality experiences a distinct seasonal cycle heavily influenced by meteorological conditions. Spring (March-May) sees a gradual improvement as temperatures rise and winds increase, dispersing accumulated winter pollutants. However, pollen season can significantly impact respiratory health, creating a separate air quality challenge. Summer (June-August), characterized by the humid subtropical climate and the East Asian monsoon, generally offers the best air quality due to frequent rainfall and robust winds that effectively flush out pollutants. The monsoon brings periods of heavy rain, naturally cleansing the atmosphere. Autumn (September-November) marks a transition period, with decreasing rainfall and a potential for stagnant air, particularly in October and November. This period can see a rise in particulate matter as agricultural activities resume and cooler temperatures lead to less atmospheric mixing. Winter (December-February) presents the most challenging period for air quality. Cold, stable air masses frequently lead to temperature inversions, trapping pollutants near the ground within the valley. Fog, common during these months, further exacerbates the problem by reducing visibility and hindering pollutant dispersion. Sensitive groups, including children, the elderly, and individuals with respiratory conditions, should limit outdoor activity during prolonged periods of fog or stagnant air in winter. While summer generally provides respite, even then, brief periods of stagnant weather can lead to localized pollution spikes. Careful monitoring and public health advisories are essential throughout the year, with particular emphasis on winter preparedness.