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Katsushika-ku is a special ward in northeastern Tokyo, Japan, situated on the Kanto Plain along the eastern bank of the Arakawa River, which flows into Tokyo Bay approximately 15 kilometres to the south. This low-lying urban district sits at an average elevation of just 2–4 metres above sea level, creating a flat terrain that contributes to stagnant air conditions, particularly during temperature inversions. Geographically, Katsushika-ku lies within Tokyo's expansive metropolitan region, positioned between the dense urban core to the west and more suburban and agricultural areas to the north and east in Chiba and Saitama prefectures. The ward's location places it downwind of prevailing westerly and northwesterly winds that can transport pollutants from Tokyo's industrial zones and vehicle emissions across the city. Proximity to Tokyo Bay influences local humidity and can lead to sea breeze effects that occasionally disperse pollutants, though the urban heat island effect intensifies ozone formation. The surrounding landscape is predominantly built-up residential and commercial areas with limited green space, though the Arakawa River provides some open corridors. Katsushika-ku's position within the Greater Tokyo Area's urban–rural gradient means it experiences air quality impacts from both local sources—such as traffic along major routes like the Keiyō Road—and regional pollution transported from neighbouring industrial belts in Kawasaki and Yokohama.

In Katsushika-ku, air quality follows distinct seasonal patterns shaped by Tokyo's temperate climate and meteorological conditions. Spring (March to May) brings moderate pollution levels, with increasing sunlight and temperatures promoting ozone formation, particularly in April and May when stagnant conditions occur; however, occasional rain showers help cleanse the air. Summer (June to August) sees mixed effects: the rainy season (June to mid-July) reduces particulate matter through precipitation, but high heat and humidity in July and August exacerbate ozone pollution, especially during windless days when sea breezes from Tokyo Bay fail to disperse pollutants. Autumn (September to November) generally offers the cleanest air, with September benefiting from typhoon-related winds that sweep away contaminants, and October to November experiencing cool, breezy conditions that minimise pollution buildup—ideal months for outdoor activities. Winter (December to February) presents the greatest challenges: temperature inversions trap pollutants near the ground, leading to elevated levels of PM2.5 and nitrogen oxides from heating and traffic; January and February are particularly problematic, with cold, calm days fostering haze. Sensitive groups, such as those with respiratory issues, should monitor forecasts and limit outdoor exertion during winter inversions and summer ozone peaks, while planning activities for autumn or spring mornings when air quality tends to be better.