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Tokyo, situated on the eastern coast of Honshu Island at the head of Tokyo Bay, occupies the Kanto Plain—Japan's largest lowland region. This sprawling metropolis, with its core at approximately 40 meters above sea level, is flanked by mountains to the west, including the Okutama and Chichibu ranges, which rise to over 2,000 meters. The city's geography profoundly influences its air quality: the surrounding mountains act as a natural barrier, trapping pollutants from local sources like vehicle exhaust and industrial emissions within the basin, especially during periods of atmospheric stability. Tokyo Bay moderates the climate but also contributes to humidity that can exacerbate photochemical smog formation. The city lies within the Keihin Industrial Zone, stretching along the bay from Tokyo to Yokohama, where concentrated manufacturing and power plants emit particulates and nitrogen oxides. The urban–rural gradient is stark, with dense skyscrapers in central wards like Shinjuku giving way to suburban sprawl and then agricultural areas in western Saitama and Chiba prefectures, though these farmlands offer little respite from transboundary pollution like Asian dust (kosa) blown from China across the Sea of Japan. Tokyo's position in East Asia exposes it to regional air masses, making it a receptor for continental pollutants, while its coastal location subjects it to sea breezes that occasionally disperse local emissions but can also bring marine aerosols.

Tokyo's air quality follows a distinct seasonal pattern shaped by its humid subtropical climate and meteorological conditions. In winter (December to February), pollution peaks due to temperature inversions that trap vehicle exhaust and industrial emissions in the Kanto Basin, compounded by cold, stagnant air and increased heating demand. Sensitive groups, such as those with respiratory issues, should limit outdoor activities during these months, especially on calm, clear days when inversions are strongest. Spring (March to May) sees a gradual improvement, but March remains problematic due to lingering inversions and the arrival of Asian dust (kosa) from China, which can spike particulate levels; wearing masks is advisable during dust events. Summer (June to August) brings cleaner air as monsoon rains from the East Asian rainy season (tsuyu) wash pollutants away, and southerly winds from the Pacific enhance dispersion, though high temperatures and humidity can intensify photochemical smog from precursors like nitrogen oxides, making midday outdoor exercise less ideal. Autumn (September to November) is generally the best season for air quality, with mild temperatures, steady winds, and lower pollution levels, favouring outdoor activities for all residents. Throughout the year, typhoons in late summer and early autumn can temporarily clear the air but pose other weather hazards.