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Nestled in the Kathmandu Valley at an elevation of approximately 1,400 meters, Kathmandu is a densely populated urban hub in Nepal's Bagmati Province, surrounded by the towering peaks of the Himalayas to the north and the Mahabharat Range to the south. This bowl-shaped topography creates a natural basin that traps pollutants, exacerbating air quality issues as emissions from the city's congested streets, brick kilns, and industrial activities accumulate within the valley. The city's urban character is marked by rapid, unplanned expansion, with a stark urban–rural gradient where peri-urban areas blend into agricultural zones and forested hills. Proximity to the Bagmati and Bishnumati rivers, though historically significant, contributes to dust from sand mining and seasonal flooding. The valley's location in South Asia places it within a regional airshed where transboundary pollution from the Indo-Gangetic Plain can drift northward, particularly during pre-monsoon months. This geographic setting, combined with the city's high population density and limited green spaces, creates a challenging environment for air quality management, as the surrounding mountains inhibit wind dispersion and foster temperature inversions that lock pollutants close to the ground.

Kathmandu's air quality follows a distinct seasonal pattern driven by the monsoon cycle and meteorological conditions. During the winter months from November to March, pollution peaks due to temperature inversions that trap emissions from vehicles, brick kilns, and domestic biomass burning in the valley, compounded by low wind speeds and frequent fog. This period poses significant health risks, especially for sensitive groups like children, the elderly, and those with respiratory conditions, who should limit outdoor activities and use masks. The pre-monsoon season from April to June sees some improvement as rising temperatures and occasional winds help disperse pollutants, though dust from construction and sand mining remains an issue. The monsoon from July to September brings heavy rainfall that scrubs the air, leading to the cleanest conditions of the year, ideal for outdoor pursuits. Post-monsoon months in October experience a gradual deterioration as dry weather returns and agricultural burning in surrounding areas adds to local emissions. To minimize exposure, visitors and residents are advised to plan outdoor activities during the monsoon and early post-monsoon period, while staying informed about air quality forecasts year-round.