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Osasco occupies a strategic position within Brazil's most populous metropolitan region, situated just 16 kilometers west of São Paulo's city center at an elevation of approximately 760 meters above sea level. This densely urbanized municipality forms part of Greater São Paulo's continuous urban fabric, characterized by intense vertical development mixed with industrial zones and residential neighborhoods. The city's geography is defined by its location on the São Paulo Sedimentary Basin, with gently rolling terrain that slopes toward the Tietê River basin to the north. Osasco's air quality is profoundly influenced by its position within the Metropolitan Region of São Paulo, where it shares atmospheric conditions with neighboring cities through the phenomenon of atmospheric transport. The city sits within an industrial belt that extends westward from São Paulo, hosting manufacturing facilities, logistics centers, and commercial enterprises that contribute to regional pollution. While not directly coastal, Osasco's climate and air circulation patterns are affected by its proximity to the Serra do Mar mountain range to the southeast, which can trap pollutants during temperature inversions. The urban-rural gradient shows minimal transition, as Osasco merges seamlessly with adjacent municipalities, creating a vast urban heat island that exacerbates pollution retention. The Tietê River, while historically significant, currently functions more as a drainage channel than a moderating water body for local microclimates. This geographic context positions Osasco as both a contributor to and recipient of metropolitan-scale air quality challenges, with limited natural ventilation pathways to disperse accumulated pollutants.

Osasco experiences distinct seasonal variations in air quality driven by São Paulo metropolitan region's subtropical climate patterns. During summer (December-February), increased rainfall from convective afternoon thunderstorms provides natural cleansing of atmospheric pollutants, though high temperatures and intense sunlight can accelerate photochemical reactions that generate secondary pollutants like ozone. This season offers relatively better air quality days, particularly following rain events. Autumn (March-May) brings transitional conditions with decreasing precipitation and more frequent temperature inversions that trap pollutants near the surface, creating gradual deterioration in air quality as the season progresses. Winter (June-August) represents the peak pollution period, characterized by persistent high-pressure systems, minimal rainfall, and frequent radiation inversions that prevent vertical mixing of the atmosphere. Cold, dry air settles in the basin, accumulating particulate matter from vehicle emissions, industrial activities, and biomass burning from agricultural regions upwind. Sensitive groups should limit outdoor exertion during winter mornings when inversion layers are strongest. Spring (September-November) shows improving conditions as increasing temperatures and occasional frontal systems begin to break up stagnant air masses, though biomass burning from sugarcane harvesting in surrounding regions can cause episodic pollution spikes. The best months for outdoor activities are typically December through February, while June through August require heightened awareness for those with respiratory or cardiovascular conditions. Wind patterns, predominantly from the southeast, occasionally bring cleaner marine air but more often circulate pollutants within the metropolitan basin.