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Santiago, Chile's capital and largest city, occupies a dramatic geographic setting in the Región Metropolitana that profoundly shapes its air quality challenges. Nestled within the Central Valley at approximately 520 meters above sea level, the city is encircled by the towering Andes Mountains to the east, reaching over 6,000 meters, and the lower Coastal Range to the west, creating a natural basin that traps pollutants. This topographic bowl effect is exacerbated by Santiago's position in a semi-arid Mediterranean climate zone, with limited natural ventilation. The city sprawls across the Mapocho River valley, historically centered along the Mapocho River, but has expanded into surrounding foothills and plains, creating a dense urban core of over 7 million people. Proximity to the Andes influences local wind patterns, with mountain breezes often insufficient to disperse accumulated smog. The region includes significant industrial belts in the city's southern and western peripheries, such as the communes of Maipú and Pudahuel, home to manufacturing, mining support services, and a major international airport. Agricultural zones in the surrounding valleys, particularly vineyards and fruit farms, contribute occasional biomass burning and dust. The urban–rural gradient shows decreasing pollution toward the Andean foothills and eastern suburbs like Las Condes, but thermal inversions frequently blanket the entire basin, especially in winter, making Santiago one of South America's most polluted cities due to this geographic confinement.

Santiago's air quality follows a distinct seasonal pattern driven by its Mediterranean climate and topographic constraints. In autumn (March to May), pollution begins to rise as temperatures cool, reducing atmospheric mixing, but winds from the Andes provide some relief; this is a moderate time for outdoor activities, though sensitive groups should monitor daily forecasts. Winter (June to August) brings the peak pollution months, with cold, stable air leading to frequent thermal inversions that trap vehicle emissions, industrial output, and residential wood-burning smoke—particularly from low-income areas using heaters—creating thick smog layers; avoid strenuous outdoor exertion, especially in mornings when inversions are strongest, and consider wearing masks if air quality warnings are issued. Spring (September to November) sees gradual improvement as warming temperatures increase vertical mixing and occasional rainfall helps cleanse the air, though pollen from surrounding vegetation can aggravate allergies; this season favors outdoor activities, but remain cautious during calm, sunny days when ozone levels may spike. Summer (December to February) offers the best air quality, with warmer, drier conditions and stronger mountain breezes dispersing pollutants, making it ideal for hiking in the nearby Andes; however, occasional heatwaves can elevate ground-level ozone, so sensitive individuals should limit midday exposure. Year-round, children, the elderly, and those with respiratory conditions should use air purifiers indoors during high-pollution episodes and follow local health advisories.