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Santiago, Chile's capital and largest city, occupies a strategic position in the country's central valley, nestled within the Santiago Basin at approximately 520 meters above sea level. This basin is dramatically framed by the Andes Mountains to the east, which rise to over 6,000 meters, and the Chilean Coastal Range to the west, creating a natural amphitheater that profoundly influences air quality. The city's Mediterranean climate, characterized by hot, dry summers and cool, wet winters, interacts with this topography to trap pollutants. Santiago's urban sprawl extends across the valley floor, merging with industrial zones in the southern and western sectors, particularly around the Maipo River, where mining, manufacturing, and construction activities concentrate emissions. The Maipo River itself, flowing from the Andes, provides limited ventilation, while the surrounding agricultural belts in the valley's outskirts contribute occasional biomass burning. The urban–rural gradient sharpens near the foothills, where affluent neighborhoods at higher elevations experience slightly better air quality due to elevation, though thermal inversions common in winter exacerbate pollution at lower altitudes. Proximity to the Pacific Ocean, about 100 kilometers west, offers some coastal breezes, but these are often blocked by the Coastal Range, limiting dispersion. This geographic setting makes Santiago one of South America's most polluted cities, with emissions from vehicles, industry, and domestic sources accumulating in the basin, worsened by altitude inversions that cap the valley like a lid.

In Santiago, air quality follows a distinct seasonal pattern driven by Mediterranean climate dynamics and topographic constraints. During autumn (March to May), temperatures cool, and rainfall decreases, leading to the onset of thermal inversions that trap pollutants, with May marking the start of peak pollution months as stagnant air dominates. Winter (June to August) is the most critical period, with cold, dry conditions and frequent temperature inversions—especially in the mornings—capping the valley and preventing dispersion; this is when PM2.5 and PM10 levels soar due to vehicle exhaust, road dust, and domestic biomass cooking for heating. Sensitive groups, such as children, the elderly, and those with respiratory conditions, should limit outdoor activities from May to July, particularly on calm, clear days when inversions are strongest. Spring (September to November) brings gradual improvement as increasing temperatures and occasional rainfall help clear the air, though wildfires in surrounding areas can cause sporadic spikes in pollution. Summer (December to February) offers the best air quality, with warmer temperatures, stronger solar heating that breaks inversions, and southerly winds from the Pacific providing ventilation; this is the ideal time for outdoor activities. Throughout the year, monitoring local air quality indices and avoiding high-traffic areas during rush hours can mitigate exposure, with health guidance emphasizing indoor air purifiers and mask use during peak pollution episodes.