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Santos is a coastal city in southeastern Brazil, situated on the island of São Vicente within the Santos Metropolitan Region. Its geography is defined by the Atlantic Ocean to the southeast and the Serra do Mar mountain range to the northwest, creating a narrow coastal plain where the urban area is concentrated. The city lies at low elevation, mostly near sea level, with the Port of Santos—Latin America's largest port—dominating its eastern shoreline. This port complex, along with adjacent industrial zones in Cubatão and São Vicente, forms a significant pollution source, with emissions from shipping, petrochemical plants, and vehicle traffic affecting local air quality. The urban–rural gradient is sharp, as the city transitions quickly from dense urban fabric to protected Atlantic Forest on the mountain slopes, though this natural barrier can trap pollutants when winds are weak. Proximity to the ocean moderates temperatures but also brings sea breezes that can disperse or, conversely, recirculate contaminants depending on atmospheric conditions. The region's terrain, with mountains blocking inland airflow, occasionally leads to stagnation episodes, particularly during cooler months, exacerbating pollution from urban and industrial activities.

In Santos, air quality varies seasonally due to meteorological influences and human activities. During summer (December to February), higher temperatures and increased solar radiation enhance photochemical reactions, potentially elevating ozone levels, while frequent sea breezes from the southeast help disperse pollutants, making this a favourable time for outdoor activities despite occasional heat-related smog. Autumn (March to May) sees a transition with cooler temperatures and reduced rainfall, leading to more stable atmospheric conditions; temperature inversions can occur, trapping pollutants near the surface, especially in early mornings, so sensitive groups should avoid strenuous outdoor exercise during these periods. Winter (June to August) is the peak pollution season, as cooler, drier air and persistent high-pressure systems foster frequent temperature inversions, limiting vertical mixing and allowing particulate matter from vehicles, port operations, and industrial sources to accumulate; fog and low clouds are common, further reducing air quality, and residents with respiratory issues are advised to limit exposure on calm days. Spring (September to November) brings increasing rainfall and windier conditions, which help cleanse the air, though transitional weather can still cause sporadic pollution spikes; overall, this season offers improving conditions for outdoor pursuits as the year progresses.