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Jacareí occupies a strategic position within Brazil's most economically dynamic region, nestled in the Paraíba Valley of southeastern São Paulo state. Situated approximately 80 kilometers northeast of São Paulo city, this municipality lies at the crossroads between the sprawling São Paulo metropolitan area and the industrial corridor extending toward Rio de Janeiro. The city's coordinates (-23.3050, -45.9658) place it at an elevation of approximately 560 meters above sea level, within the transitional zone between the coastal Serra do Mar mountain range and the interior plateau. This topography creates a natural basin effect that can trap pollutants, particularly during atmospheric inversions common in the valley. Jacareí straddles the Paraíba do Sul River, which flows through the urban center, providing some natural ventilation but also serving as a corridor for transportation emissions. The surrounding landscape features a mix of urban development, agricultural zones (primarily sugarcane and eucalyptus plantations), and remnants of Atlantic Forest, creating a distinct urban-rural gradient. Proximity to major highways like the Presidente Dutra Highway (BR-116) and the Carvalho Pinto Highway (SP-070) subjects the city to significant vehicular emissions, while its position within São Paulo's industrial belt means contributions from manufacturing facilities in neighboring municipalities. The valley's orientation—running northeast-southwest—channels both clean oceanic breezes from the coast and polluted air masses from the São Paulo metropolitan region, creating complex air quality dynamics influenced by regional transport of pollutants.

Jacareí's air quality follows distinct seasonal patterns shaped by the subtropical climate of Brazil's Southeast region. During summer (December-February), increased rainfall from convective afternoon thunderstorms effectively scrubs particulate matter from the atmosphere, making this the cleanest period for outdoor activities despite occasional high-ozone episodes from intense sunlight reacting with vehicle emissions. Autumn (March-May) brings transitional conditions with decreasing precipitation and more frequent temperature inversions in the valley, particularly during calm mornings when cold air settles, trapping pollutants near the surface—sensitive groups should limit early outdoor exercise during this season. Winter (June-August) represents the most challenging period, with dry conditions, persistent high-pressure systems, and frequent radiation inversions that can last for days, accumulating pollutants from vehicles, residential wood burning, and regional industrial sources; the coldest months of July and August typically see the highest particulate concentrations, warranting precautions for children, elderly, and those with respiratory conditions. Spring (September-November) begins with lingering dry conditions before the return of cleansing rains, creating a gradual improvement in air quality; however, agricultural burning in surrounding regions during the sugarcane harvest season (typically August-October) can occasionally bring smoke plumes into the valley. Throughout the year, wind patterns play a crucial role—northeasterly winds bring relatively cleaner air from the interior, while southwesterly flows can transport pollution from the São Paulo metropolitan area, particularly during winter anticyclones.