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Montijo is strategically positioned on the southern shore of the Tagus estuary, serving as a vital link within the Lisbon Metropolitan Area. Its terrain is characterized by a predominantly flat, low-lying coastal plain that slopes gently toward the riverbanks. This geography creates a distinct urban-rural gradient, where dense residential clusters blend into fertile agricultural zones and salt marshes. The town's proximity to the vast water body of the Tejo is the primary driver of its local microclimate, introducing significant humidity and regulating temperature fluctuations. However, this same low elevation makes the area susceptible to stagnant air masses during specific meteorological events. The surrounding landscape is marked by the presence of the Montijo Air Base and nearby industrial belts in the Setúbal district, which introduce anthropogenic emissions into the local atmosphere. Because Montijo sits in a transitional zone between the urban sprawl of the south bank and the rural hinterlands of Setúbal, it experiences a complex interplay of pollutants. Nitrogen oxides from heavy commuter traffic crossing the bridge to Lisbon mingle with particulate matter from local industry and agricultural burning. The absence of significant topographic barriers to the south allows for some ventilation, yet the river valley can trap pollutants during temperature inversions, particularly in the cooler months. Consequently, the air quality is a direct reflection of its dual identity as both a residential suburb and an industrial-logistical hub within the Portuguese Atlantic corridor and estuary.

Air quality in Montijo follows a distinct Mediterranean rhythm, dictated by the interaction between the Atlantic breeze and seasonal temperature shifts. During the summer, the town faces its most challenging period for ozone levels. High solar radiation and stagnant air masses catalyze the formation of ground-level ozone, particularly in July and August. While the sea breeze often provides relief by flushing the coast, inland pockets can see pollutant accumulation, making midday outdoor activities risky for asthmatics. Autumn brings a transition where shifting wind patterns may occasionally bring industrial plumes from the wider Setúbal region into the residential core. Winter is generally the cleanest period due to frequent Atlantic depressions and heavy rainfall, which effectively scrub particulate matter from the atmosphere. However, December and January can experience temperature inversions, where a layer of warm air traps cooler, polluted air near the ground, leading to spikes in nitrogen dioxide from heating and traffic. Spring is characterized by a rise in biological aerosols, as pollen counts peak alongside moderate increases in commuter emissions. For sensitive groups, the late winter inversions and peak summer ozone windows are the primary times to limit prolonged exertion outdoors. Health guidance emphasizes monitoring local alerts during the hot, dry months when the air feels heavy and stagnant. By favoring early morning walks in the spring or the rain-cleansed air of winter, residents can best avoid the periodic troughs in air quality common to the Tagus basin and riverfront areas.