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San Andres, Rizal, occupies a strategic transition zone between the dense urban sprawl of the Greater Manila Area and the rugged wilderness of the Sierra Madre mountain range. This municipality is characterized by a diverse terrain, shifting from flat lowland plains to rolling hills as one moves eastward. Its regional position makes it a critical corridor for commuters and logistics, increasing the density of vehicular emissions along its primary arterial roads. The urban character is a blend of residential clusters and agricultural patches, creating a distinct urban-rural gradient that influences pollutant dispersal. Because the town is nestled against the foothills of the Sierra Madre, it often experiences a "basin effect" where topographic barriers prevent the efficient flushing of particulate matter, especially during stagnant weather conditions. The proximity to the Laguna de Bay watershed further introduces humidity and moisture, which can interact with nitrogen oxides and sulfur dioxide to form secondary aerosols. While the surrounding greenery of the Rizal uplands acts as a natural carbon sink, the increasing encroachment of industrial belts from neighboring municipalities often pushes smog toward the valley. The elevation varies slightly, but the primary driver of air quality is the interaction between the mountain breeze and the polluted air masses drifting from the west. Consequently, the local atmosphere reflects a tension between the pristine air of the highlands and the anthropogenic stressors of the rapidly expanding Calabarzon region, making its atmospheric profile highly variable and sensitive to regional shifts.

In San Andres, air quality is governed by the rhythmic oscillation between the dry season and the wet season. From December to May, the Amihan or northeast monsoon brings cool, dry air from the Pacific, which generally aids in dispersing local pollutants. However, during the peak of the dry season in March and April, temperature inversions often occur. These atmospheric lids trap particulate matter and vehicle exhaust near the ground, leading to stagnant air and higher concentrations of smog. This period is the most challenging for respiratory health, and sensitive groups, such as children and the elderly, should limit outdoor exertion during midday peaks. Conversely, the wet season, spanning from June to November, is dominated by the Habagat or southwest monsoon. Heavy rainfall acts as a natural scrubbing mechanism, washing pollutants from the atmosphere through wet deposition, which significantly lowers particulate levels. Despite this, high humidity during the monsoon can exacerbate the feeling of air heaviness and promote mold growth, affecting those with asthma. The transition months of May and November are often unpredictable, characterized by shifting winds that can either clear the skies or pull in pollution from the Metro Manila basin. For optimal air quality, outdoor activities are best scheduled during the rainy months or early mornings during the Amihan. Health guidance emphasizes the use of masks during the dry season's peak smog events to mitigate the inhalation of fine particulates and other harmful airborne irritants.