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San Mateo, Rizal occupies a distinctive transitional position in the Philippines' Luzon region, situated at the foothills of the Sierra Madre mountain range to the east and bordered by the Marikina River to the west. This geography creates a unique urban-rural gradient where the municipality blends into Metro Manila's eastern fringe, with coordinates placing it approximately 15 kilometers northeast of Manila's urban core. The terrain slopes gently from the mountainous Sierra Madre foothills down toward the Marikina Valley, with elevations ranging from about 20 to 200 meters above sea level, creating natural channels for air movement. Proximity to the Marikina River and Laguna de Bay to the south influences local humidity and occasional fog formation, while the Sierra Madre acts as a partial barrier to easterly winds. This location within the greater Manila industrial belt means San Mateo experiences pollution transport from Metro Manila's urban emissions, compounded by local sources including transportation along major arteries like Marcos Highway and Ortigas Avenue Extension, small-scale manufacturing, and residential combustion. The urban character is rapidly densifying, with green spaces diminishing as residential and commercial development expands, reducing natural air filtration. The valley-like setting can trap pollutants during certain meteorological conditions, particularly when winds are light and from the west, carrying Metro Manila's pollution into the area. Agricultural zones persist in northern barangays, adding occasional biomass burning to the pollution mix, though urban sources dominate the air quality narrative.

San Mateo's air quality follows a distinct seasonal pattern shaped by the Philippines' tropical monsoon climate, with pollution levels fluctuating based on rainfall, wind patterns, and temperature. During the dry season from March to May, pollution typically peaks due to increased vehicular and industrial activity, combined with stable atmospheric conditions that limit vertical mixing. High temperatures and occasional temperature inversions trap pollutants near the surface, making these months particularly challenging for air quality, with sensitive groups advised to limit prolonged outdoor exertion, especially during midday when ozone formation is highest. The southwest monsoon (Habagat) from June to September brings substantial rainfall that scrubs pollutants from the air, leading to generally better air quality, though humidity can exacerbate respiratory discomfort for some. October to February marks the northeast monsoon (Amihan) period, characterized by cooler temperatures and moderate winds that often improve dispersion, but occasional fog and low-level clouds can sometimes concentrate pollutants. Wind direction plays a crucial role—westerly winds transport pollution from Metro Manila into San Mateo, while easterly winds from the Sierra Madre bring cleaner air. The transition months of November and February often see the best balance of comfortable weather and reasonable air quality for outdoor activities. Sensitive populations, including those with asthma or cardiovascular conditions, should monitor local air quality reports year-round, use air purifiers indoors during peak pollution periods, and schedule outdoor activities for early mornings when pollution levels are typically lower.