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San Mariano, nestled within the province of Isabela in the Philippines, occupies a geographically significant position in the Cagayan Valley region. Located at coordinates 16.9800, 122.0200, the municipality is characterized by a predominantly rural landscape transitioning into more concentrated settlements around the town proper. The terrain is largely flat to gently rolling, typical of the valley’s alluvial plains, formed by the sediment deposition of the Cagayan River – the largest river basin in the Philippines. This proximity to the Cagayan River, while vital for irrigation and agriculture, also contributes to humidity and potential for localized fog formation, impacting air dispersion. San Mariano’s economy is heavily reliant on agriculture, primarily rice and corn cultivation, with some livestock farming. The surrounding areas feature extensive agricultural lands, which, during planting and harvest seasons, can contribute to particulate matter pollution from field preparation and crop residue burning. While not a major industrial hub, the increasing presence of small-scale processing facilities and transport networks linking San Mariano to larger urban centers like Santiago City and Tuguegarao City introduce localized emissions. The urban-rural gradient is pronounced, with air quality generally better in the more remote agricultural areas compared to the town center, where traffic and commercial activity concentrate pollutants. The elevation is relatively low, further influencing air circulation patterns.

San Mariano experiences a distinct wet and dry season, heavily influenced by the prevailing wind patterns and the Philippine monsoon. Air quality generally improves during the wet season (roughly June to November) as rainfall effectively washes particulate matter from the atmosphere. However, increased humidity can also promote the formation of mold and allergens, impacting respiratory health. The northeast monsoon, or ‘Amihan’, brings cooler temperatures and relatively cleaner air, making November to February the most favourable months for outdoor activities. Pollution levels tend to peak during the dry season (March to May), coinciding with agricultural activities like land preparation and crop burning, releasing significant amounts of smoke and particulate matter. High temperatures during this period also contribute to the formation of ground-level ozone. March and April are particularly challenging for those with respiratory conditions. Temperature inversions, where a layer of warm air traps cooler air near the surface, can exacerbate pollution events, especially in the mornings. Sensitive groups – children, the elderly, and individuals with pre-existing respiratory illnesses – should limit strenuous outdoor activity during peak pollution periods and consider using protective masks. Monitoring wind direction is crucial, as pollutants can be transported from neighboring agricultural areas.