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Castillejos, Zambales, Philippines, occupies a strategic position on the western coast of Luzon Island, nestled between the Zambales Mountains and the South China Sea. Its coordinates (14.9300, 120.2000) place it within a dynamic interplay of geographical features that significantly influence its air quality. The terrain is characterized by a coastal plain gradually rising towards the foothills of the Zambales range, creating a complex topography that can trap pollutants. The town’s proximity to Subic Bay, a former US naval base and now a bustling economic zone, introduces both industrial activity and maritime influences. Surrounding Castillejos are extensive agricultural lands, primarily dedicated to rice and fruit cultivation, with potential for seasonal burning contributing to particulate matter. The urban-rural gradient is pronounced, with denser settlements concentrated along the national highway and coastal areas, transitioning to more dispersed agricultural communities inland. This gradient affects pollution dispersion; coastal breezes can offer some relief, but temperature inversions, common in the mornings, can exacerbate pollutant concentrations near populated areas. The prevailing winds, influenced by the monsoon patterns, play a crucial role in transporting pollutants from both local sources and potentially from the larger Subic Bay Metropolitan Authority area, impacting air quality throughout the year. Elevation varies, but generally remains low-lying, limiting natural ventilation.

Castillejos experiences a distinct wet and dry season, heavily influencing its air quality. The dry season, typically from November to May, sees a peak in pollution. Lower rainfall means less atmospheric cleansing, and agricultural practices like land preparation often involve burning, releasing significant particulate matter. The northeast monsoon (Amihan) brings relatively cooler, drier air, but can also trap pollutants due to stable atmospheric conditions. February and March are often the most challenging months for respiratory health. The transition period in May-June sees increasing humidity and sporadic rainfall, offering temporary relief. The wet season, from June to October, generally brings improved air quality. The southwest monsoon (Habagat) delivers frequent rainfall, effectively washing away pollutants. However, increased humidity can also promote the formation of mold and allergens. September and October, while still wet, can experience tail-end effects of lingering dry season pollutants. Sensitive groups – children, the elderly, and those with pre-existing respiratory conditions – should limit strenuous outdoor activity during the peak of the dry season. Monitoring wind direction is crucial, as pollutants can be transported from nearby industrial areas during calm periods. Fog, common during the cooler months, can also trap pollutants near the ground.