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Calasiao, nestled in the heart of Pangasinan province, Philippines, occupies a largely flat, alluvial plain formed by the Agno River basin. Its coordinates (16.0200, 120.3700) place it within a predominantly agricultural landscape, characterized by extensive rice paddies – Pangasinan being the ‘Rice Bowl of the Philippines’. The terrain is gently sloping towards the Lingayen Gulf, approximately 15 kilometers to the west, influencing local wind patterns and humidity. Calasiao’s urban character is that of a developing municipality, transitioning from a purely rural setting with increasing commercial activity along the major thoroughfares like the McArthur Highway. This highway acts as a crucial transport corridor, contributing to vehicular emissions. Surrounding Calasiao are other agricultural towns, with pockets of small-scale industries related to rice milling and processing. The urban-rural gradient is relatively sharp, with immediate surroundings remaining intensely agricultural. This proximity to agricultural lands introduces potential for seasonal biomass burning, impacting air quality. The lack of significant topographical features means pollutants can accumulate easily, particularly during periods of calm weather. The coastal influence moderates temperatures but also contributes to high humidity, potentially exacerbating the effects of pollutants.

Calasiao experiences a distinct wet and dry season, heavily influencing its air quality. The dry season, typically from March to May, sees a peak in particulate matter due to agricultural practices like land preparation and rice straw burning after harvest. Lower rainfall means pollutants aren’t efficiently washed away, and stable atmospheric conditions can lead to stagnation. March and April are generally the least favourable months for outdoor activity, particularly for those with respiratory conditions. The onset of the wet season in June brings a significant improvement, with rainfall cleansing the atmosphere. However, increased humidity can also promote the formation of mold and allergens. The southwest monsoon, dominant from June to November, brings frequent rainfall and generally good ventilation, dispersing pollutants. December to February represents a transition period, with cooler temperatures and relatively lower pollution levels, though occasional haze can occur. Sensitive groups – children, the elderly, and those with pre-existing respiratory illnesses – should limit strenuous outdoor activity during the dry season and monitor their health closely. During the wet season, while air quality improves, precautions against waterborne diseases are essential.