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San Juan, nestled on the island of Siquijor in the Philippines, presents a unique geographic context influencing its air quality. The municipality occupies a coastal strip, with terrain rising gently inland. Its coordinates (9.1590, 123.4940) place it within a tropical marine climate, heavily influenced by the surrounding waters of the Visayan Sea and the Bohol Sea. Siquijor is largely rural, with San Juan being one of its more populated centres, boasting a population of 16,363. The island’s landscape is characterised by limestone cliffs, caves, and lush vegetation, transitioning from coastal mangroves to inland forests. While Siquijor isn’t known for heavy industry, localised sources of pollution include small-scale agriculture – primarily coconut farming – and increasing tourism. The urban-rural gradient is relatively sharp; San Juan’s built-up areas are concentrated along the coast, quickly giving way to agricultural land and forested hills. Proximity to the sea provides some natural ventilation, but can also trap pollutants during periods of calm weather. The island’s relatively small size means pollutants can disperse quickly, but also accumulate rapidly with increased emissions. The prevailing winds, typically from the southeast during the wet season, play a crucial role in pollutant transport.

San Juan experiences a distinct wet and dry season, profoundly impacting air quality. The dry season, generally from March to May, sees increased dust and particulate matter from agricultural activities and unpaved roads, exacerbated by stronger winds. This period often coincides with increased tourist activity, leading to higher vehicle emissions. While rainfall is minimal, temperature inversions can occur, trapping pollutants near the ground. The transition months of June and November are characterised by fluctuating weather patterns and moderate air quality. The wet season, from June to February, brings relief from dust, as rainfall effectively washes pollutants from the atmosphere. However, increased humidity can promote the formation of mould and other biological pollutants. The peak of the wet season, typically August to October, often experiences the cleanest air due to consistent rainfall and stronger winds dispersing emissions. Sensitive groups – children, the elderly, and those with respiratory conditions – should be particularly cautious during the dry season. Avoiding strenuous outdoor activity during peak hours in March-May is advisable. Monitoring local conditions and staying indoors when air feels stagnant is recommended year-round.