San Juan Air Quality Index (AQI)

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San Juan, La Union, Philippines, is a coastal municipality nestled along the western coastline of Luzon Island. Situated at approximately 16.6700°N, 120.3300°E, the city’s geography is defined by its proximity to the South China Sea. The terrain is a mix of flat coastal plains and gently rising hills, characteristic of the Ilocos Region. This relatively low elevation, combined with its coastal location, makes San Juan particularly vulnerable to marine aerosols and the transport of pollutants from wider regional sources. The city’s urban character is evolving, transitioning from a primarily agricultural and fishing economy to one increasingly influenced by tourism, particularly surfing. Surrounding San Juan are extensive rice paddies and agricultural lands inland, contributing to potential particulate matter from farming activities, especially during dry seasons when fields are prepared for planting or harvested. The urban-rural gradient is relatively sharp, with agricultural areas bordering the city proper. The West Philippine Sea influences wind patterns, often providing some dispersion, but can also trap pollutants during periods of calm. Industrial activity is limited within San Juan itself, but the broader La Union province has some pockets of small-scale manufacturing that could contribute to regional air pollution. The city’s location along a major transport route also means vehicle emissions are a significant factor.

San Juan experiences a pronounced wet and dry season, dictating the patterns of air quality throughout the year. The dry season, typically from November to May, sees a peak in particulate matter pollution. Reduced rainfall means less atmospheric cleansing, and agricultural activities like land preparation and harvesting contribute dust and smoke. The northeast monsoon (Amihan) brings cooler, drier air, but can also trap pollutants near the coast due to stable atmospheric conditions. February and March are often the months with the poorest air quality. The onset of the wet season, around June, brings significant improvement. Heavy rainfall effectively washes pollutants from the atmosphere, and increased humidity suppresses dust. However, the southwest monsoon (Habagat), while cleansing, can also bring in pollutants from other areas of Luzon. July and August often experience localized flooding, which can stir up mold and other biological pollutants. September and October see a transitional period, with fluctuating air quality. Sensitive groups – children, the elderly, and those with 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 areas. Fog, common during the cooler months, can exacerbate respiratory issues.