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Khao Yoi, nestled in Phetchaburi Province, Thailand, occupies a geographically significant position within the central plains region. Located at coordinates 13.2402°N, 99.8243°E, the town’s terrain is characterised by a gentle undulating landscape transitioning from the coastal plains towards the foothills of the Tanaosri Range. This proximity to both the Gulf of Thailand and the mountainous interior profoundly influences local weather patterns and, consequently, air quality. The town’s elevation is relatively low, contributing to potential temperature inversions, particularly during the cooler months. Surrounding Khao Yoi are extensive agricultural lands, primarily dedicated to sugarcane, mango, and other tropical fruit cultivation. While not a major industrial hub itself, Khao Yoi benefits from, and is potentially impacted by, industrial activity concentrated further east towards Phetchaburi city and the Bangkok metropolitan area. The urban-rural gradient is pronounced; the town centre represents a concentrated area of activity, while the surrounding areas remain largely agricultural. This mix creates localised pollution sources alongside regional influences. The prevailing winds, often shifting with the monsoon seasons, play a crucial role in dispersing or concentrating pollutants. The town’s location within a relatively sheltered valley can exacerbate pollution build-up under stagnant atmospheric conditions.

Khao Yoi experiences a distinct tropical monsoon climate, shaping its air quality throughout the year. The cool, dry season (November to February) often sees the poorest air quality. Lower temperatures lead to temperature inversions, trapping pollutants from agricultural burning and vehicle emissions near the ground. Wind speeds are typically lower during this period, hindering dispersion. March to May brings the hot season, with increased dust from agricultural activities and potential for localised wildfires, impacting particulate matter levels. The southwest monsoon arrives in June, ushering in the rainy season (June to October). While rainfall effectively washes pollutants from the air, increased humidity can promote the formation of secondary pollutants. The monsoon winds, however, generally improve air circulation, offering a temporary respite. November marks a transition period, with decreasing rainfall and a return to drier conditions. Sensitive groups – children, the elderly, and those with respiratory conditions – should be particularly cautious during the cool, dry season. Outdoor activity is generally more favourable during the monsoon months, though vigilance regarding potential flooding and associated mould growth is advised. Monitoring wind direction is key, as pollutants can be transported from regional sources.