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Fang, nestled in the northern Thai province of Chiang Mai, occupies a geographically complex position within a mountainous landscape. Situated at coordinates 19.9189, 99.2136, the town lies in a valley carved by the Fang River, a tributary of the Ping River. The surrounding terrain is dominated by the rugged, forested hills of the Chiang Mai range, rising steeply to the east and west. This topography significantly influences air quality, creating natural basins where pollutants can accumulate, particularly during periods of atmospheric stability. Fang’s elevation, around 580 meters above sea level, contributes to cooler temperatures but also increases the likelihood of temperature inversions trapping pollutants near the ground. The urban-rural gradient is pronounced; Fang itself is a relatively small town of 30,000, but is surrounded by extensive agricultural land, primarily rice paddies and fruit orchards, alongside patches of teak forest. While not a major industrial centre, regional agricultural practices – including seasonal burning of agricultural waste – and increasing road traffic contribute to localised pollution sources. Its proximity to the Myanmar border also introduces transboundary influences on air quality, as prevailing winds can carry pollutants across the international boundary. The valley location limits natural ventilation, exacerbating pollution episodes.

Fang’s air quality follows a distinct seasonal pattern dictated by the monsoon and dry seasons. The most challenging period is typically from February to April, during the dry season. Agricultural burning to clear land for the next planting cycle is rampant during these months, releasing vast quantities of particulate matter (PM2.5) into the atmosphere. This coincides with stable atmospheric conditions and reduced rainfall, allowing pollutants to linger. Temperature inversions are common, trapping smoke close to the ground. November to January offers comparatively clearer air, as the cool, dry season begins with less burning. The onset of the monsoon season in May-October brings significant rainfall, effectively washing pollutants from the air and improving air quality dramatically. However, increased humidity can also contribute to the formation of haze. August and September, while wet, can still experience localised pollution from ongoing agricultural activities. Sensitive groups – children, the elderly, and those with respiratory conditions – should limit outdoor exposure during February-April and monitor conditions closely. During the monsoon, mould spores can become an issue for allergy sufferers. Overall, the wet season provides the most favourable conditions for outdoor activity.