Seasonal Influence of Road Proximity and Classroom Type on School Respirable Dust Levels in Chiang Rai Province, Thailand

This study aims to investigate respirable dust concentrations using personal air samplers with PVC filters at six schools, both proximal and distal to roadways, during dry and wet seasons in Chiang Rai Province, Thailand. Indoor/Outdoor ratios (I/O ratios) were calculated using respirable dust samples in both indoor and outdoor classrooms of schools situated both near and far from roadways to evaluate particle infiltration. Seasonal effects on respirable dust concentrations were examined using meteorological parameters. Respirable dust concentrations during the dry season were substantially higher (80.50±0.94–127.40±1.25 µg/m3) than during the wet season (16.68±0.60–25.98±1.51 µg/m3), highlighting the influence of biomass combustion, dust resuspension, and the absence of precipitation. Schools located near major roadways showed higher concentrations than those farther away, particularly in the wet season (p < 0.05). During the dry season, indoor/outdoor ratios approached unity (0.96–1.00), indicating nearly complete infiltration. I/O ratios varied from 0.86 to 1.00 during the wet season, with nearly covered classrooms providing some protection. This study found that students and educators in northern Thailand are exposed to elevated levels of respirable dust throughout the year. Biomass burning is the primary source in the dry season, while traffic-related emissions and classroom design play a larger role in the wet season. These findings highlight the need for integrated policies that combine regional biomass combustion reduction strategies with localized interventions, such as improved classroom enclosures and air purification systems, to better protect vulnerable schoolchildren.