Vertical Distribution and Health Risk Assessment of SBTEX in Urban Microenvironments: A Comparative Study Between...

This study investigates the spatiotemporal distribution and health risks of styrene, benzene, toluene, ethylbenzene, and xylenes (SBTEX) in Chiang Mai, Thailand, under haze and non-haze conditions. Air samples were collected at ground level (high-traffic parking area) and rooftop height (30 m) to evaluate the influence of sampling height and source proximity, and seasonal biomass burning. The mean SBTEX concentration was 6.67 ± 3.46 μg m^-3, with toluene (44%) and benzene (18%) as the dominant species. Concentrations were higher at ground level (7.99 ± 4.20 μg m^-3) than at rooftop level (5.35 ± 1.72 μg m^-3), reflecting the combined influence of source proximity and vertical dispersion. During haze, ground-level concentrations increased by up to 2.4-fold, particularly in the morning, driven by atmospheric stability and boundary-layer suppression. Diagnostic ratios and principal component analysis identified vehicular emissions as dominant SBTEX sources, with enhanced photochemical processing and vertical stratification during haze. Elevated m,p-X/E ratios further indicated fresher emissions under reduced atmospheric mixing. Health risk assessments showed that non-carcinogenic risk remained negligible (HI < 0.1) for both students and staff. However, lifetime cancer risk (LCR) exceeded the acceptable threshold (10^-6) for staff at ground level, with benzene as the dominant risk contributor. Sensitivity analysis identified benzene concentration as the influential factor controlling risk variability. These findings highlight the role of vertical pollutant gradients and haze in shaping exposure patterns and health risks, while supporting the need for extended-duration and vertically resolved monitoring to characterize exposure and health risks in Upper Southeast Asia.