Health Risk Assessment of Ambient PM2.5-Bound Metals During Haze Seasons in Urban and Rural Chiang Mai, Thailand

Seasonal haze in northern Thailand raises concerns about the health impacts of PM2.5-bound metals. This study assessed PM2.5-bound metals and inhalation health risks across three contrasting land use environments during the haze season in Chiang Mai Province, Thailand: Chiang Mai University (CMU; an urban site), Mae Chaem (MC; an agricultural rural site), and Chom Thong (CT; a roadside rural site). In total, 101 24 h PM2.5 samples were collected during the haze season from 15 January to 22 April 2020. CT showed the highest median PM2.5 concentrations (64.9 µg m−3), metal enrichment, and overall estimated health risks. Notably, although MC (42.1 µg m−3) had lower PM2.5 than CMU (50.3 µg m−3), its non-carcinogenic hazard index (HI = 2.10) and cancer risk (TCR = 2.37 × 10−4) exceeded those of CMU (HI = 1.34; TCR = 1.18 × 10−4), indicating that PM2.5 mass alone may not adequately capture differences in estimated health risks. HI exceeded the reference level (HI > 1) at all sites, with the highest HI at CT (2.69), and TCR exceeded the benchmark (1 × 10−4) at all sites, with the highest TCR at CT (3.63 × 10−4) and chromium as the dominant contributor. Se, Cd, and As showed the strongest anthropogenic enrichment, and PCA identified potential source-related associations consistent with biomass burning, traffic emissions, agricultural activities, and crustal or road dust. These findings show that agricultural and roadside rural environments may present distinct health risk profiles that are not captured by PM2.5 mass alone, underscoring the need for composition-specific mitigation and public health interventions across the urban–rural continuum in northern Thailand.