Airborne microplastics and associated heavy metal contamination in indoor and outdoor environments: Pollution levels and health risks in Metro Manila, Philippines.

Airborne microplastics (MPs) have emerged as environmental pollutants that can co-occur with hazardous contaminants; however, evidence of heavy-metal associations with atmospheric MPs remains limited. This study investigated the abundance, characteristics, metal loadings, and health risks of airborne MPs in Metro Manila, Philippines. Air samples were collected from classroom, laboratory, dining room, office, living room, and store environments, followed by polymer identification via μFTIR and metal quantification using ICP-MS. MP concentrations ranged from 0.074 to 0.595 MP/m3 indoors (mean: 0.297 ± 0.20 MP/m3) and from 0.057 to 0.262 MP/m3 outdoors (mean: 0.173 ± 0.07 MP/m3), with no statistically significant difference. Fibers dominated the MP assemblage (52.38%), followed by fragments and films. PP, PET, and PE were the most prevalent polymers. Heavy metal concentrations ranged from 0.35 to 27521.66 mg/kg indoors and from 0.11 to 30556 mg/kg outdoors, with notable contributions from Zn, Cu, Ba, Pb, and Ag. These showed site-specific variations associated with room function and anthropogenic activity, with elevated Cu and Zn in high-activity indoor spaces. Pollution indices (PLI, MPI) indicated moderate pollution levels, while the ecological risk index indicated moderate ecological risk. Spearman analysis showed strong positive correlations between MPI-HM and As, Fe, and Zn (r = 0.70, p < 0.05), while PCA and HCA differentiated sampling sites by metal- and MP-related profiles. Inhalation risk assessment indicated low-to-moderate non-carcinogenic risk; however, the co-occurrence of toxic metals with respirable MPs highlights the occurrence of hazardous substances within the recovered airborne MP fraction. This study provides one of the first assessments of heavy metals associated with airborne MPs in indoor-outdoor urban environments.