Microplastics bound to fine particulate matter in subway air: A pathway for toxic metal transport to enhanced carcinogenicity in human lungs

This study investigates the role of microplastics (MPs; components of inhalable particulate matter (PM)) in promoting cancer development in the human respiratory system. Findings reveal that fine PM-bound MPs significantly influence inhalation cancer risk (ICR) by transporting carcinogenic metals (CM) deep into the lungs. Airborne PM, MPs, and trace metals were measured in underground and subsurface subway stations in Korea, as well as at control and outdoor sites. The study further assessed ICR, lung deposition, and molecular interactions between CM and PM-bound MPs to clarify how MPs enhance PM toxicity during short-term subway exposure. Underground stations exceeded Korean air quality standards, with PM10 and PM2.5 levels 1.6 and 4.4 times higher, respectively, than outdoor sites. CM and MPs accounted for 76 % of PM10 and were 4.8-6.4 times more concentrated than outdoors. Lung deposition of PM2.5 and PM2.5-bound MPs reached 73-78 %, about twice that of PM10. Negatively charged MPs bound toxic metals, disrupted lung surfactant, and induced oxidative stress, inflammation, and epithelial damage, contributing to chronic respiratory diseases. PM2.5-bound metals contributed 67-84 % of the total ICR, while microplastics (polyamide, polyethylene, polystyrene, and polyethylene terephthalate) made up 88-90 % of lung deposition and showed a strong correlation with Cr and Ni, which together accounted for 90 % of the ICR. Density functional theory (DFT) analyses showed strong van der Waals interactions between CMs and MPs. The high deposition and enrichment of CM-bound to MPs in fine PM pose a severe health threat to subway commuters. This study offers key insights into the noble inherent causes of PM toxicity to protect commuters' health in subway environments.