Correlation of α/γ-Fe2O3 nanoparticles with the toxicity of particulate matter originating from subway tunnels in Seoul stations, Korea

[Display omitted] •Particulate matter (PM) consists of α/ γ-Fe2O3 nanoparticles (NPs) in large quantities.•Synthetic α/γ-Fe2O3 NPs have properties similar to those of Fe2O3 presented in PM.•PM and the α-Fe2O3 NPs show considerable toxic effects, compared to the γ-Fe2O3 NPs.•α-Fe2O3 NPs show the closest correlation with PM harmful effects. According to the increasing concern about particulate matter (PM) pollution at subway systems, particularly its potentially severe effects on human health, this study investigated the constituents, characteristics, and toxicity of PM collected at underground subway stations in Seoul, Korea. It was found that α/γ-Fe2O3 NPs, which are considered as thermal products derived from the brake-wheel-rail interface, were the main components of PM (57.6% and 48% of PM10 and PM2.5, respectively). In addition, hydrothermally synthesized α/γ-Fe2O3 NPs, proposing to possess similar properties to those of Fe2O3 contained in PM, were used to investigate the correlation of these oxides with PM toxicity. In particular, the synthesized γ-Fe2O3 NPs induced a negligibly toxic, while the synthesized α-Fe2O3 NPs and PM showed remarkably toxic effects on HeLa cells and zebrafish embryos, specifically in reducing cell proliferation to 85% and 72% survival, causing high apoptosis of 29.8% and 29.3%, and inhibiting the development of embryos up to 60% and 8% after prolonged exposure, respectively. It is considered that α-Fe2O3 NPs were primarily responsible for the harmful effects of PM, resulting in significant damage to DNA due to their capacity of producing high reactive oxygen species (ROS) and, thus, deleterious effects on the human body.