BACKGROUND: Epidemiologic studies have linked exposure to airborne pollutant particulate matter (PM) with increased cardiopulmonary mortality and morbidity. The mechanisms of PM-mediated lung pathophysiology, however, remain unknown. OBJECTIVES: We tested the hypothesis that PM, via enhanced oxidative stress, disrupts lung endothelial cell (EC) barrier integrity thereby enhancing organ dysfunction. METHODS: Utilizing PM collected from Ft. McHenry Tunnel (Baltimore, MD), we assessed PM-mediated changes in transendothelial electrical resistance (TER) (a highly sensitive measure of barrier function), reactive oxygen species (ROS) generation and p38 MAP kinase activation in human pulmonary artery EC. RESULTS: PM induced significant dose (10-100 µg/ml)- and time (0-10 hrs)-dependent EC barrier disruption reflected by reduced TER values. Exposure of human lung EC to PM resulted in significant ROS generation which was directly involved in PM-mediated EC barrier dysfunction as N-acetyl-cysteine (NAC, 5 mM) pretreatment abolished both ROS production and barrier disruption induced by PM. Furthermore, PM induced p38 MAP kinase activation and HSP27 phosphorylation, events which were both attenuated by NAC. Additionally, PM-induced EC barrier disruption was partially prevented by the p38 MAP kinase inhibitor SB203580 (10 µM) as well as by reduced expression of either p38 MAPK or HSP27 (siRNA). CONCLUSIONS: These results demonstrate that PM induces ROS generation in human lung endothelium resulting in oxidative stress-mediated EC barrier disruption via p38 MAP kinase- and HSP27-dependent pathways. These findings support a novel mechanism for PM-induced lung dysfunction and adverse cardiopulmonary outcomes. Funded by EPA/Johns Hopkins PM Center RD832417-010