PM (100 μg/ml) induced substantial time-dependent ROS production in microvascular ECs as measured by DCFDA oxidation, which peaked around 30-60 min (Additional file 1: Figure S4 ). EC pretreatment with N-acetyl-cysteine (NAC, 5 mM, 1 hr), an ROS scavenger, or PEG-catalase (PEG-CAT, 250 U/ml, 1 hr), which degrades H 2 O 2 , prevented PM-induced DCFDA oxidation by ROS (Additional file 1: Figure S4 ). PM produced a sustained timedependent decrease in TER, with a maximal effect observed at 100 μg/ml (80% decrease in TER), which is similar to its effects on other endothelial cell types, as we have reported previously [13] . NAC pretreatment (5 mM, 1 hr pretreatment) prevented PM-induced ZO-1 degradation ( Figure 1B ), while NAC (5 mM, 1-24 hr) does not change ZO-1 protein levels by itself (Additional file 1: Figure S5 ). After ECs were treated with PM (100 μg/ml, 6 hr), ZO-1 was relocated from the cell periphery and degraded, which was followed by gap formation between ECs. VE-cadherin, on the other hand, underwent no relocation or degradation ( Figure 1C ). NAC pretreatment (5 mM, 1 hr pretreatment) prevented PM-induced ZO-1 relocation and gap formation ( Figure 1C ). These data strongly indicate that PM causes EC barrier disruption selectively via oxidative tight junction protein degradation. NAC (5 mM) or PEG-CAT (250 U/ml) pretreatment significantly inhibited PM-induced EC barrier disruption as measured by TER ( Figure 1D ). We also examined the effects of another ROS scavenger EUK-134 on PMchallenged ECs. As NAC, EUK (5 μM, 1 hr pretreatment) attenuated PM-induced ZO-1 degradation and TER reduction (Additional file 1: Figure S6 ). These results further confirmed that PM induces ROS-dependent EC barrier disruption.