A549 cells were pretreated with 5 µM H2-DCFDA for 2 h prior to exposure to Nano-TiO2 or Nano-Co for 12 h (A). A549 cells were treated with Nano-Co or Nano-TiO2 for 12h and 24h, respectively. 8-OHdG level was assayed by OxiSelect™ Oxidative DNA Damage ELISA Kit. (B). For antioxidant experiments, NAC or catalase was added 2 h prior to adding H2-DCFDA and Nano-Co (C). A549 cells without treatment were used as controls. Values are mean ± SE of six experiments. * Significant difference compared with the control, p < 0.05; # significant difference from the same dose of Nano-TiO2-treated group, p < 0.05; ** significant difference as compared with Nano-Co alone, p < 0.05.