Dihydroethidium (DHE) was used to image superoxide in Generation 1 A549 cells. DHE (10 μM) was incubated with cells for 30 min prior to imaging and then rinsed with clear MEM. A. Untreated control cells. B. TiO2 NP-treated cells (994 μg/mL, 24 hr, 37°C). C. Co-incubation of TiO2 NPs (994 μg/mL, 24 hr, 37°C) with SOD (50 U/mL), a superoxide scavenger. D. Incubation with polystyrene (PS) NPs (40 pM, 24 hr, 37°C). Cold-bound TiO2 NPs (994 μg/mL, 10 min, 4°C) did not result in a DHE signal (Figure S7).