XCELLigence experiment. Real-time cell index (CI) monitoring of A549 cells (n = 3) exposed to 1.5, 3.0, 6.0, 25, and 50 μg/cm2 fumed silica AEROSIL® 200 NPs for 72 h. The black arrow represents the starting point of exposure. Cell index was normalized on baseline (control cells) to ensure inter-dose comparison. The highest concentrations (i.e. 25 and 50 μg/cm2) induced a drastic CI decrease compared to control cells. The lowest concentrations (1.5, 3.0 and 6.0 μg/cm2) indicated a biphasic CI evolution with an early increase followed by a decrease up to the end of the experiment.