Transcriptomic dose–response curve compared with ATP assay. A549 cells (n = 3) were exposed for 24 h to increasing concentrations of fumed silica AEROSIL® 200 NPs (0.1, 1.0, 1.5, 3.0, and 6.0 μg/cm2). Blue histograms represent the number of differentially expressed genes after statistical analysis with Genespring software (Agilent). The black curve indicates the cell death percentage obtained with the ATP viability test related to the concentration of silica nanoparticles. The NOTEL (no observed transcriptional effect level) was determined to be 1.0 μg/cm2 and the LOTEL (lowest observed transcriptional effect level) as 1.5 μg/cm2. The NOAEL (no observed adverse effect level) was determined as 1.5 μg/cm2 and the LOAEL (lowest observed adverse effect level) at 3.0 μg/cm2.