Comparison of the viability of the cell lines A549 and NCIH441 exposed to gold nanoparticles. The cell lines A549 (A) and NCIH441 (B) in the logarithmic growth phase were exposed to AuNPs for 24, 48 and 72 hr. The cell viability was measured by the MTT assay as described in the Experimental Section. Each result represents the mean viability ± standard deviation (SD) of three independent experiments and each of these was performed in triplicate. Cell viability was calculated as the percentage of the viable cells compared to the untreated controls.