HPLC and viability studies to select the CSE concentration for the study. A) HPLC analysis to determine nicotine concentration in CSEshowed that 5% CSE had nicotine concentration comparable to the physiological concentration in a chronic smoker (100 ng/ml) n = 10. (B) Cell viability studies following increasing concentration of nicotine at 24 and 48 h and (C) 5% diluted CSE from ultralow, 1R5F (equivalent to ultralight cigarettes), 3R4F (equivalent to full flavor cigarettes), ultralow nicotine and tobacco free (nicotine free - NF) using MTT assay. Note that 5% CSE from all tested brand but NF did not cause a statistically significant decrease in cell viability. n = 3 individual experiments.