The effect of e-liquids on cell viability. MG-63 cells were treated for 48 h with e-liquid containing a final nicotine concentration of 0.01, 0.1, 0.5, or 1.0 mg/ml or an equivalent volume of e-liquid without nicotine at 0.04%, 0.4%, 2.0% and 4.0%, respectively. Cell viability was determined using the MTT assay. Results are expressed as percent cell viability. Each bar represents the mean ± SEM of at least 3 independent experiments. * denotes significant difference from culture medium only control (p <0.05). † denotes significant difference from the matched control without nicotine (p <0.05). The e-liquids tested were (A) Vapor Emporium brand, (B-E) Lotus brand, (F-J) Mister-E-Liquid brand.