Cytotoxic and oxidative effects of commercially available propylene glycol (PG) and vegetable glycerin (VG): Common humectants in electronic cigarettes

The effects of commercially available brands of common e-liquid humectants - propylene glycol (PG) and vegetable glycerin (VG) are not known. This study evaluates the toxicities of commercially available PG and VG mixtures compared to their analytically pure grade chemicals. The pH and the viscosity of 50:50 PG/VG mixtures from four different commercially available brands and a pure chemical mixture were measured. Commercially available brands of PG/VG generally had higher pH (up to ∼6.25) and viscosity values (up to 16.4 %) than the pure chemical mixture (pH = 5.5; viscosity = 12.9 %). GC-MS spectra provide evidence of impurities in commercially available PG, but not VG liquids when compared to pure chemicals. While we observe variation in the level of acellular reactive oxygen species generated by commercial mixtures as compared to the pure chemical, the protein carbonylation levels were comparable for both the mixtures, thus indicating variation in the nature of the oxygen species generated upon aerosolization. No noticeable change was observed upon studying the human monocytic U937 cellular responses upon treatment with sub-toxic concentrations of PG/VG mixtures; however, a decrease in the basal IL-8 production was observed at higher concentrations (1 %) of PG/VG treatment. The commercially available PG/VG mixtures show comparable oxidative and biological effects to the pure chemical. However, GC-MS data identified impurities which may explain our observation of a variable ROS profile in each PG/VG mixture. Future work on the brand-specific toxicity of PG/VG or its usage is warranted to inform regulatory guidelines for e-cigarettes.