Effect of Flavoring Chemicals on Free Radical Formation in Electronic Cigarette Aerosols

Abstract

Flavoring chemicals, or flavorants, have been used to flavor electronic cigarettes (e-cigarettes) since they first appeared on the market. Despite their inclusion in nearly every e-liquid available, little is known about their toxicological effects. Aerosols generated by e-cigarettes contain reactive carbonyls and free radicals capable of inducing oxidative stress that can damage inflammation, proliferation, and survival cellular pathways. Common e-liquid solvents, propylene glycol (PG) and glycerol (GLY), alone produce high levels of free radicals in e-cigarette aerosols; however, few studies have investigated the modulating effect of flavorants on these solvent-derived radicals. The radical production from 49 different nicotine-free e-liquids flavors was evaluated using a temperature controlled e-cigarette device. The free radicals found in the aerosols were captured and analyzed using electron paramagnetic resonance (EPR). Of the 49 flavors analyzed, nearly half of them appeared to modulate the production of free radicals as compared to a base of PG:GLY (60:40). The constituent flavorants in each flavored e-liquid were identified using gas chromatography mass spectroscopy (GC-MS). The relative abundance of each flavorant was correlated with the free radical production from that flavorant's parent e-liquid. Ten compounds with the strongest (positive or negative) correlations were analyzed for their individual impact on free radical production. The flavorants identified and analyzed were β-damascone, δ-tetradecalactone, γ-decalactone, citral, dipentene, ethyl maltol, ethyl vanillin, ethyl vanillin PG acetal, linalool, and piperonal. The aerosols generated from these flavorants were also analyzed for their ability to oxidize biologically relevant lipids in vitro. Concentration-dependent free radical increases were observed with the dipentene, ethyl maltol, citral, linalool, or piperonal. Free radical formation was inhibited in a concentration-dependent manner with the addition of ethyl vanillin. Increases in lipid oxidation byproducts, thiobarbituric acid-reactive substances (TBARS), were seen with the addition of linalool, piperonal, and citral. Increases in 8-isoprostane formation were seen with the addition of δ-tetradecalactone, linalool, dipentene, citral, and γ-decalactone. The addition of ethyl vanillin significantly reduced 8-isoprostane formation. Our results suggest that flavorants play an important role in modulating free radical production in the aerosols of flavored e-cigarettes.