Effects of Solvent and Temperature on Free Radical Formation in Electronic Cigarette Aerosols

Abstract

The ever-evolving market of electronic cigarettes (e-cigarettes) presents a challenge for analyzing and characterizing the harmful products they can produce. Earlier we reported that e-cigarette aerosols can deliver high levels of reactive free radicals; however, there are few data characterizing the production of these potentially harmful oxidants. Thus, we have performed a detailed analysis of the different parameters affecting the production of free radical by e-cigarettes. Using a temperature-controlled e-cigarette device and a novel mechanism for reliably simulating e-cigarette usage conditions, including coil activation and puff flow, we analyzed the effects of temperature, wattage, and e-liquid solvent composition of propylene glycol (PG) and glycerol (GLY) on radical production. Free radicals in e-cigarette aerosols were spin-trapped and analyzed using electron paramagnetic resonance. Free radical production increased in a temperature-dependent manner, showing a nearly 2-fold increase between 100 and 300 °C under constant-temperature conditions. Free radical production under constant wattage showed an even greater increase when going from 10 to 50 W due, in part, to higher coil temperatures compared to constant-temperature conditions. The e-liquid PG content also heavily influenced free radical production, showing a nearly 3-fold increase upon comparison of ratios of 0:100 (PG:GLY) and 100:0 (PG:GLY). Increases in PG content were also associated with increases in aerosol-induced oxidation of biologically relevant lipids. These results demonstrate that the production of reactive free radicals in e-cigarette aerosols is highly solvent dependent and increases with an increase in temperature. Radical production was somewhat dependent on aerosol production at higher temperatures; however, disproportionately high levels of free radicals were observed at ≥100 °C despite limited aerosol production. Overall, these findings suggest that e-cigarettes can be designed to minimize exposure to these potentially harmful products.

Figure 1.

Logic flow diagram for a temperature control e-cigarette.

Figure 2.

E-Cigarette setup. (a) The relay timer switches the e-cigarette on and opens the solenoid valve. (b) The e-cigarette’s pushbutton switch has been shorted enabling it to be turned on when the relay switch is closed. (c) The e-cigarette aerosol is pulled into the impinger that is filled with a spin trap to capture free radicals. (d) The solenoid valve opens, which pulls a vacuum when the relay switch is closed allowing the aerosol to be pulled through the impinger. (e) The flow meter is adjusted to ensure that the volume pulled for each puff is consistent.

Figure 3.

Effects of solvent on radical production. (a) Radical production under constant-temperature mode at different temperatures and different solvent compositions. (b) Radical production under constant-wattage mode at different temperatures and different solvent ratios. Different letters indicate significant differences within the temperature or wattage group.

Figure 4.

Effects of constant temperature and constant wattage on radical production for a typical e-liquid solvent (75:25). (a) Effects of different constant temperatures on radical generation in a 75:25 (PG:GLY) solution. (b) Effects of different constant wattages on radical generation in a 75:25 PG/GLY solution. Different letters indicate significantly different values. (c) Sample PBN spectra of the different constant temperatures. (d) Sample PBN spectra of the different constant wattages.

Figure 5.

Coil temperatures based on modes. (a) Temperature changes based on different coil resistances and different wattages using a constant-wattage mode. (b) Temperature changes based on different coil resistances and different set temperatures using a constant-temperature mode. Different letters indicate significant differences.

Figure 6.

Effects of constant-temperature and constant-wattage radical production on a per gram solvent consumption basis for a typical e-liquid solvent (75:25). (a) Effects of different constant temperatures on radical generation in a 75:25 PG/GLY solution on a per gram solvent consumed basis. (b) Effects of different constant wattages on radical generation in a 75:25 PG/GLY solution on a per gram solvent consumed basis. Different letters indicate significantly different values between temperatures and wattages only.

Figure 7.

Effects of solvent on lipid peroxidation markers under constant-temperature conditions (200 °C). (a) Thiobarbituric acid-reactive substances (TBARS) formed by different PG:GLY ratios and numbers of puffs. (b) 8-Isoprostane formation from different PG:GLY ratios and numbers of puffs. Different letters indicate significantly different values.