Transport phenomena governing nicotine emissions from electronic cigarettes: model formulation and experimental investigation

Abstract

Electronic cigarettes (ECIGs) electrically heat and aerosolize a liquid containing propylene glycol (PG), vegetable glycerin (VG), flavorants, water, and nicotine. ECIG effects and proposed methods to regulate them are controversial. One regulatory focal point involves nicotine emissions. We describe a mathematical model that predicts ECIG nicotine emissions. The model computes the vaporization rate of individual species by numerically solving the unsteady species and energy conservation equations. To validate model predictions, yields of nicotine, total particulate matter, PG, and VG were measured while manipulating puff topography, electrical power, and liquid composition across 100 conditions. Nicotine flux, the rate at which nicotine is emitted per unit time, was the primary outcome. Across conditions, the measured and computed nicotine flux were highly correlated (r = 0.85, p<.0001). As predicted, device power, nicotine concentration, PG/VG ratio, and puff duration influenced nicotine flux (p<.05), while water content and puff velocity did not. Additional empirical investigation revealed that PG/VG liquids act as ideal solutions, that liquid vaporization accounts for more than 95% of ECIG aerosol mass emissions, and that as device power increases the aerosol composition shifts towards the less volatile components of the parent liquid. To the extent that ECIG regulations focus on nicotine emissions, mathematical models like this one can be used to predict ECIG nicotine emissions and to test the effects of proposed regulation of factors that influence nicotine flux.

Figure 1

Schematic of an ECIG “tank” or “clearomizer” unit (battery not shown).

Figure 2

Schematic of the ECIG aerosol generation process.

Figure 3

One-zone model (dashed line). Heat and mass fluxes across the control volume are computed in a time-resolved manner, as is its mean temperature and composition.

Figure 4

Nicotine yield vs TPM multiplied by the nicotine mass fraction. r is the correlation coefficient, N is the sample size.

Figure 5

Effects of power, puff duration, and liquid composition (nicotine concentration, PG/VG ratio, and water content) on normalized nicotine flux. Unless otherwise shown, experimental conditions were: 4 Watts, 4 sec puff duration, 1 L/min flow rate, 10 sec interpuff interval, zero water content, and 8 mg/ml nicotine concentration. Lines represent model predictions, while points represent measurements. Error bars: 95% CI. r is the correlation between the measured and predicted nicotine fluxes, N is the sample size.

Figure 6

Effect of power on aerosol composition for binary liquid solutions. PG fraction in the aerosol (wt%) for a 50/50 PG/VG liquid. (Error Bars: 95% CI). 4 sec puff duration, 10 sec interpuff interval and 1L/min flow rate.

Figure 7

Measured vs predicted nicotine flux for three ECIG devices with varying liquid composition, power, and puff duration. (1:2 parity line shown for reference. Error bars: SD)

Figure 8

Model predictions of temperature, liquid composition, and vaporization rate for a 50/50 PG/VG liquid at two ECIG power levels (2W, left; 11 W right). 4 sec puff duration, 10 sec interpuff interval and 1L/min flow rate.