Hot Wires and Film Boiling: Another Look at Carbonyl Formation in Electronic Cigarettes

Abstract

Electronic cigarettes (ECIGs) are a class of tobacco products that emit a nicotine-containing aerosol by heating and vaporizing a liquid. Apart from initiating nicotine addiction in nonsmokers, a persistent concern about these products is that their emissions often include high levels of carbonyl species, toxicants thought to cause most noncancer pulmonary diseases in smokers. This study examined whether the phenomenon of film boiling can account for observations of high carbonyl emissions under certain operating conditions and, if so, whether film boiling theory can be invoked to predict conditions where high carbonyl emissions are likely. We measured the critical heat flux for several common heating materials and liquids and carbonyl emissions for several ECIG types while varying the power. We found that emissions rise drastically whenever the power exceeds the value corresponding to the critical heat flux. While limiting the heat flux to below this threshold can greatly reduce carbonyl exposure, ECIG manufacturer operating instructions often exceed it. Product regulations that limit heat flux may reduce the public health burden of electronic cigarette use.

Passive Vaping from Sub-Ohm Electronic Cigarette Devices

“…Under the CHF condition, the resistance temperature increases abruptly, because of the formation of a film of vapor on its surface, which reduces the heat transfer coefficient [ 32 ]. The 365 kW m −2 heat flux of this study is slightly below the range of 389–662 kW m −2 , measured by Talih et al [ 21 ] by submerging kanthal wires in a beaker containing PG; however, deviation from such values can be expected, given the differences of the authors’ experimental setting from a real e-cig atomizer.…”

“…Therefore, increasing the heat flux, by increasing the coil temperature, will also increase the amount of liquid volatilized that, once cooled at room temperature, depending on the vapors pressure, can condense into droplets. The important role played by the coil surface area [ 21 ] is clearly described by the N Tot values at 50 W (133 kW m −2 ) power setting, which, due to the higher coil surface area, are lower than those observed for the 40 W (208 kW m −2 ) vaping session, carried out with a double coil single aerosol exit hole atomizer, for both liquids.…”

“…Table 1 shows the main characteristics of the coils and the relevant wattage tested, together with the relevant heat fluxes, used with the fourth-generation e-cig. Heat fluxes (kW m −2 ) have been calculated as the ratio of electrical power and the heating coil surface area [ 21 ].…”

Highlighting Specific Features to Reduce Chemical and Thermal Risks of Electronic Cigarette Use through a Technical Classification of Devices

“…Furthermore, we highlight an average heat flux (0.43 ± 0.17 W mm −2 ) that looks to be a constant of devices (excepted for Veco plus). In a recent publication [4], authors carried boiling experiments using wire immersed in propylene-glycol and glycerol. They found a critical heat flux of 0.34 W mm −2 for PG and 0.87 W mm −2 for VG.…”

“…In a recent publication [4], authors link these regimens to the produced aldehydes and highlight a significant increase in their productions using powers higher than the maximal power. They also suggest a link between the boiling process and vaping.…”