Chemical characterisation of the vapour emitted by an e-cigarette using a ceramic wick-based technology

E-cigarettes are battery-powered devices designed to deliver nicotine and/or other substances including, in some cases, flavourings. Although e-cigarettes were first proposed in 1927 by Joseph Robinson¹, it was only in the early 2000s that the 1st generation of e-cigarettes or ‘cig-a-likes’ became commercially available^2,3,4. Subsequent generations of devices have evolved since then, ranging from e-cigarettes with prefilled or refillable cartridges (2nd generation) to rechargeable tank-style devices (3rd generation) with modifiable or ‘‘Mods’’ components^3,4,5,6. The 4th generation of devices, known as ‘Pods’, has been driven by advances in electronic atomization technology^3,7,8,9.

E-cigarettes consist of a mouthpiece, an e-liquid chamber, an atomiser and a battery. The atomiser has a wicking material that draws the e-liquid onto a battery-powered heating coil. Optimal vapour production depends on an efficient supply of e-liquid to the heating coil, which is limited by the wicking and rate of e-liquid evaporation^10,11,12. Power levels that produce aerosol beyond the ability of the wick to resupply the liquid to the coil may result in overheating of the atomizer coil and consequently overheating of the e-liquid^10,11. Different types of wicking material, varying in size and shape, have been used in e-cigarettes^3,13. Silica was commonly the first material to be used as a wick, followed by cotton and ceramic^3,13,14,15. Cotton has good wicking properties but is less thermally stable than silica^14,16,17, while ceramic is chemically stable and heat-resistant¹⁸. The use of microporous ceramic as a wicking material has increased in the past few years^{14,16,18,19,20}. Its application has been reported to improve heating efficiency and reduce charring^{14,16,18,19,20}.

E-liquids are an important part of any vaping system and their composition, together with the characteristics of the device, may have an impact on nicotine delivery²¹. They mainly constitute a mix of propylene glycol (PG), glycerol (vegetable glycerine or VG) and nicotine. E-liquids may include flavouring compounds and usually come in different nicotine strengths or concentrations.

To help adult users to completely switch to alternative nicotine products, it is important the other alternatives provide effective nicotine delivery comparable or close to that of conventional/combustibles cigarettes^22,23. Heavy smokers (12.4 ± 8.4 cigarettes per day, n = 11) have found that e-cigarettes, especially those from the 1st generation, were unsatisfactory because delivery of nicotine was ineffective as compared with conventional cigarettes²². Later generations of devices have achieved improved nicotine delivery by using different product designs and power settings, innovative materials, and nicotine salts in e-cigarette formulations^{3,21,22,24,25}. For example, Bowen and Xing²⁴ reported that a combination of nicotine with some weak organic acids, such as benzoic, lauric, levulinic, salicylic or sorbic acid, provides satisfaction comparable to that of conventional cigarettes. They suggested that the satisfaction effect was consistent with an efficient transfer of nicotine to the user’s lungs and a rapid rise in nicotine absorption in the plasma²⁴. Use of lactic acid and pyruvic acid has been investigated by other authors, who reported nicotine absorption kinetics that are similar to those of conventional cigarettes and associated with acceptable sensory qualities and relief of craving^23,25,26,27. A combination of nicotine with weak organic acids to form nicotine salts has also been applied in pharmaceutical formulations used in Metered Dose Inhalers (MDIs) therapy equipment²⁸. Its application in e-cigarette formulations has the potential to mimic cigarette smoking’s nicotine pharmacokinetics, which may help cigarette smokers to transition to e-cigarettes^{22,23,25,26,27,29,30,31,32}.

E-cigarettes do not burn tobacco and may produce less harmful and potentially harmful constituents (HPHCs) as compared with combustible cigarettes^{6,33,34,35,36,37}. HPHCs have been defined by the US Food and Drug Administration (U.S FDA) as chemicals or chemical compounds in tobacco products or tobacco smoke that cause or might cause harm to smokers or non-smokers^38,39. E-cigarettes have been recognised as an alternative for adult smokers who are unable or unwilling to quit smoking^{35,37,40,41,42,43,44,45,46}. The most recent Public Health England evidence review highlights, as a key finding, a study suggesting that the cancer potencies of e-cigarettes were largely less than 0.5% of those of smoking⁴². The risks of cardiovascular disease and lung disease have not been quantified for e-cigarettes, but are also likely to be substantially less than those from smoking⁴². Because e-cigarettes do not burn tobacco, the reduction of harmful substances depends on the chemical composition of the e-liquid, as well as the characteristics of the device^{4,5,15,47,48,49}. For example: overheating of e-liquid on the coil and poor wicking performance may lead to an increase in carbonyls to levels higher than observed in cigarette smoke^{11,15,47,50,51}.

Compared with silica and cotton wicking materials, there are fewer studies on ceramic wick-based e-cigarette systems, and their impact on e-cigarettes emissions is less documented in the literature. To address this gap, the aim of this study was to characterise the vapour emitted by a 4th-generation pod e-cigarette designed with a ceramic wick-based technology using ISO 20768:2018 standard puffing regime (55 mL puff volume/3 s puff duration/30 s puff frequency; rectangular puff profile)⁵². The emissions of two Berry Blast flavoured e-liquids with different levels of nicotine and different nicotine salts (BB57 with 57 mg mL^–1 of nicotine containing lactic acid and BB18 with 18 mg mL^–1 of nicotine containing benzoic acid) were tested for a total of 89 organic compounds covering different classes of compounds (e.g., nicotine and non-nicotine toxicants). From those, 55 compounds have been listed by the U.S. FDA as relevant to tobacco products and with 19 compounds proposed by the FDA as HPHCs of specific concern in e-cigarette aerosols^{38,39,44,53,54}. We also focused on the nine toxicants (acetaldehyde, acrolein, benzo[a]pyrene, benzene, 1,3-butadiene, carbon monoxide (CO), formaldehyde, nitrosonornicotine (NNN) and 4-(N-nitrosomethylamino)-1-(3-pyridyl)-1-butanone (NNK) recommended for mandated reduction in cigarette smoke by the WHO Tobacco Product Regulation Group (WHO TobReg) which are also part of the HPHCs U.S.FDA list^53,54,55. To provide context, e-cigarette vapour emissions were compared with smoke yields from a reference cigarette (Kentucky 1R6F (Ky1R6F)) smoked under ISO 20778:2018 puffing regime (55 mL puff volume/2 s puff duration/30 s puff frequency; bell-shaped puff profile, 100% ventilation blocked)^56,57.

Carbon monoxide, aerosol mass and water

Table 1 summarizes the per-puff levels of CO, aerosol collected mass (ACM), water and nicotine in the emissions from two e-cigarettes: namely, Berry Blast 57 mg mL⁻¹ of nicotine containing lactic acid (BB57); and Berry Blast 18 mg mL⁻¹ of nicotine containing benzoic acid (BB18). CO, which is associated with combustion of organic material, was below the limit of detection (< LOD) for both e-cigarettes, with a percentage reduction of 99.8% relative to Ky1R6F cigarette smoke (Table 1). ACM, which comprises mainly PG, VG, water, nicotine and other minor constituents, was in the same range for both e-cigarettes. ACM results were found to be reproducible across all methods as demonstrated by the low standard deviation of ACM in both e-cigarette emissions (6.58 ± 0.39 mg puff⁻¹ and 6.46 ± 0.36 mg puff⁻¹ for BB57 and BB18 respectively), accounting for a coefficient of variation of 5.9% and 5.5% for BB57 and BB18, respectively (n = 85). This is an indication of sampling robustness and puffing consistency. The nicotine-free dry particulate matter (NFDPM) or ‘tar’, a parameter associated with cigarette smoke, consists predominantly of combustion by-products^36,58. The level of NFDPM, 3.67 ± 0.30 mg puff⁻¹ equivalent to 33 ± 3 mg cig⁻¹, was in accordance with the Ky1R6F certified value of 29 ± 2 mg cig⁻¹ (ISO Intense smoking regime)