Chemicals are present in the aerosol produced by electronic cigarettes (e-cigarette) when the e-liquid it contains is heated and aerosolized (Goniewicz et al., 2014; European Parliament, 2014). The e-cigarette refers to the aerosol-generating device, which uses the “e-liquid” to create aerosol by an evaporation–condensation method. If in sufficient quantities, and dependent on the hazard type, chemicals present in an e-cigarette-produced aerosol have the potential to negatively impact health when inhaled (European Association for the Co-ordination of Consumer Representation in Standardisation, 2021). For example, group 1 carcinogens acetaldehyde and formaldehyde have been found in e-cigarette aerosols and are degradation products of the e-liquid base components, propylene glycol and glycerol (Goniewicz et al., 2014). Other chemical ingredients found in unheated e-liquids that pose a risk to human health (e.g. respiratory irritants, sensitizers) may include flavours or solvents added to the e-liquids. These chemicals can be present in unheated e-liquid, heated e-liquid, aerosolized e-liquid, or any combination of the three forms. Yet, as recently as 2015, no country in the world regulated e-liquid ingredients beyond nicotine levels.

As more evidence emerges suggesting that e-cigarette aerosols negatively impact health, regulation is rapidly evolving in this area (European Parliament, 2014; Budzyńska et a., 2020; Therapeutic Goods Administration, 2021). For example, the European Union (EU) Tobacco Product Directive (TPD) states that only ingredients in nicotine-containing e-liquids that do not pose a risk to human health in heated or unheated forms can be used (European Parliament, 2014), and many countries (including United Kingdom, Germany, and France) banned some ingredients as a result (European Parliament, 2014; European Association for the Co-ordination of Consumer Representation in Standardisation, 2021; Budzyńska et al., 2020). A modest ingredient ban for nicotine-containing e-liquids came into effect in October 2021 in Australia, prior to a complete ban on non-prescription e-cigarette (and e-liquid) importation and sale in Australia being announced in May 2023 (Nogrady, 2023; Therapeutic Goods Administration, 2021). Known health effects of banned ingredients include e-cigarette- or vaping-associated pulmonary (or lung) injury (a.k.a VAPI or VALI/EVALI). There is emerging evidence of self-reported lung conditions associated with e-cigarette use (Winnall et al., 2023; Osei et al., 2020; Bircan et al., 2021). Health concerns are amplified by a multibillion-dollar market driving product sales and increasing use in young people. Consequently, pre-market approvals (including unheated ingredient listing), which allow independent determination of product safety (Australian Government, 2023; United States Food and Drug Administration, 2023), are preferred by regulators.

Whilst chemical testing of unheated e-liquids is relatively common, the safety of e-liquids remains largely unassessed due to the sheer scale of the market. Additionally, except for the EU TPD guidance (to the best of our knowledge), testing heated e-liquids for inclusion in pre-market product approval is not required in Australia or elsewhere (European Parliament, 2014; Winnall et al., 2023; Scientific Committee on Health Environmental and Emerging Risks, 2021; Larcombe et al., 2022). Methods to assess chemical content generally involve testing of unheated e-liquids or the e-cigarette aerosol generated, but not the heated e-liquid (Scientific Committee on Health Environmental and Emerging Risks, 2021). However, the heated e-liquid is more representative of what the user inhales compared to the unheated e-liquid and easier to assess when compared to e-cigarette aerosol (Larcombe et al., 2022; Erythropel et al., 2019). Importantly, chemicals present in e-liquids are known to degrade due to heating, by either boiling or evaporative convection (depending on wetted-wick temperature), and this can be exacerbated by the presence of catalytic surfaces such as Kanthal. The secondary products formed, at high (> 200 ∘C) or low temperatures (< 200 ∘C), may have increased or decreased toxicity compared to the parent compound (Erythropel et al., 2019; Floyd et al., 2019; Goniewicz et al., 2014; Jaegers et al., 2021; Li et al., 2021; Zhao et al., 2016; Saliba et al., 2018).

Many approaches have been trialled to address the difficulties in directly assessing the chemical composition of e-cigarette-produced aerosol. Such challenges include collecting enough aerosol to perform an assay (and therefore detect potentially toxic compounds) and overload of the main excipients (propylene glycol and vegetable glycerin). These difficulties induce increasingly complicated test methods, yet current methods have been shown to be outdated or limited (Floyd et al., 2019; Herrington and Myers, 2015; Scientific Committee on Health Environmental and Emerging Risks, 2021). Despite the expansive range of tests, a simple test capable of assessing heated and unheated e-liquids, and the aerosol produced all at once, is yet to be established, but vital.

In this study we aimed to (1) establish and validate a simple “accelerated”, method of heating and aerosolizing e-liquids (i.e. “vaping”) that would be comparable to, but quicker than, the “typical” method of heating and aerosolizing e-liquid and (2) assess the chemical composition of the heated e-liquid and the aerosol produced all at once. We hypothesized that the accelerated method and the commonly used typical vaping method would result in similar heating-induced chemical changes in the e-liquids. The two heating methods were compared by measuring the presence and concentration of a range of chemicals. We hope that this accelerated test methodology can form the critical first step in establishing a rapid test for screening of e-liquids for banned substances.

Over all e-liquids, 3 of the 13 compounds tested for were not detected in any e-liquid form (4-(4-methoxyphenyl)-2-butanone, thymol, 2-chlorophenol) (Table 1).

The accelerated method used here is simple and cost-effective and has the potential to produce heated e-liquid and aerosol in a similar manner to an e-cigarette but on an accelerated time frame, allowing chemical assessment in a single experiment. Due to the prohibitive costs of commercially available vaping machines, many “in-house” simplified methods and set-ups (e.g. e-cigarette puffing machines) have been developed. However, to the best of our knowledge, they all focus on generation of e-cigarette aerosol and not on assessment of the heated e-liquid (Palazzolo et al., 2021).

Comparison of the accelerated and typical heated samples with their unheated counterpart showed that in ∼ 70 % of the e-liquids tested the heating methods demonstrated a similar trend, i.e. increase or decrease in chemical concentration. The four chemicals implicated in the five differing comparisons were mostly aldehydes (ethyl vanillin (2/11), furfural (1/11), and benzaldehyde (1/11)) except for ethyl maltol (1/11), being an alcohol. In two chemicals – ethyl vanillin (2/11) and ethyl maltol (1/11) – an increase in chemical concentration with the accelerated heating method compared to the typical method was detected. The observed “increase” with the accelerated method can be attributed to a loss of aerosol with the typical vaping experimental method. Rainout (recondensation of the aerosol as it cools) of the liquid aerosol was observed in the three-way tap system and the very thin tubing (ID ∼ 4 mm) connecting the tap system to the 27 L chamber (Fig. 1a). A modification of the design (e.g. larger tubing) would reduce the rainout losses, thus increasing the yield from the impinger with the typical method and improving reproducibility between methods. It is also possible that the flavour aldehydes were present in their propylene glycol acetal form instead of their aldehyde form, as aldehydes are known to form acetals readily (Erythropel et al., 2019). The inability to fully capture the aerosol from the typical heating method due to rainout meant we are unable to confirm the suitability of our accelerated heating method as an impinger for aerosol, but only its validity to compare heating methods.

The remaining two discrepancies involved furfural (1/11) and benzaldehyde (1/11), and these compounds were found in increased quantities with the typical method compared to the accelerated method. Because these two discrepancies contain low-molecular-weight and low-boiling-point products we suspect they may have evaporated more readily (compared to the typical method) and that our accelerated method was simply unable to capture compounds with low boiling point that volatilize easily (Erythropel et al., 2019). For example, furfural has the lowest boiling point and molecular weight of all chemicals detected (162 ∘C, 96.09 g mol-1) and was detected to have decreased from the unheated sample in both butterscotch and tiramisu flavours. While our study design angled the beaker at 45∘ to allow re-condensation of any vapour on the beaker wall, the experiment was carried out in a ventilated fume hood, which may have increased the loss of highly volatile compounds. In future studies, the addition of a lid on the angled beaker as well as monitoring the liquid temperature on the wick (or surrounding the wick) may help to reduce discrepancies to allow full validation of the method for detection of aerosols (Li et al., 2021; Palazzolo et al., 2021; Bitzer et al., 2018). Further studies should also be designed to consider the solubility of the flavouring compound in the base excipients (i.e. propylene glycol or glycerol) and not the boiling point, as this is suggested to be a major indicator of whether a compound will be detected in and therefore carried over into an aerosol (Erythropel et al., 2019).

Undetected chemicals included 4-(4-methoxyphenyl)-2-butanone, thymol, and 2-chlorophenol. Considering that no “fruity” flavours were assessed, it is less surprising that 4-(4-methoxyphenyl)-2-butanone was undetected, as it is a raspberry ketone methyl ether – a common flavouring in “berry”-flavoured e-liquids. Although we were looking for thymol because of its use as a precursor for racemic menthol (produced from m-cresol), its absence may be explained because pulegone and other terpenoids are also used as the precursor (Dylong et al., 2022). Additionally, menthol was not detected in either tiramisu or choc caramel flavours, so perhaps synthetic analogues were present (where menthol was not detected) such as N-ethyl 2-isopropyl-5-methylcyclohexanecarboxamide (trade name WS-3). As the demand for menthol increases, alternative methods to produce L-menthol are on the rise, such as from citronellal (Dylong et al., 2022).

Chlorophenols like 2-chlorophenol have previously been detected in e-liquids probably because they are notorious environmental contaminants. Particularly, 2-chlorophenol is a priority contaminant in both the US and EU and has previously been found in e-liquids (Larcombe et al., 2022; Chivers et al., 2019; Igbinosa et al., 2013); 2-chlorophenol is used for many applications, predominantly as a detergent but also as an intermediate in the manufacturing of agricultural chemicals, pharmaceuticals, biocides, and dyes. Therefore, it is commonly detected in environmental water samples after being discharged from industrial effluents (Igbinosa et al., 2013; Yahaya et al., 2019). It has previously been suggested that 2-chlorophenol may be a contaminant from the glycerin excipient for two reasons: (1) vegetable glycerin is made from plant crops such as canola, and 2-chlorophenol has been found in canola as a pesticide residue, and (2) glycerin (not from plants) is a by-product of biodiesel production, and biodiesel can be made with canola (Abdel-Gawad and Hegazi, 2010; Yahaya et al., 2019). It is possible that derivatives of 2-chlorophenol or other phenolic derivatives known to be priority contaminants were present. However, it was not within the scope of the study to assess these compounds as they are not commonly reported to be found and/or tested for in e-liquids. Eugenol and trans-cinnamaldehyde were the least detected compounds; they were found in only one flavour e-liquid. Cinnamaldehyde might not be detected because it is known to form propylene glycol acetals (αβ, unsaturated aldehyde), and this form is known to be relatively reactive (Erythropel et al., 2019).

In summary, the accelerated method described here is a suitable screening tool for rapid chemical assessment of heated e-liquids and their aerosols that mimics typical e-cigarette vaping on an accelerated time frame. The EU TPD recommendation is to assay heated e-liquids; however (to the best our knowledge) there has only been one previously published study on the effects of ageing and/or heating on e-liquids. We propose that this method (with our recommended improvements) can be used as a standardized screening tool for e-liquids and their aerosols to identify potentially harmful chemicals, such as those recently banned in Australia or previously banned in Europe and the United Kingdom. With minor modification, this test could be used prior to importation or sale to ensure that only tested products, containing approved ingredients, reach the consumer. In the absence of an approved therapeutic goods status for e-cigarettes, the type of high-throughput testing described here is necessary as a minimal precaution to assess and reduce the potential harms of a consumer product that is generally accepted in the public to be a less harmful alternative to smoking.