18.5 Chemicals in e-liquids and e-cigarette aerosols

Last updated: Aug 2022 Suggested citation: Winnall, WR, Greenhalgh, EM and Scollo, MM. 18.5 Chemicals in e-liquids and e-cigarette aerosols. In Greenhalgh EM, Scollo, MM and Winstanley, MH [editors]. Tobacco in Australia: Facts and issues. Melbourne: Cancer Council Victoria; 2023. Available from  https://www.tobaccoinaustralia.org.au/chapter-18-e-cigarettes/18-5-chemicals-in-e-liquids-and-e-cigarette-aerosols

 

E-cigarette use in Australia and many other countries is rapidly increasing, particularly in school-aged children and young adults (see Section 18.3). E-cigarettes have been promoted as a lower-risk substitutes for cigarettes for those smokers who have been unable to quit nicotine, but many cigarette smokers who take up vaping continue to use both products (known as ‘dual use’). Non-smoking adults and children are also using e-cigarettes in concerning numbers.

In all but one Australian jurisdiction (Western Australia), non-nicotine e-cigarettes may be sold by retailers to people aged 18+, however, nicotine has been detected in many brands that don’t state it on the label in Australia. ¹ E-cigarettes containing nicotine may only be sourced legally in Australia by people with a valid prescription who purchase a product from a pharmacy or import a product via the Personal Importation Scheme (see Section 18.13).

This section focuses on e-cigarette users’ exposure to chemicals found in e-liquids and in the aerosol emitted during the use of an e-cigarette, referred to as ‘vaping’. People may also be exposed to chemicals in a variety of ways during mixing and handling of e-liquids, via the inhalation of secondhand aerosol and through accidental ingestion. This section examines:

• E-liquids and aerosols from e-cigarettes
• Ingredients, reaction products and contaminants in e-liquids and e-cigarette aerosols
• Chemicals detected in e-liquids and e-cigarette aerosols, including ‘ingredients’; chemical reaction products; contaminants; and comparisons with tobacco cigarettes
• E-cigarette device types and modes of use that affect exposure to chemicals
• Health concerns associated with specific chemicals in e-liquids and e-cigarette aerosols, and
• DNA adducts and biomarkers for e-cigarette exposure

A summary of the main points from this section:

E-cigarette aerosols contain hundreds of chemicals, with many having associated health and safety concerns. Most of the chemicals with potential toxic effects are present at very low levels, so the dose of each chemical during a single puff of an e-cigarette is quite low. The low doses of most of these chemicals are not associated with known toxicities, but for many, there is little data available—so this lack of known harms is not a good indicator of risk. Dose, however, is not the only factor affecting the risk of health effects from these chemicals: more frequent use, longer duration of use, the route of entry through the lungs and the mix of many chemicals with potential to do harm in e-cigarettes—all increase the risk of harm.

The health effects of almost one-third of the chemicals in e-cigarette aerosols are unknown, and over 80% have little information on inhalation toxicity. For chemicals where the health concerns have been investigated, they are usually tested in the short term only. For most of the chemicals in e-cigarette aerosols, the effects via inhalation are also unknown, and cannot be assumed to be safe. There is also great variety in the unregulated construction and use of e-cigarette devices, some of which might be shedding heavy metals from the internal construction and/or heating coil. ² The use of custom- and home-made e-liquid mixes, and the decay of their constituent chemicals over time also makes it difficult to determine the exact types and doses of chemicals to which users are exposed.

Some chemicals with potential toxic effects have been banned by Australia’s Therapeutic Goods Administration (TGA) in prescribed (i.e., nicotine-containing) e-cigarettes. Two of these (benzaldehyde and cinnamaldehyde) are found in a high proportion of e-cigarettes available internationally and in Australia, both in nicotine and non-nicotine e-cigarettes (see Section 18.5.5.3).

Most chemicals with toxicity concerns in e-cigarettes are present at considerably lower levels than in conventional cigarettes but this does not mean that e-cigarettes can be assumed to be harmless. Non-smokers, including children and young adults, are using e-cigarettes in increasing numbers. E-cigarette use increases the risk of harm in this group compared with no e-cigarette use at all. E-cigarettes may be beneficial for individuals who smoke and use them to quit smoking completely. However, a false sense of safety resulting from comparing chemical emissions of cigarettes with e-cigarettes and a sustained nicotine addiction may lead to long-term vaping, the health risks of which are unknown. The most common pattern of use is vaping combined with smoking (‘dual use’), which may further increase health risks beyond use of either product alone.

There are known pulmonary and cardiovascular effects for e-cigarette users (see Section 18.6). These health effects are likely to be caused by exposure to the chemicals in e-cigarette aerosols.

Given the lack of data on long-term inhalation toxicity for many e-cigarette chemicals, and the known pulmonary and cardiovascular effects after short-term use, there is currently insufficient evidence to support the safety of long-term e-cigarette use.  

18.5.1 E-liquids and aerosols from e-cigarettes

E-cigarettes heat a mix of chemicals, called an e-liquid, to produce an aerosol that is inhaled by the user. E-cigarettes expose users and those in the vicinity to a variety of chemicals, many of which have specific health concerns.

E-liquids used in e-cigarette devices vary in their contents, mostly due to the wide variety in flavouring chemicals mixed to create e-liquid flavours. E-liquids usually contain numerous flavouring chemicals and solvents and some contain nicotine and/or other drugs, such as those derived from cannabis. ³ Some e-liquids contain naturally extracted tobacco liquids (NET liquids) mixed with other chemicals. ⁴

The emissions from an e-cigarette are an aerosol, consisting of tiny droplets of chemicals that float in gases, as described for tobacco in Section 12.4.1.1. Tobacco smoke is also an aerosol, but the range and amounts of the chemicals in tobacco emissions differ to those in e-cigarette emissions. ³ Mainstream e-cigarette emissions are those that are drawn by the user and inhaled—either through ‘direct-to-lung’ use or collected in the mouth then inhaled into the lungs (‘mouth-to-lung’ use). ⁵

E-cigarettes produce aerosol particles in similar numbers and sizes to those produced by conventional cigarettes. E-cigarette aerosol particles are typically between 11 nm and 560 nm in diameter; ^{3 , 6-8} a particle size that is expected to be deposited in the small airways and alveoli (air sacs) of the lungs. ³ Modelling studies predict that billions of e-cigarette aerosol particles would be deposited throughout the airways after a single two-second puff, more than double the number for conventional cigarettes, with deposition primarily in the alveoli. ^{8 , 9}

Aerosols that are exhaled by the user potentially expose non-users to chemicals in the secondhand aerosol. Unlike cigarettes and other tobacco products that are lit, e-cigarettes produce minimal side-stream emissions (described in Section 12.4.1.2 for tobacco smoke), so secondhand aerosols mostly consist of those exhaled from users. There is conclusive evidence that e-cigarette use results in increased airborne particulate matter pollution in indoor environments. ¹⁰ Significant increases in propylene glycol, glycerol, nicotine and volatile organic compounds (VOCs) have been detected after indoor e-cigarette use. ³

18.5.2 Ingredients, reaction products and contaminants in e-liquids and e-cigarette aerosols

E-liquids and aerosols from e-cigarettes contain intentionally added chemicals (often referred to as ‘ ingredients’), contaminants (such as those emanating from the e-cigarette device), and reaction products from chemical reactions occurring during storage of the e-liquid or during heating to form aerosols. ^{3 , 11}

E-liquids contain hundreds of intentionally added chemical ingredients, including flavours and solvents such as glycerol or propylene glycol, and often contain drugs such as nicotine. Aside from the chemicals intentionally added to e-liquids, they contain contaminants (such as metals, that may have entered from the heating coil or other parts of the e-cigarette device) and reaction products (from chemical reactions occurring in the e-liquid during storage). ³

Aerosols produced by e-cigarettes contain chemicals from the e-liquids (intentionally added chemicals, reaction products and contaminants) as well as new reaction products produced by chemical reactions during heating that produces the aerosol. ³ Most of the chemicals in e-cigarette aerosols are glycerol, propylene glycol, water and nicotine, and with roughly 3% of constituents being flavours, contaminants and reaction products.

18.5.3 Chemicals detected in e-liquids and e-cigarette aerosols

Several comprehensive reviews from Australian and international agencies have examined the evidence predicting the risks of exposure to the chemicals in e-liquids and e-cigarette aerosols. Readers are directed to comprehensive reports published in 2018 by the US National Academies of Science, Engineering and Medicine, ¹² in 2019 from the National Industrial Chemicals Notification and Assessment Scheme, ³ the 2022 report of the National Centre for Epidemiology and Population Health at the ANU ¹⁰ and the 2022 CEO’s statement from the National Health and Medical Research Council (NHMRC). ^{13 , 14}

In 2019, the National Industrial Chemicals Notification and Assessment Scheme (NICNAS) published a report on the chemistry and health concerns of chemicals that are found in non-nicotine e-cigarettes (referred to here as the ‘NICNAS report’). ³ In Australia, non-nicotine e-cigarette chemicals are regarded as industrial chemicals. While it does not formally regulate e-cigarettes, NICNAS has provided in this report an assessment and advice regarding the safety of use of the detected chemicals. The NICNAS report produced lists of chemicals found in e-cigarettes based on thorough literature searches. This report listed chemicals found in one or more brands of e-liquid; not all these chemicals are in every e-cigarette. Chemicals were reported if detected in ready-to-use e-liquids or e-liquid concentrates for dilution. ³

The NICNAS report included chemicals that have been detected in e-liquids with and without nicotine: NICNAS considered that the same chemicals are found in both types. ³ it considered that the concentration and reactivity of nicotine in e-liquids is such that any reactions are unlikely to substantially alter the chemical composition of other substances in e-liquids. The NICNAS report therefore states that ‘…chemical information obtained from nicotine-containing e-cigarette liquids is also applicable to liquids without nicotine, although consideration should be given to potential confounding chemical effects of nicotine.’ ³

18.5.3.1 Chemical ‘ingredients’ found in e-liquids and e-cigarette aerosols

The NICNAS report identified a total of 243 different chemical ‘ingredients’ in e-liquids and/or e-cigarette aerosols. ³ Most of these chemicals were flavourings (235 out of 243). Eight chemicals had other roles such as solvents. Solvents such as glycerol and propylene glycol constitute the majority of e-liquids, with one study detecting them at 80–97% of the e-cigarette liquid (by weight). ¹⁵ Individual flavouring chemicals are commonly detected at between 1 mg/ml and 10 mg/ml, but the highest (limonene) was found at over 70 mg/ml (7% by weight). One study found that the typical total proportion of flavourings in e-liquids constituted between 0.26% to 4.3% of the contents. ¹⁶

The full list of these 243 chemicals can be found in Tables A2 and A4 of the NICNAS report. ³

18.5.3.2 Chemical reaction products in e-cigarette aerosols

The NICNAS report lists 27 chemicals in e-cigarette aerosols predicted to be reaction products— those produced from chemical reactions during the heating and use of the e-cigarette and during storage of the e-liquid after mixing. ³ Most of these 27 reaction products are aldehydes (a type of carbonyl compound) such as acetaldehyde, acrolein and formaldehyde, the effects of which are described in Section 12.4 3.2. Aldehydes produced during e-cigarette use are predicted to come from chemical reactions that involve the solvents glycerol and propylene glycol ¹⁷ and there is evidence that they are also derived from some flavouring compounds. ³ A list of these 27 reaction products is available in the NICNAS report. ³

Reaction products may also be produced by chemical reactions between flavourings and propylene glycol solvent during storage or as degradation products of flavouring chemicals such as limonene. ³

18.5.3.3 Contaminants in e-liquids and e-cigarette aerosols

The NICNAS report identified 106 chemicals considered to be contaminants. These were detected in the e-liquid, aerosols or both. ³  

Contaminants found in e-liquids and aerosols mostly consisted of volatile organic compounds (VOCs), pesticides, metals and phthalates. ³ VOCs are carbon-based chemicals that release molecules as gases, often producing scents. Many VOCs can be toxic at sufficient doses. Metals such as nickel and silicon may have entered the e-liquids from the e-cigarette device. Phthalates are small carbon-based molecules used to make plastics, many of which have reproductive toxicity and endocrine disrupting activity with exposure at high doses. Carcinogenic (cancer-causing) polycyclic aromatic hydrocarbons (see Section 12.4.3.6) and N-nitrosamines () have also been detected in e-cigarette aerosols at low levels. ^18-20

Contaminants in e-liquids and e-cigarette aerosols are usually present at much lower levels than the purposely added ‘ingredients’ and reaction products. However, some contaminants with toxic potential are present at concerning levels. Examples are diethyl phthalate detected at 1.8 mg/mL (0.18%) and propylene oxide at 6.7 mg/mL (0.67%) in e-cigarette liquids. ³

Lists of the contaminants in the e-liquids (Table A3) and in e-cigarette aerosols (Table A6) are available in the NICNAS report. ³  

18.5.3.4 Chemicals in e-cigarette aerosols compared to conventional cigarettes

The chemicals present in e-cigarette aerosols are an indicator of the risks of their use, but there are many disadvantages to using the information to predict health effects from e-cigarette use, as detailed in Section 18.5.5.1. E-cigarettes are commonly and increasingly used by non-smokers, including children and young adults. Their use increases the risk of harm in this group compared with no e-cigarette use at all. Perceptions that e-cigarettes are less harmful than smoking predict use of e-cigarettes among young people, even among those who have never smoked—see Section 18.9.7.

E-cigarettes may be beneficial for smokers who use them to quit smoking completely. Most chemicals with toxicity concerns in e-cigarettes are present at considerably lower levels than in conventional cigarettes but this does not mean that e-cigarettes can be assumed to be harmless. A false sense of safety can result from comparing the chemicals in conventional cigarettes with e-cigarettes, as the effects of long-term exposure to the chemicals found in e-cigarettes, even at low doses, are unknown. Further, the most common pattern of use is e-cigarettes combined with smoking (‘dual use’), which may further increase health risks beyond use of either product alone.

E-cigarette aerosols contain considerably lower amounts of harmful and toxic chemicals compared to conventional cigarette aerosols. An extensive study from 2021 tested carbonyl compounds (including aldehydes such as acetaldehyde and formaldehyde, Section 12.4.3.2) and polycyclic aromatic hydrocarbons (PAHs, Section 12.4.3.6) levels in e-cigarette compared to conventional cigarette and heated tobacco products aerosols. Among the 19 carbonyl compounds tested, 17 were lowest in e-cigarette aerosols. Among the 22 PAHs that were detected, 21 were lowest in e-cigarette aerosols compared to conventional cigarette and heated tobacco products. When comparing average levels of all the compounds in each class, e-cigarettes had a 99% reduction per puff of carbonyl compounds compared to conventional cigarettes, and a 98% reduction per puff of PAHs, many of which are known causes of cancer. ²¹ This study used three different types of e-cigarettes in these comparisons. But there is a great variety of e-cigarettes and of the ways that they are used, such as differing power settings, as described below in Section 18.5.4. More extensive studies are necessary to comprehensively compare e-cigarette aerosols to conventional cigarettes that take into account the variety of products and modes of use.   

There is a lack of research directly comparing the metals found in e-cigarette aerosols to conventional cigarettes (see Section 12.4.3.10). However, studies of metals in human samples, such as urine and blood, have mostly found similar, if not higher, amounts of metals in samples from e-cigarette users compared to conventional cigarettes. ²² These findings are consistent with studies showing similar or higher levels of some metals in the urine of e-cigarette users compared to conventional cigarettes, as described in Section 18.5.6. Numerous metals present in e-liquids and/or e-cigarette aerosols have associated health concerns, as detailed in Table 18.5.1.

18.5.4 E-cigarette device types and modes of use that affect exposure to chemicals

E-cigarette devices generally use battery power to heat a metal coil, that heats the e-liquid, forming an aerosol that is inhaled by the user. The temperature of the heating element varies from device to device and has been measured at between approximately 150°C to 350°C under typical usage conditions. ²³ Chemical exposure from e-cigarette aerosols is affected by the power output and temperature of the heated coil. An increase in temperature can increase the production of harmful free radicals, carbonyl compounds and benzene produced, as well as some metals. ⁵ Increasing the power settings also increased the damaging ‘free radical’ compounds in human bronchial cells exposed to e-cigarette aerosols in laboratory experiments. ²¹

The toxic gas carbon monoxide was detected in e-cigarette aerosols when high power settings were used. ²⁴ Under maximal power (200W) and 4-second puff conditions, the carbon monoxide concentration in aerosols was detected at over 180 ppm, a concentration that exceeds the US National Ambient Air Quality Standards for outdoor carbon monoxide concentration (35 ppm for 1 hour). Lower power settings are recommended by the authors of this study to reduce the risk from inhaling carbon monoxide. ²⁴

Under some conditions, a low supply of e-liquid reaches the heating coil leading to an unpleasant user experience called a ‘dry puff’. Dry puff conditions lead to an increased concentration of carbonyls such as formaldehyde in the aerosols. ²⁵ If dry puff conditions were always used, this could lead to intake of formaldehyde at much higher levels than cigarettes. It’s believed that these conditions are aversive, and therefore avoided by users. However, there remains some debate over the ability of e-cigarette to release higher levels of formaldehyde under non dry puff conditions. ²⁶

E-cigarette devices set up for ‘direct-to-lung’ use commonly use a higher power setting than devices used for ‘mouth-to-lung’ inhalation, where the aerosols are gathered in the mouth first, before inhalation into the lungs. ⁵ The high surface area of the lungs means that direct-to-lungs vaping leads to a higher amount of exposure to the chemicals. However, direct-to-lung e-liquid mixes are usually less concentrated to compensate for this.

The concentration of toxic carbonyl compounds in the aerosols may be increased during dripping (adding e-liquid directly to the heating coil) and ‘squonking’ (where an inbuilt squeeze bottle is used to add e-liquid to the heating coil). This effect may be due to an increase in the temperature of the coil. ²⁵

18.5.5 Health concerns associated with specific chemicals in e-liquids and e-cigarette aerosols

18.5.5.1 Challenges and limitations in predicting the health effects of e-cigarette use

The health effects of long-term e-cigarette use have not been directly measured due to the relatively recent introduction of these products into the market. In the absence of direct measurement, the health concerns associated with e-cigarette use have been predicted using information on 1) the individual chemicals present in liquids and aerosols, 2) the extent of the exposure and 3) the known health effects of these chemicals. There are multiple issues with this approach that make these predictions challenging.

The risk of toxic effects from exposure to a specific chemical is affected by factors such as the dose of the chemical, the frequency of exposure, the duration of exposure and the route of entry into the body. ²⁷ Toxicity may also vary for people of different sexes, ethnicity and age, and be complicated by experiences such as pregnancy or chronic diseases. Furthermore, the effects of chemicals in e-cigarette aerosols occur in the context of a mixture of many different chemicals, which could modify their risks in ways that are difficult to quantify. For example, a mix of numerous chemicals that are capable of mutagenesis (changing the sequence of DNA) can increase the risk of cancer above that of one only, but these risks cannot simply be added together to estimate an overall risk. Similarly, the uptake into the lungs or the bloodstream of one chemical can be modified by the presence of another, modifying its potential to cause harm. ²⁸

The extent of exposure is an important factor affecting toxicity. ²⁷ For many chemicals, the acute toxicity (from one exposure) and short-term effects have been measured. But the health concerns from long-term and high-frequency exposure, such as from 10 to 20 years of daily low dose exposure, are unknown. The frequency and duration of exposure also differ considerably between users and modes of e-cigarette use, as described in Section 18.5.4. A greater amount of some toxic chemicals has been detected from e-cigarettes used at higher power settings. With higher powered and variable-powered devices becoming more popular, ²⁹ it’s possible that future users will be exposed to higher concentrations of chemicals with potential toxicity. Examining the chemicals in e-liquids and e-cigarette aerosols is further complicated by a lack of regulation and manufacturing standards, and significant inconsistency between the labelled content and the actual content and concentrations. ^{30 , 31}

Toxicological risk assessments use knowledge of the risks from chemicals exposure to predict the toxic effects of specific chemicals through different routes of exposure. ²⁷ Many of these risks are tested in experiments using animals and cells growing in laboratories. The doses that cause toxicity in humans are often quite different to those in animals, and experiments using cell lines do not take into account the effects on the many different cell types present in the body. These experiments are usually only conducted over the short or mid-term, so extrapolating to long-term use at lower dosages is problematic.

For many chemicals present in e-cigarette aerosols, toxicological risk assessments have not been performed for inhalation. ³² This is problematic, because the risks from inhalation may be quite different to those from ingestion or skin contact. Many chemicals that are generally regarded as safe at low doses in food cannot be assumed to be safe when inhaled. The lungs are uniquely designed to transport gases quickly into the bloodstream. They have the largest exposed surface area of any organ, at 70 to 100 m ² compared to 10 m ² for the digestive system. ²⁷ Chemicals that are deposited into the airways and lungs can cause localised damage such as irritation and construction, inflammation, cell death, fibrosis (scar formation) and DNA damage that can lead to cancer. ²⁷  

18.5.5.2 Risk assessments of chemicals in e-liquids and e-cigarette aerosols

In 2022, the National Health and Medical Research Council (NHMRC) produced a report assessing the toxicological risks of the chemicals in e-liquids and their aerosols ³² that were identified by NICNAS ³ as described in Section 18.5.3. This report examined existing toxicological risk assessments for the 369 individual chemicals and metals identified by NICNAS (the ingredients, contaminants and reaction products discussed above). The toxicological risk assessments provide an indication of potential harms of these e-cigarette chemicals on an individual level, but do not measure the health effects of long-term e-cigarette use.

Of the 369 substances detected in e-liquids and e-cigarette aerosols: ³²

• 116 had no risk assessments available, so their risks upon inhalation or other exposure are unknown, and cannot be assumed as safe
• 42 were harmful by inhalation (described in Table 18.5.5.1)

 

• Eight were known or suspected respiratory sensitisers (that trigger long term lung inflammatory conditions)
• 203 had other known or suspected health risks.

Over 80% of the 369 substances had no risk assessments specific to inhalation, so the health concerns of their inhalation are unknown, and cannot be assumed safe.

Of the 253 substances with known or suspected health risks, there were a variety of known or suspected risks, and many had more than one associated risk. Thirty-nine had known or suspected acute toxicity, 82 were known or suspected carcinogens (chemicals that cause cancer), 53 were known or suspected skin irritants and 114 were described as harmful if swallowed. ³²

Of the 42 chemicals identified as having inhalation toxicity (listed in Table 18.5.5.1): ³²

• 16 were designated as harmful to inhale, such as arsenic, benzyl alcohol and benzaldehyde
• 11 as being fatal by inhalation (when at sufficient dose), such as acrolein, formaldehyde and mercury
• 10 are potentially fatal to inhale (when at sufficient dose), such as benzene, toluene and hexane
• three are described as causing irreversible lung damage, being diacetyl, acetoin and acetyl propionyl
• two are known to cause damage to organs through prolonged or repeated exposure, being manganese and nickel.

A study tested for some of these 42 chemicals in 65 e-liquids (labelled as non-nicotine) that were sold in Australia. ³³ This study detected benzyl alcohol, benzaldehyde, furfuraldehyde, nicotine and phenol in one or more of the e-liquids tested.

It is important to note that these toxicological effects would only occur with sufficient exposure doses, durations and frequencies. For many of these chemicals, the extent of exposure is poorly characterised.

18.5.5.3 Chemicals prohibited in prescribed e-cigarettes in Australia

In Australia, e-cigarettes and e-liquids containing nicotine can only be sourced legally on prescription (see Section 18.13 for more details). The Therapeutic Goods Administration (TGA) describes these as ‘nicotine vaping products’ (NVPs). The TGA has not assessed the safety, quality and efficacy of any nicotine vaping product, meaning there are no TGA approved products. However, the TGA  has released a product standard that sets out minimum safety and quality requirements for prescribed nicotine vaping products. ³⁴ The TGA’s Standard for Nicotine Vaping Products (TGO 110) prohibits the addition of eight substances in e-liquids due to known health risks: ³⁵ Acetyl propionyl, acetoin, benzaldehyde, cinnamaldehyde, diacetyl, diethylene glycol, DL-alpha-tocopheryl acetate and ethylene glycol. To comply with TGO 110, the presence of any of these compounds in e-liquids must be below a level of 10 parts per million (ppm, i.e. 10mg/g). Testing of e-liquids sold in Australia containing nicotine, conducted by the TGA and published online in 2022, showed that 35% of the 190 nicotine-containing e-liquids did not comply with the 10 ppm limit for one or more of these chemicals. ¹

Seven of the eight prohibited substances listed above have been detected in e-liquids and/or their emissions assessed by NICNAS ³ and the NHMRC. ³² Each of these seven have been detected at higher than 10 ppm (10 mg/g, which is approximately 10 mg/ml). These are: acetyl propionyl, ³⁶ acetoin, ³⁷ benzaldehyde, ¹⁶ cinnamaldehyde, ³⁸ diacetyl, ³⁶ diethylene glycol and ethylene glycol. ³⁹ However, the NICNAS data mostly come from international studies, so do not necessarily reflect the concentrations of these chemicals in e-liquids sold in Australia.

A study of 65 fresh and aged e-liquids sold in Australia, which were labelled as non-nicotine, detected substances prohibited in nicotine e-liquids above the 10 ppm threshold: ³³

• benzaldehyde in 60 fresh and 61 aged e-liquids, with concentrations ranging from 11.4 ng/ml to 17.3 mg/ml,
• cinnamaldehyde in 48 fresh e-liquids (maximum 97.9 mg/ml) and 38 aged e-liquids (maximum 142.5 mg/ml)

Cinnamaldehyde is a flavouring chemical that may degrade into toxic styrene. In laboratory experiments, cinnamaldehyde has been shown to impair the function of some immune cells, but the effects of long-term low dose exposure to cinnamaldehyde through e-cigarette use are unknown. ⁴⁰

The levels of cinnamaldehyde and benzaldehyde found in Australian products are only prohibited by the TGA in nicotine e-liquids. These results demonstrate that Australians are exposed to specific chemicals in non-nicotine e-liquids at levels that are considered unsafe by the TGA (but in products not regulated by the TGA).

18.5.5.4 Health concerns associated with specific chemicals from e-cigarettes

A scoping review of 89 studies was conducted by the NHMRC to estimate the potential for e-cigarette aerosols and/or the individual chemicals in them to cause specific health effects. ³² These publications examined general toxicity, cellular toxicity, toxicity to the lungs and to the cardiovascular system. This review concluded that long-term data on inhalation toxicity of e-cigarettes (both nicotine-containing and non-nicotine-containing) remained limited. The evidence on the differential health impacts of a specific flavour, solvent or humectant could not be determined. Likewise, the evidence on the differential health impacts of nicotine-containing or nicotine-free e-cigarettes or e-liquids could also not be determined. ³²

The inconclusive findings from this report have stemmed from the low doses of these chemicals in e-liquids and aerosols, as well as the sparse information on long-term and inhalation toxicity. This does not mean that there are no health effects of e-cigarette use. The effects described below only occur under sufficient conditions of exposure, which are poorly defined for e-cigarettes.  

18.5.5.4.1 Chemicals with known or potential health effects

Most of the chemicals in e-cigarettes that had toxicology assessments had known or suspected health concerns, such as acute toxicity (effects from a single exposure or short period of exposure), cancer-causing activity (carcinogenicity), skin irritation or were described as harmful if swallowed. ³²

Among the chemicals known to have fatal effects when at a high enough dose, there are numerous toxic metals and aldehydes (chemicals with carbonyl groups, described in Section 12.4.3.2) (see Table 18.5.5.1). The metals are most likely to be contaminants and are described further below. Aldehydes, such as formaldehyde, acrolein and crotonaldehyde, can be formed as reaction products during the heating of the e-liquid to create an aerosol. They are also detected in tobacco smoke, but at significantly higher concentrations. ¹¹ For tobacco smoke, acrolein and formaldehyde are priority toxicants for regulation recommended by the World Health Organization (see Section 12.5.3). Formaldehyde is a preservative used in embalming solutions and for preserving dead tissue in laboratories. ⁴¹ It is toxic to the cardiovascular system and has a genotoxic (mutating DNA to cause cancer) effect in lung cells. ^{42 , 43} Acrolein is an irritant to the human respiratory system, toxic to lung cilia and is likely to be a carcinogen in the lungs. ⁴³ Acrolein is toxic to the cardiovascular system and causes oxidative stress in the heart and increases cardiovascular disease risk. ^{42 , 43} Although the long term health effects of exposure to these aldehydes via e-cigarette aerosols is not yet known, long-term exposure to carcinogenic compounds, even at low concentrations, should be a concern for e-cigarette users. When such exposure occurs via the respiratory system, there are not the same detoxification steps undertaken by the body as those occurring within the digestive system that afford some protection from compounds such as formaldehyde.

Known and suspected carcinogens (cancer-causing chemicals) found in e-cigarette aerosols include butylated hydroxytoluene, limonene, diacetyl, furans, creosol and cyclohexanone. ³² Many of these are present at relatively low doses. However, the consequences of inhalation of a mix of many of these chemicals in low doses, but high frequencies for a long duration, are not known. Some carcinogens are believed to be able to promote the formation of cancer with no known lower limit, called linear non-threshold action (LNT). Examples of chemicals with LNT or assumed LNT that have been detected in e-liquids or their aerosols are 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and arsenic. ^{32 , 44}

Solvent chemicals, usually propylene glycol and glycerol are the most common constituent of most e-liquids. When heated, these solvents can undergo reactions to form other chemicals, some of which have health concerns. For example, propylene glycol can break down into diacetyl and formaldehyde when heated during e-cigarette use. These chemicals have considerable toxicity but are generally produced at very low levels in e-cigarette aerosols. A study of propylene glycol breakdown products in JUUL emissions (13 different flavours) detected diacetyl at an average of 20 µg/m ³ and methylglyoxal at an average of 4,219 µg/m ³ (range of 677 to 15,342 µg/m ³). ⁴⁵ Methylglyoxal, a respiratory irritant, is therefore present at much higher levels in aerosols, but the long term health effects of exposure to this chemical via the respiratory tract are unknown. One study has shown that methylglyoxal is capable of increasing pro-inflammatory responses in human nasal cells grown in the laboratory. ⁴⁶

18.5.5.4.2 Metal contamination in e-liquids and e-cigarette aerosols

E-cigarettes use batteries and metallic heating elements of varying designs to heat e-liquids, forming the aerosol. The composition of the metal heating elements is un-standardised and reported to have considerable variation. ⁴⁷ Some metal particles from the heating elements and the tank holding the e-liquid seep into the e-liquid and are detected in the aerosols. Metals such as aluminium, barium, cadmium, chromium, cobalt, copper, iron, lead, magnesium, manganese, nickel, tin, tungsten and zinc, as well as the metalloids antimony and arsenic, have been detected in e-liquids and aerosols. ^{32 , 47-49} As power settings increase, many of these metals increase in concentration. ⁴⁹

An analysis of the risks from long term exposure to these metals used published cancer potency factors and non-cancer reference exposure concentrations from the US Environmental Protection Agency and the California Environmental Protection Agency to estimate the range of cancer and non-cancer risks from these metals. ⁴⁷ This study predicted that chromium and nickel were the leading contributors to cancer risk, with minor contributions from cadmium, lead, and arsenic. Assuming an exposure to 2 ml/day, the risks ranged from 5.7 to 30,000 additional cancers per million e-cigarette users. Cancer risks in the mid to upper end of these ranges were stated to exceed acceptable levels. Nickel, chromium, and manganese were the most influential metals driving the risk of non-cancer diseases, with nickel producing the highest risk. The authors concluded that e-cigarettes contaminated with metals in the mid and upper end of the reported ranges pose a significant cancer and non-cancer health risk. ⁴⁷

18.5.6 DNA adducts and biomarkers for e-cigarette exposure

Biomarkers for e-cigarette aerosols are substances in the human body that can be measured in a test that will indicate exposure to specific chemicals from e-cigarettes. Biomarkers of e-cigarette exposure can be detected in samples such as saliva and urine. Biomarkers have been very useful in the study of tobacco exposure. They can indicate the dose of a specific chemical exposure, the biological activity, damage or potential for damage and the risk of disease. ⁵⁰ Researchers are starting to use biomarkers to detect exposure to chemicals in e-cigarette users. See Section 12.5.6 for more information about biomarkers.

A 2021 scoping review evaluated the current data from biomarkers and compared exposure of people who were smokers, e-cigarette users and non-users of both. ⁵¹ This review found consistent evidence suggesting that biomarkers of some volatile organic compounds (VOCs) (acrylamide and acrylonitrile), metals (beryllium, cadmium, selenium, uranium and zinc) and propylene glycol are higher in e-cigarette users compared with non-users. However, conflicting data meant that it is currently unclear whether e-cigarette users have similar or higher biomarkers of acrolein, benzene, crotonaldehyde, formaldehyde, propylene oxide, toluene, xylene and 1,3-butadiene, and some metals (chromium, lead, nickel, strontium and manganese) compared to non-users. ⁵¹

In comparisons of e-cigarette users to cigarette smokers: biomarkers of VOCs (acrolein, acrylamide, acrylonitrile, ethylene oxide, vinyl chloride and 1,3-butadiene) were lower in e-cigarette users. However, e-cigarette users and cigarette smokers had similar urinary levels of beryllium, zinc and uranium. There were insufficient data to draw conclusions on the levels of biomarkers for other VOCs (benzene, crotonaldehyde and propylene oxide) and metals (cadmium, chromium, lead, nickel, manganese and strontium). ⁵¹

Mutagenic chemicals, such as carcinogens (chemicals that can cause cancer), damage DNA by changing its sequence. Once they enter the body, they are often modified in biochemical reactions before binding to DNA at specific sites (called DNA adducts, see Section 3.3.2.2). DNA adducts could be considered as biomarkers indicating the potential for damage. E-cigarette aerosols can induce DNA strand breaks and oxidative DNA damage cells grown in the laboratory and the formation of DNA adducts in the lung, heart and bladder of mice. ⁵² Consistent with these findings, long-term (54 weeks) exposure to e-cigarette aerosols has been shown to induce lung adenocarcinoma and bladder urothelial hyperplasia in mice. ⁵³ DNA-acrolein adducts have been detected in mouth brush samples (taken at dental visits) of e-cigarette users at 9-fold higher amounts than those in non-e-cigarette users. ⁵⁴    

 

Relevant news and research

For recent news items and research on this topic, click  here. ( Last updated January 2023)

 

References 

1. Therapeutic Goods Administration. Testing of nicotine vaping products. Australian Government, Department of Health,  2022. Available from: https://www.tga.gov.au/testing-nicotine-vaping-products

2. Olmedo P, Goessler W, Tanda S, Grau-Perez M, Jarmul S, et al. Metal concentrations in e-cigarette liquid and aerosol samples: The contribution of metallic coils. Environmental Health Perspectives, 2018; 126(2):027010. Available from: https://www.ncbi.nlm.nih.gov/pubmed/29467105

3. National Industrial Chemicals Notification and Assessment Scheme (NICNAS). Non-nicotine liquids for e-cigarette devices in Australia: chemistry and health concern. Australian Government Department of Health, 2019. Available from: https://www.industrialchemicals.gov.au/sites/default/files/2020-08/Non-nicotine%20liquids%20for%20e-cigarette%20devices%20in%20Australia%20chemistry%20and%20health%20concerns%20%5BPDF%201.21%20MB%5D.pdf.

4. Farsalinos KE, Gillman IG, Melvin MS, Paolantonio AR, Gardow WJ, et al. Nicotine levels and presence of selected tobacco-derived toxins in tobacco flavoured electronic cigarette refill liquids. International Journal of Environmental Research and Public Health, 2015; 12(4):3439-52. Available from: https://www.ncbi.nlm.nih.gov/pubmed/25811768

5. Korzun T, Lazurko M, Munhenzva I, Barsanti KC, Huang Y, et al. E-cigarette airflow rate modulates toxicant profiles and can lead to concerning levels of solvent consumption. ACS Omega, 2018; 3(1):30-6. Available from: https://www.ncbi.nlm.nih.gov/pubmed/29399647

6. Ingebrethsen BJ, Cole SK, and Alderman SL. Electronic cigarette aerosol particle size distribution measurements. Inhalation Toxicology, 2012; 24(14):976-84. Available from: https://www.ncbi.nlm.nih.gov/pubmed/23216158

7. Mikheev VB, Brinkman MC, Granville CA, Gordon SM, and Clark PI. Real-time measurement of electronic cigarette aerosol size distribution and metals content analysis. Nicotine & Tobacco Research, 2016; 18(9):1895-902. Available from: https://www.ncbi.nlm.nih.gov/pubmed/27146638

8. Manigrasso M, Buonanno G, Fuoco FC, Stabile L, and Avino P. Aerosol deposition doses in the human respiratory tree of electronic cigarette smokers. Environmental Pollution, 2015; 196:257-67. Available from: https://www.ncbi.nlm.nih.gov/pubmed/25463721

9. Manigrasso M, Buonanno G, Stabile L, Morawska L, and Avino P. Particle doses in the pulmonary lobes of electronic and conventional cigarette users. Environmental Pollution, 2015; 202:24-31. Available from: https://www.ncbi.nlm.nih.gov/pubmed/25796074

10. Banks E, Yazidjoglou A, Brown S, Nguyen M, Martin M, et al. Electronic cigarettes and health outcomes: systematic review of global evidence. Report for the Australian Department of Health. Canberra: National Centre for Epidemiology and Population Health, 2022. Available from: https://nceph.anu.edu.au/research/projects/health-impacts-electronic-cigarettes#health_outcomes.

11. Margham J, McAdam K, Cunningham A, Porter A, Fiebelkorn S, et al. The chemical complexity of e-cigarette aerosols compared with the smoke from a tobacco burning cigarette. Frontiers in Chemistry, 2021; 9:743060. Available from: https://www.ncbi.nlm.nih.gov/pubmed/34660535

12. National Academies of Sciences Engineering and Medicine. Public health consequences of e-cigarettes. The National Academies Press, Washington, DC 2018. Available from: http://nationalacademies.org/hmd/Reports/2018/public-health-consequences-of-e-cigarettes.aspx.

13. National Health and Medical Research Council. CEO statement on electronic cigarettes.  2022. Available from: https://www.nhmrc.gov.au/health-advice/all-topics/electronic-cigarettes/ceo-statement

14. National Health and Medical Research Council. CEO statement on electronic cigarettes: Plain english summary.  2022. Available from: https://www.nhmrc.gov.au/health-advice/all-topics/electronic-cigarettes/ceo-statement-summary

15. Han S, Chen H, Zhang X, Liu T, and Fu Y. Levels of selected groups of compounds in refill solutions for electronic cigarettes. Nicotine & Tobacco Research, 2016; 18(5):708-14. Available from: https://www.ncbi.nlm.nih.gov/pubmed/26568061

16. Tierney PA, Karpinski CD, Brown JE, Luo W, and Pankow JF. Flavour chemicals in electronic cigarette fluids. Tobacco Control, 2016; 25(e1):e10-5. Available from: https://www.ncbi.nlm.nih.gov/pubmed/25877377

17. Klager S, Vallarino J, MacNaughton P, Christiani DC, Lu Q, et al. Flavoring chemicals and aldehydes in e-cigarette emissions. Environmental Science & Technology, 2017; 51(18):10806-13. Available from: https://www.ncbi.nlm.nih.gov/pubmed/28817267

18. Beauval N, Antherieu S, Soyez M, Gengler N, Grova N, et al. Chemical evaluation of electronic cigarettes: Multicomponent analysis of liquid refills and their corresponding aerosols. Journal of Analytical Toxicology, 2017; 41(8):670-8. Available from: https://www.ncbi.nlm.nih.gov/pubmed/28985322

19. Margham J, McAdam K, Forster M, Liu C, Wright C, et al. Chemical composition of aerosol from an e-cigarette: A quantitative comparison with cigarette smoke. Chemical Research in Toxicology, 2016; 29(10):1662-78. Available from: https://www.ncbi.nlm.nih.gov/pubmed/27641760

20. Goniewicz ML, Knysak J, Gawron M, Kosmider L, Sobczak A, et al. Levels of selected carcinogens and toxicants in vapour from electronic cigarettes. Tobacco Control, 2014; 23(2):133-9. Available from: https://www.ncbi.nlm.nih.gov/pubmed/23467656

21. Dusautoir R, Zarcone G, Verriele M, Garcon G, Fronval I, et al. Comparison of the chemical composition of aerosols from heated tobacco products, electronic cigarettes and tobacco cigarettes and their toxic impacts on the human bronchial epithelial BEAS-2B cells. Journal of Hazardous Materials, 2021; 401:123417. Available from: https://www.ncbi.nlm.nih.gov/pubmed/32763707

22. Zhao D, Aravindakshan A, Hilpert M, Olmedo P, Rule AM, et al. Metal/metalloid levels in electronic cigarette liquids, aerosols, and human biosamples: A systematic review. Environmental Health Perspectives, 2020; 128(3):36001. Available from: https://www.ncbi.nlm.nih.gov/pubmed/32186411

23. Chen W, Wang P, Ito K, Fowles J, Shusterman D, et al. Measurement of heating coil temperature for e-cigarettes with a "top-coil" clearomizer. PLoS One, 2018; 13(4):e0195925. Available from: https://www.ncbi.nlm.nih.gov/pubmed/29672571

24. Casebolt R, Cook SJ, Islas A, Brown A, Castle K, et al. Carbon monoxide concentration in mainstream E-cigarette emissions measured with diode laser spectroscopy. Tobacco Control, 2020; 29(6):652-5. Available from: https://www.ncbi.nlm.nih.gov/pubmed/31771993

25. Farsalinos KE, Voudris V, and Poulas K. E-cigarettes generate high levels of aldehydes only in 'dry puff' conditions. Addiction, 2015; 110(8):1352-6. Available from: https://www.ncbi.nlm.nih.gov/pubmed/25996087

26. Shihadeh A, Talih S, and Eissenberg T. Commentary on Farsalinos et al. (2015): E-cigarettes generate high levels of aldehydes only in 'dry puff' conditions. Addiction, 2015; 110(11):1861-2. Available from: https://www.ncbi.nlm.nih.gov/pubmed/26395030

27. University of Nebraska Lincoln. Toxocology and exposure guidelines.  2002. Available from: https://ehs.unl.edu/documents/tox_exposure_guidelines.pdf.

28. Stahlmann R and Horvath A. Risks, risk assessment and risk competence in toxicology. German Medical Science, 2015; 13:Doc09. Available from: https://www.ncbi.nlm.nih.gov/pubmed/26195922

29. Farsalinos K. Electronic cigarette evolution: from first to fourth generation products and beyond.  2015. Available from: https://web.archive.org/web/20150708172614/http://gfn.net.co/downloads/2015/Plenary%203/Konstantinos%20Farsalinos.pdf.

30. Brown CJ and Cheng JM. Electronic cigarettes: product characterisation and design considerations. Tobacco Control, 2014; 23 Suppl 2(Suppl 2):ii4-10. Available from: https://www.ncbi.nlm.nih.gov/pubmed/24732162

31. Herrington JS, Myers C, and Rigdon A. Analysis of nicotine and impurities in electronic cigarette solutions and vapor. Restek,  2017. Available from: http://www.restek.com/Technical-Resources/Technical-Library/Foods-Flavors-Fragrances/fff_FFAN2127-UNV.

32. National Health and Medical Research Council. Inhalation toxicity of non-nicotine e-cigarette constituents: risk assessments, scoping review and evidence map.  2022. Available from: https://www.nhmrc.gov.au/file/18287/download?token=Z5D5_sam.

33. Larcombe A, Allard S, Pringle P, Mead-Hunter R, Anderson N, et al. Chemical analysis of fresh and aged Australian e-cigarette liquids. Medical Journal of Australia, 2022; 216(1):27-32. Available from: https://www.ncbi.nlm.nih.gov/pubmed/34528266

34. Therapautic Goods Administration. Therapeutic goods (standard for nicotine vaping products) (TGO 110) Order 2021. Canberra, Australia: Australian Government, 2021. Available from: https://www.legislation.gov.au/Details/F2021L00595.

35. Therapeutic Goods Administration. Nicotine vaping products and vaping devices: Guidance for the therapeutic goods (standard for nicotine vaping products) (TGO 110) Order 2021 and related matters Version 1.2. Canberra: Government of Australia, 2021. Available from: https://www.tga.gov.au/sites/default/files/nicotine-vaping-products-and-vaping-devices_0.pdf.

36. Farsalinos KE, Kistler KA, Gillman G, and Voudris V. Evaluation of electronic cigarette liquids and aerosol for the presence of selected inhalation toxins. Nicotine & Tobacco Research, 2015; 17(2):168-74. Available from: https://www.ncbi.nlm.nih.gov/pubmed/25180080

37. Varlet V, Farsalinos K, Augsburger M, Thomas A, and Etter JF. Toxicity assessment of refill liquids for electronic cigarettes. International Journal of Environmental Research and Public Health, 2015; 12(5):4796-815. Available from: https://www.ncbi.nlm.nih.gov/pubmed/25941845

38. Behar RZ, Luo W, Lin SC, Wang Y, Valle J, et al. Distribution, quantification and toxicity of cinnamaldehyde in electronic cigarette refill fluids and aerosols. Tobacco Control, 2016; 25(Suppl 2):ii94-ii102. Available from: https://www.ncbi.nlm.nih.gov/pubmed/27633763

39. Hutzler C, Paschke M, Kruschinski S, Henkler F, Hahn J, et al. Chemical hazards present in liquids and vapors of electronic cigarettes. Archives of Toxicology, 2014; 88(7):1295-308. Available from: https://www.ncbi.nlm.nih.gov/pubmed/24958024

40. Hickman E, Herrera CA, and Jaspers I. Common e-cigarette flavoring chemicals impair neutrophil phagocytosis and oxidative burst. Chemical Research in Toxicology, 2019; 32(6):982-5. Available from: https://www.ncbi.nlm.nih.gov/pubmed/31117350

41. Britannica. Formaldehyde.  Available from: https://www.britannica.com/science/formaldehyde.

42. World Health Organization. Report on the scientific basis of tobacco product regulation: Seventh report of a WHO study group. Geneva: WHO, 2019. Available from: https://www.who.int/publications/i/item/who-study-group-on-tobacco-product-regulation-report-on-the-scientific-basis-of-tobacco-product-regulation-seventh-report-of-a-who-study-group.

43. US Department of Health and Human Services. How tobacco smoke causes disease: the biology and behavioral basis for smoking-attributable disease. A report of the US Surgeon General, Atlanta, Georgia: US Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health, 2010. Available from: https://www.ncbi.nlm.nih.gov/books/NBK53017/.

44. Bevan RJ and Harrison PTC. Threshold and non-threshold chemical carcinogens: A survey of the present regulatory landscape. Regulatory Toxicology and Pharmacology, 2017; 88:291-302. Available from: https://www.ncbi.nlm.nih.gov/pubmed/28119000

45. Azimi P, Keshavarz Z, Lahaie Luna M, Cedeno Laurent JG, Vallarino J, et al. An unrecognized hazard in e-cigarette vapor: Preliminary quantification of methylglyoxal formation from propylene glycol in e-cigarettes. International Journal of Environmental Research and Public Health, 2021; 18(2). Available from: https://www.ncbi.nlm.nih.gov/pubmed/33419122

46. Kwak S, Choi YS, Na HG, Bae CH, Song SY, et al. Glyoxal and methylglyoxal as e-cigarette vapor ingredients-induced pro-inflammatory cytokine and mucins expression in human nasal epithelial cells. American Journal of Rhinology & Allergy, 2021; 35(2):213-20. Available from: https://www.ncbi.nlm.nih.gov/pubmed/32746708

47. Fowles J, Barreau T, and Wu N. Cancer and non-cancer risk concerns from metals in electronic cigarette liquids and aerosols. International Journal of Environmental Research and Public Health, 2020; 17(6). Available from: https://www.ncbi.nlm.nih.gov/pubmed/32213824

48. Kapiamba KF, Hao W, Adom S, Liu W, Huang YW, et al. Examining metal contents in primary and secondhand aerosols released by electronic cigarettes. Chemical Research in Toxicology, 2022; 35(6):954-62. Available from: https://www.ncbi.nlm.nih.gov/pubmed/35385266

49. Ko TJ and Kim SA. Effect of heating on physicochemical property of aerosols during vaping. International Journal of Environmental Research and Public Health, 2022; 19(3). Available from: https://www.ncbi.nlm.nih.gov/pubmed/35162914

50. US Department of Health and Human Services. A report of the Surgeon General: How tobacco smoke causes disease. US Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health, 2010. Available from: https://www.ncbi.nlm.nih.gov/books/NBK53017/.

51. Hiler M, Weidner AS, Hull LC, Kurti AN, and Mishina EV. Systemic biomarkers of exposure associated with ENDS use: a scoping review. Tobacco Control, 2021. Available from: https://www.ncbi.nlm.nih.gov/pubmed/34732539

52. Tang MS, Lee HW, Weng MW, Wang HT, Hu Y, et al. DNA damage, DNA repair and carcinogenicity: Tobacco smoke versus electronic cigarette aerosol. Mutation Research - Reviews in Mutation Research, 2022; 789:108409. Available from: https://www.ncbi.nlm.nih.gov/pubmed/35690412

53. Tang MS, Wu XR, Lee HW, Xia Y, Deng FM, et al. Electronic-cigarette smoke induces lung adenocarcinoma and bladder urothelial hyperplasia in mice. Proceedings of the National Academy of Sciences of the U S A, 2019; 116(43):21727-31. Available from: https://www.ncbi.nlm.nih.gov/pubmed/31591243

54. Cheng G, Guo J, Carmella SG, Lindgren B, Ikuemonisan J, et al. Increased acrolein-DNA adducts in buccal brushings of e-cigarette users. Carcinogenesis, 2022; 43(5):437-44. Available from: https://www.ncbi.nlm.nih.gov/pubmed/35239969