18.5.4.1 Challenges and limitations in predicting the health effects of e-cigarette use
The health effects of long-term e-cigarette use cannot be directly measured due to the relatively recent introduction of these products into the market. In the absence of direct measurement, 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.1 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.2
The extent of exposure is an important factor affecting toxicity.1 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 18.5.6. 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,3 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 illegal sales of products that do not meet the required product standards, and significant inconsistency between the labelled content and the actual content and concentrations.4,5
Toxicological risk assessments use knowledge of the risks from chemicals exposure to predict the toxic effects of specific chemicals through different routes of exposure.1 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.6 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.7 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 m2 compared to 10 m2 for the digestive system.1 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.1
18.5.4.2 Methods for detecting the chemicals in e-cigarettes
Researchers use a variety of laboratory tests to detect specific chemicals in e-cigarettes. The simpler approach is to detect chemicals in e-liquids, before this liquid is heated into an aerosol for inhalation. Analysis of e-liquid refills (where the liquid is sold separately) is more easily and commonly performed than analysis of e-liquids inside disposable devices, which need to be extracted from a sealed device. However, analysis of e-liquids has limitations; the range of chemicals inhaled by the user from the aerosol is different to that found in the e-liquid. E-cigarette aerosols contain the chemicals in e-liquids and the products of thermal degradation, including pyrolysis, and other chemical reactions that occur upon heating, many of which are toxic. Detection of chemicals in the aerosols produced by e-cigarettes is a better indication of the chemicals to which a user is exposed. See also Section 18.6.9 for the chemicals detected in secondhand e-cigarette emissions.
Another approach is to detect chemicals in the urine or saliva of e-cigarette users (see Section 18.5.8 for a discussion of biomarkers). Detecting biomarkers can show which chemicals are taken up into the body. But the drawback to biomarker detection in human samples is that these chemicals could have come from numerous sources, not just e-cigarettes.
Detection of chemicals in e-liquids
Hundreds of independent laboratories have developed bespoke methods to detect specific chemicals in e-liquids.8-10 Nicotine, glycerol (vegetable glycerin) and propylene glycol are commonly measured, as are specifical flavourants and toxic contaminants/reaction products such as aldehydes (formaldehyde etc) and polycyclic aromatic hydrocarbons (benzo[a]pyrene etc.).9 Techniques used in these laboratories include gas chromatography and mass spectroscopy, as described in more detail in Section 12.5.3. Methods have been developed to determine the fraction of nicotine that is free-base or protonated (as in nicotine salts).11,12 Modified mass spectroscopy techniques have been developed to measure a range of toxic metals in e-liquids,13 coolants10 and flavourants,14 and the harmful and potentially harmful constituents that are also found in tobacco smoke.15 Non-targeted approaches using mass spectrometry can identify as a many chemicals as possible in a sample, as an alternative to individual tests for specific compounds.16
An Australian study has used an ageing technique to analyse the effects of time in storage on e-liquids.8 Chemical reactions during e-liquid storage may include thermal decomposition, oxidation, and polymerisation of e-liquid components.8
Standard Operating Procedures (SOPs) have been developed to improve the accuracy of detection of chemicals in e-liquids and reduce variability between different laboratories.17 TobLabNet (Tobacco Laboratory Network, under the World Health Organization) is a global network of government, academic, and independent laboratories that collaborate to strengthen capacity for the testing of tobacco product contents and emissions.18 In addition to measuring chemicals in tobacco smoke, TobLabNet has recently developed methods for measuring chemicals in e-liquids. Their Standard Operating Procedure SOP11 details a method for determining the concentrations of nicotine, propylene glycol and glycerol (vegetable glycerin) in e-liquids.19 This method uses gas chromatography, in which a mixture of chemicals is converted into a gas prior to binding and release from an adsorbent material. This can be coupled with either mass spectrometry or analysis with a flame ionization detector. See Section 12.5.3 for more details. A range of TobLabNet laboratories were able to use both approaches and generate consistent results.20
Detection of chemicals in e-cigarette aerosols
The aerosols created from e-cigarettes consist of gases as well as tiny droplets containing chemicals that float in these gases. Both components need to be collected and analysed for the detection of chemicals in the aerosols that are inhaled by users. An ‘automated vaping machine’ is used to generate and collect aerosols from an e-cigarette. These machines take puffs using a programmed puff duration, volume, puff intervals and device settings such as power usage.21,22 There is difficulty in establishing a standard protocol for these settings, given the wide variety of devices available and the variety in the ways in which people use them. Once aerosols have been collected directly from e-cigarette devices, the analysis of chemicals, contaminants and reaction products in these samples uses similar techniques to those described above for e-liquids.9,15,23-27
18.5.4.3 Reports listing the chemicals 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. In this section, three major reports were used to inform a list of the hazardous chemicals found in e-cigarettes through research across the world. It’s important to recognise that the chemical content of these products varies widely and changes over time. Not every e-cigarette has each of these chemicals.
The list of toxic chemicals presented in Table 18.5.1 uses data from 2019 from the National Industrial Chemicals Notification and Assessment Scheme (NICNAS),28 which identified 369 chemicals in e-cigarettes, and its companion report, the 2022 NHMRC risk assessments, scoping review and evidence map, which identified 42 of these chemicals that are harmful by inhalation and that are 8 respiratory sensitisers. Table 18.5.1 also includes chemicals cited in a recent comprehensive analysis of hazardous substances in e-cigarettes published in 2025.29 This study identified 1,585 chemicals reported as being present in e-cigarettes or their emissions, with 134 that are potentially hazardous to health.
Chemicals in e-cigarettes and their aerosols are derived from three different sources. The “ingredients”, which are purposely added by manufacturers, are described in Section 18.5.3. Chemical reaction products are produced from chemical reactions occurring during the heating and use of the e-cigarette or during storage of the e-liquid.28 Contaminants are chemicals (or micro-organisms) that were unintentionally added to e-liquids. These chemicals are usually present in small amounts, and include toxic metals that slough from the heating coil or other components.28
Many of the contaminants and chemical reaction products have known toxicity and therefore are a concern for health. Most of the “ingredients” are not known to be toxic, at least for ingestion, but the toxicity of these chemicals by frequent inhalation is mostly unstudied. However, some of the “ingredients”, such as nicotine, benzaldehyde and cinnamaldehyde are present at levels where toxic effects are possible.
NICNAS report on chemicals found in e-cigarettes (2019)
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 e-cigarettes, with or without nicotine (referred to here as the ‘NICNAS report’).28 Chemicals were reported if detected in ready-to-use e-liquids or e-liquid concentrates for dilution. This report listed chemicals found by researchers worldwide in a wide variety of e-cigarette brands, so not all these chemicals are in every e-cigarette. This report listed all chemicals detected and did not identify those with toxicity concerns. Identifying harmful chemicals from the NICNAS list was done by the NHMRC report, describe below.
There were a total of 369 chemicals listed by the NICNAS report, with 243 identified as “ingredients”, 106 as contaminants and 27 as chemical reaction products (some chemicals fell into more than one group).28
The NICNAS report identified a total of 243 different chemical “ingredients” in e-liquids and/or e-cigarette aerosols (see Tables A2 and A4 of this report).28 Most of these chemicals were flavourings (235 out of 243). Eight chemicals had other roles, for instance, as solvents.
Of the 27 chemical reaction products, most are aldehydes (more information in Sections 18.5.4.4 and 12.4.3.2) such as acetaldehyde, acrolein and formaldehyde. A list of these 27 reaction products is available in Table A5 the NICNAS report.28
Contaminants found in e-liquids and aerosols mostly consisted of volatile organic compounds (VOCs), pesticides, metals and phthalates.28,30 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.28
Lists of the contaminants in the e-liquids (Table A3) and in e-cigarette aerosols (Table A6) are available in the NICNAS report.28
NHMRC report on inhalation toxicity of e-cigarettes (2022)
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 aerosols6 that were identified by NICNAS28 as described above. 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: 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 (listed in Table 18.5.1), 8 were known or suspected respiratory sensitisers (that trigger long term lung inflammatory conditions), (listed in Table 18.5.1) and 203 had other known or suspected health risks.6 These numbers are presented in Figure 18.5.1.
See Section 18.5.4.4 below for descriptions of some of the individual chemicals and their health risks.
Of the 42 chemicals identified as having inhalation toxicity (see above):6 16 were designated as harmful by inhalation, such as arsenic, benzyl alcohol and benzaldehyde, 11 as those that may be 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, and two are known to cause damage to organs through prolonged or repeated exposure, being manganese and nickel. These chemicals are summarised in Figure 18.5.2.
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 potential 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.6
A scoping review of 89 studies was also conducted by the NHMRC to estimate the potential for e-cigarette aerosols and/or the individual chemicals in them to cause specific health effects.6 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.6
The inconclusive findings from this 2022 report have stemmed from the low doses of these chemicals in e-liquids and aerosols, as well as the sparse information on long-term toxicity 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.
El Bouz et al comprehensive analysis of hazardous substances in e-cigarettes (2025)
An extensive study from 2025 identified chemicals present in e-cigarettes or their emissions and identified those with known toxicity. The data came from two sources: 1) manufacturers declarations of chemical present in e-liquids and in their emissions (from mandatory testing), and 2) literature review of scientific studies from which data was used from 206 studies on emissions.29
In this study, 1301 chemicals were identified in e-liquids and 284 in e-cigarette aerosol emissions. Chemicals with known toxicity concerns were identified from this list. These were substances with carcinogenic (cancer causing), mutagenic (cause changes to DNA that may or may not lead to cancer), reproduction toxicity, endocrine disruptors (which interfere with hormone function) or other long-term toxicities. Many of these chemicals, such as carcinogens, are dangerous even at very low concentrations if there is regular exposure for a long period of time. However, most of these chemicals are present at very low concentrations. This comprehensive report lists the chemicals of with toxicity concerns, but does not indicate which ones are the cause of health effect in people who use e-cigarettes.
Of the 1301 chemicals in e-liquids, 45 had known toxicities. Of the 284 chemicals found in e-cigarette emissions, 103 had known toxicity. A total of 134 chemicals were listed as toxic, given that some were present in both emissions and e-liquids (Table 18.5.1, see column 7 for toxicity of chemicals reported by El Bouz, et al). These data come from many different types of e-cigarettes, so not every e-cigarette contains every chemical, or every toxic chemical.29
One notable conclusion from this data is that there are a considerable number of toxic chemicals present in the emissions, but not as many in the e-liquids. Many of these are likely to be breakdown products of the “ingredients” added to e-liquids. Therefore, the toxicity of e-cigarette use for users should be assessed by examining the chemicals in the emissions, not just those in e-liquids.
The toxic substances detected in e-cigarettes and their emissions mostly include volatile organic compounds, tobacco-specific nitrosamines (TSNAs), polycyclic aromatic hydrocarbon (PAHs), and toxic heavy metals (Table 18.5.1). For more information about the risks from these chemicals see Section 18.5.4.4 below and Section 12.4.3.
18.5.4.4 Toxic chemicals in e-cigarettes and their health risks
The chemicals in e-cigarettes with toxicity concerns that are listed in Table 18.5.1, plus some others described in this section, may come from ingredients in e-liquids, contaminants in e-liquids or be the products of chemical reactions occurring within e-liquids or when they are heated to form an aerosol for use. Most of these toxic chemicals are not considered e-liquid ingredients, and are found in the aerosol emissions. The chemicals described in this section constitute the major classes of toxic chemicals in e-cigarettes, and come from all three sources.
Many of the chemicals listed are generally known as volatile organic compounds (VOCs). These carbon-based chemicals readily evaporate and are heated into gases at low temperatures, such as those used by e-cigarettes. Many VOCs are toxic, including aldehydes and other carbonyl compounds, hydrocarbons, alcohols, terpenes, nitro compounds and others. Many scents and fragrances are VOCs and some have known toxicities. Other toxic components of e-cigarettes include heavy metals and reactive oxygen species, described below.
Nicotine
Nicotine is found in most e-cigarettes sold in Australia, often at 4% to 5% of the e-liquid. E-cigarettes contain various versions of nicotine and nicotine analogues, such as nicotine salts, synthetic nicotine, 6-methyl-nicotine and other alkaloids such as nicotinamide (see descriptions in Section 18.5.3.1).
Nicotine is highly toxic. When sufficiently concentrated, nicotine is fatal if swallowed, inhaled or spilled onto the skin. The amounts of nicotine in e-cigarette refills are toxic and may be fatal by accidental exposure (see Section 18.4.1.2). Lower concentrations inhaled during vaping can lead to mild nicotine poisoning symptoms such as vomiting, diarrhoea and seizures in some cases. The nicotine analogue 6-methyl nicotine, which binds the same receptors as nicotine, also has toxicity concerns.31
Toxic flavourants
Hundreds of different chemicals are added to e-liquids to impart flavour (see Section 18.5.3.4). On average, each e-cigarette contains 10 different flavourants.32
Most flavourants used in food are considered safe to eat. However, ingestion involved multiple steps of detoxification before the breakdown products of these flavours circulate around the body in the bloodstream. Some of these flavourants are known to be toxic when inhaled, where they are heated and enter the lungs. Through this route, these lungs are directly exposed to these chemicals without all the detoxification that occurs through ingestion.
Some flavorants have known inhalation toxicities, and others break down into toxic chemicals once heated. Examples of flavourants with toxicity in e-liquids or their emissions are acetovanillin, benzaldehyde, dihydrocoumarin, caryophyllene oxide, caffeic acid and diacetyl (Table 18.5.1). Benzaldehyde and cinnamaldehyde have been detected in illicit e-cigarettes sold in Australia at higher levels than permitted in therapeutic e-cigarettes (see Section 18.5.5.2). Experiments have shown that e-cigarettes with flavours emit higher amounts of toxic carbonyl compounds compared to unflavoured controls.33 There is currently insufficient evidence to determine whether the low levels of toxic flavourants are causing harm to people who use e-cigarettes.
Sweeteners
Sucralose, which is found in some e-liquids, can break down into potentially harmful chlorinated compounds when heated, and may contribute to the toxic chemicals found in emissions.34
Solvents: propylene glycol and glycerol (vegetable glycerin)
The solvents propylene glycol and glycerol (vegetable glycerin), which constitute most of e-liquids and aerosols, have been designated by the FDA as respiratory toxicants.35,36
When heated, these solvents undergo thermal degradation reactions to form numerous other chemicals products, many of which are toxic.37 A study has identified thermal degradation products in e-cigarette aerosols made only from propylene glycol and glycerol. These include acetaldehyde, acetone, acrolein, allyl alcohol, formic acid, glycidol, all of which are designated as toxic (Table 18.5.1) as well as acetic acid, dihydroxyacetone, lactaldehyde, glycolaldehyde, glyceraldehyde, propanal and hydroxyacetone.37 Higher temperatures or more power produced more toxic breakdown products.37 E-liquids with a higher proportion of glycerol produce higher levels of some toxic byproducts during aerosol formation.38
Propylene glycol can also 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/m3 and methylglyoxal at an average of 4,219 µg/m3 (range of 677 to 15,342 µg/m3).39 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.40
Toxic metals and metalloids
Toxic metals and metalloids found in e-cigarettes and their emissions include arsenic, beryllium, cadmium chromium, cobalt, lead, manganese, mercury, nickel and uranium (Table 18.5.1)28,29 Other metals of concern found in e-cigarettes include antimony, aluminium, barium, copper, iron, magnesium, tin, tungsten and zinc.6,24,30,41-44
Metals in e-liquids and emissions are considered contamination. There are two possible routes of entry for these substances: leaching from the metal components of e-cigarette devices or contamination of the chemicals added to e-liquids.45 There is evidence that both sources are likely to contribute to the chemicals in e-cigarette emissions. Some studies have detected toxic metals (including arsenic, cadmium, chromium, lead and nickel) in the refill e-liquids (stored in separate bottles), indicating that these were not the solely result of leaching from the e-cigarette device components.46 There is also evidence that elemental composition of the metal components in e-cigarettes matches detected particles.47 Higher chromium and aluminium levels may indicate degradation of the e-cigarette coil.48 Both lead and cadmium are common trace element contaminants of tobacco and other plants that yield nicotine and flavourants added to e-liquids.49 The extent to which these metals derive from the metal components of the e-cigarette device, or from contaminants in the e-liquid is uncertain.
E-cigarettes contain batteries, soldered joints and metallic heating coils of varying designs to heat e-liquids, forming the aerosol.24 The composition of the metal heating elements is un-standardised and reported to have considerable variation.43 Coils are reported made from metal alloys containing chromium, aluminium, iron, manganese, aluminium or nickel, whereas soldered joints and wires may contain copper, zinc, tin or lead.24 All of these metals have been found in various e-liquids or emissions. The use of leaded bronze in some e-cigarette devices may be associated with unusually high levels of lead in some e-cigarette emissions.50
The concentration of metals in e-cigarette emissions increases with higher power settings42 and with lower pH of e-liquids.51
A brand of e-cigarettes uses sonication (sound waves) to heat an e-liquid, rather than a coil, in devices known as “ultrasonic cigarettes”. A study of the metal contamination of the emissions from these devices found toxic metal levels equivalent to heated coil-style devices and high levels of the toxic metalloids arsenic and selenium.52
An analysis of the risks from long term exposure to e-cigarette 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.43 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.43
Aldehydes
A number of toxic and carcinogenic (cancer-causing) aldehydes are found in e-cigarette emissions. Aldehydes, such as formaldehyde, acrolein and crotonaldehyde, can be formed as reaction products during the heating of the e-liquid to create an aerosol. Aldehydes produced during e-cigarette use may come from chemical reactions that involve the solvents glycerol and propylene glycol53 and there is also evidence that they are also derived from some flavouring compounds.28
Aldehydes are also detected in tobacco smoke, but at significantly higher concentrations.54 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.55 It is toxic to the cardiovascular system and has a genotoxic (mutating DNA to cause cancer) effect in lung cells.56,57 Acrolein is an irritant to the human respiratory system, toxic to lung cilia and is likely to be a carcinogen in the lungs.57 Acrolein is toxic to the cardiovascular system and causes oxidative stress in the heart and increases cardiovascular disease risk.56,57 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.
Other carbonyl compounds
Carbonyl compounds include aldehydes (described above), plus other carbon-based chemical classes such as ketones, carboxylic acids and numerous types of esters. Many examples of these chemicals can be found in Table 18.5.1. Many carbonyl compounds have known toxicity, such as carcinogens (cancer-causing chemicals), reproductive toxicants and endocrine disruptors (interfere with hormone function).28,29
Nitrosamines
Carcinogenic (cancer-causing) N-nitrosamines (Section 12.4.3.7) have been detected in e-cigarette aerosols at low levels.28,29,58,59 These chemicals form as breakdown products from chemicals containing nitrogen, such as nicotine. In tobacco, nitrosamines form during the curing and ageing of tobacco. Since much of the nicotine used to make cigarettes comes from tobacco plants, it’s possible that the nitrosamines detected in e-cigarettes derive from this nicotine.
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 the tobacco-derived N-nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and arsenic.6
Polycyclic aromatic hydrocarbons (PAHs)
Polycyclic aromatic hydrocarbons (PAHs) (see Section 12.4.3.6) have also been detected in e-cigarette aerosols, albeit at low levels.29,58,59 PAHs are a range of similar compounds, having multiple rings of fused carbon atoms, which are usually formed in the production of smoke, such as tobacco smoke. PAHs are expected to be made during combustion, at higher temperatures than would usually be used in e-cigarettes. Very low levels have been detected in some e-cigarettes. Many PAHs are carcinogens, mutagen, reproductive toxicants or endocrine disruptors, as described in Table 18.5.1.
Hydrocarbons
Hydrocarbons are a wide range of molecules containing only carbon and hydrogen atoms, described in Section 12.4.3.4. Aside from PAHs, described above, a number of other toxic hydrocarbons are found in e-cigarettes or their emissions. These include carcinogens such as benzene, cyclohexane, ethyl-benzene, a- and b-pinene, styrene and toluene, which is a reproductive toxicant (Table 18.5.1). Many of these chemicals are also endocrine disruptors.6,28,29 Benzene is a hydrocarbon that can cause cancer, has been detected in e-liquids and aerosols, and found at higher levels when nicotine benzoate salts are used.60
Other organic compounds
As listed in Table 18.5.1, other organic compounds with toxicity concerns include dioxins, furans, alkenes, chlorinated substances, nitriles, phenols, alcohols, acids and epoxides. These compounds have a range of difference toxicities, such as carcinogens (furans, some acids, phenols), endocrine disruptors (phenols, some acids) and mutagens (furans) and causes of lung damage (nitriles) (details in Table 18.5.1).6,28,29
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. These chemicals have been found in e-liquids at low levels.61
Per- and polyfluoroalkyl substances (PFAS) are a group of synthetic compounds considered organic due to their carbon backbone. They are used in industrial processes, and known as “forever chemicals” as they are persistent pollutants; they do not break down easily in the human body or the environment. Low amounts of PFAS have also been found in some e-liquids refills.62
Toxic gases
E-cigarette emissions are aerosols, consisting of both gases and particles. A study that analysed the gaseous content of the aerosols from individual e-cigarette puffs from three major brands detected carbon monoxide (CO), nitrogen dioxide (NO2) and nitric oxide (NO). Single-puff amounts of all three exceeded the permissible exposure limits of the U.S. Occupational Safety and Health Administration, indicating potential toxicity.63 Carbon monoxide is made at much lower levels in e-cigarette emissions than from burning tobacco, where it is produced during combustion chemical reactions occurring at high temperatures. In e-cigarettes, lower temperature pyrolysis reactions are likely producing the carbon monoxide detected.64
Reactive oxygen species
Free radicals and reactive oxygen species (ROS) have been found in e-liquids.25 These are a range of molecules that have unpaired electrons, making them highly reactive and damaging to the body. Free radical and ROS exposure is considered to increase the risk of cancer and other diseases. Nicotine salt e-liquids containing benzoic acid have been found to have the highest level of ROS compared to other types of nicotine salts.25
Inorganic non-metals
The mineral quartz is the most common component of silica dust. When silica dust is very fine, known as respirable crystalline silica, it can cause lung cancer.65 Quartz has been detected at low levels in some e-liquids.29
Another non-metal inorganic compound found in e-cigarette emission is hydrazine, an endocrine disruptor, reproductive toxicant and carcinogen.29
Bacterial contamination in e-liquids
Numerous types of bacteria have been found in e-liquids, including Pseudomonas aeruginosa, Bacillus pumilus, B. megaterium and B. cereus.66 The risk of toxicity from these bacteria in e-liquids is uncharacterised.
18.5.4.5 Toxic emissions from e-hookah (electronic waterpipes)
An e-hookah is a device that produces an aerosol from heated e-liquid that may pass through water before inhalation (see Section 18.1.1.2). Toxic chemicals detected in the emissions from e-hookahs include the aldehydes called acetaldehyde and acrolein, and the hydrocarbon benzene, each of which are carcinogenic.67
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References
1. European Commission. Introduction to toxicology Available from: https://ec.europa.eu/health/ph_projects/2003/action3/docs/2003_3_09_a21_en.pdf.
2. 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
3. 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.
4. 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
5. Herrington JS, Myers C, and Rigdon A. Analysis of nicotine and impurities in electronic cigarette solutions and vapor. Restek, 2017. Available from: https://www.restek.com/global/en/articles/analysis-of-nicotine-and-impurities-in-electronic-cigarette-solutions-and-vapor.
6. 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.
7. Kassem NOF, Strongin RM, Stroup AM, Brinkman MC, El-Hellani A, et al. A review of the toxicity of ingredients in e-cigarettes, including those ingredients having the FDA's "generally recognized as safe (GRAS)" regulatory status for use in food. Nicotine & Tobacco Research, 2024; 26(11):1445-54. Available from: https://www.ncbi.nlm.nih.gov/pubmed/38783714
8. 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
9. Eshraghian EA and Al-Delaimy WK. A review of constituents identified in e-cigarette liquids and aerosols. Tobacco Prevention & Cessation, 2021; 7:10. Available from: https://www.ncbi.nlm.nih.gov/pubmed/33585727
10. Jabba SV, Erythropel HC, Torres DG, Delgado LA, Woodrow JG, et al. Synthetic cooling agents in US-marketed e-cigarette refill liquids and popular disposable e-cigarettes: chemical analysis and risk assessment. Nicotine & Tobacco Research, 2022; 24(7):1037-46. Available from: https://www.ncbi.nlm.nih.gov/pubmed/35167696
11. Yassine A, Antossian C, El-Hage R, and Saliba NA. A quick method for the determination of the fraction of freebase nicotine in electronic cigarettes. Chemistry Research in Toxicology, 2023; 36(7):1021-7. Available from: https://www.ncbi.nlm.nih.gov/pubmed/37406365
12. Harvanko AM, Havel CM, Jacob P, and Benowitz NL. Characterization of nicotine salts in 23 electronic cigarette refill liquids. Nicotine & Tobacco Research, 2020; 22(7):1239-43. Available from: https://www.ncbi.nlm.nih.gov/pubmed/31821492
13. Gray N, Halstead M, Gonzalez-Jimenez N, Valentin-Blasini L, Watson C, et al. Analysis of toxic metals in liquid from electronic cigarettes. International Journal of Environmental Research and Public Health, 2019; 16(22). Available from: https://www.ncbi.nlm.nih.gov/pubmed/31766137
14. Jenkins C, Powrie F, Kelso C, and Morgan J. Chemical analysis and flavor distribution of electronic cigarettes in Australian schools. Nicotine & Tobacco Research, 2025; 27(6):997-1005. Available from: https://www.ncbi.nlm.nih.gov/pubmed/39531255
15. Reilly SM, Cheng T, Feng C, and Walters MJ. Harmful and potentially harmful constituents in e-liquids and aerosols from electronic nicotine delivery systems (ENDS). Chemistry Research in Toxicology, 2024; 37(7):1155-70. Available from: https://www.ncbi.nlm.nih.gov/pubmed/38924487
16. Tehrani MW, Newmeyer MN, Rule AM, and Prasse C. Characterizing the chemical landscape in commercial e-cigarette liquids and aerosols by liquid chromatography-high-resolution mass spectrometry. Chemistry Research in Toxicology, 2021; 34(10):2216-26. Available from: https://www.ncbi.nlm.nih.gov/pubmed/34610237
17. Turina A, Passoni A, Gallus S, Lugo A, Klerx W, et al. On the extension of the use of a standard operating procedure for nicotine, glycerol and propylene glycol analysis in e-liquids using mass spectrometry. Tobacco Induced Diseases, 2024; 22. Available from: https://www.ncbi.nlm.nih.gov/pubmed/39263493
18. World Health Organization. WHO Tobacco Laboratory Network (TobLabNet). Available from: https://www.who.int/groups/who-tobacco-laboratory-network.
19. Tobacco Laboratory Network. Standard operating procedure for determination of nicotine, glycerol and propylene glycol in e-liquids. WHO TobLabNet Official Method SOP11. Geneva: World Health Organisation, 2021. Available from: https://www.who.int/publications/i/item/9789240022744.
20. Joint Action on Tobacco Control. WP8 - D8.3 Report on the results of inter-laboratory variability of EU MS emission data., Agreement n°: 761297-JATC-HP-JA-03-2016. JATC, 2021. Available from: https://jaotc.eu/wp-content/uploads/2022/03/D8.3-Report-on-the-results-of-inter-laboratory-variability.pdf.
21. Mikheev VB, Buehler SS, Brinkman MC, Granville CA, Lane TE, et al. The application of commercially available mobile cigarette topography devices for e-cigarette vaping behavior measurements. Nicotine & Tobacco Research, 2020; 22(5):681-8. Available from: https://www.ncbi.nlm.nih.gov/pubmed/30215774
22. Visser WF, Krusemann EJZ, Klerx WNM, Boer K, Weibolt N, et al. Improving the analysis of e-cigarette emissions: detecting human "dry puff" conditions in a laboratory as validated by a panel of experienced vapers. International Journal of Environmental Research and Public Health, 2021; 18(21). Available from: https://www.ncbi.nlm.nih.gov/pubmed/34770036
23. Halstead M, Gray N, Gonzalez-Jimenez N, Fresquez M, Valentin-Blasini L, et al. Analysis of toxic metals in electronic cigarette aerosols using a novel trap design. Journal of Analytical Toxicology, 2020; 44(2):149-55. Available from: https://www.ncbi.nlm.nih.gov/pubmed/31588518
24. Aherrera A, Lin JJ, Chen R, Tehrani M, Schultze A, et al. Metal concentrations in e-cigarette aerosol samples: a comparison by device type and flavor. Environmental Health Perspectives, 2023; 131(12):127004. Available from: https://www.ncbi.nlm.nih.gov/pubmed/38048100
25. Tran LN, Rao G, Robertson NE, Hunsaker HC, Chiu EY, et al. Quantification of free radicals from vaping electronic cigarettes containing nicotine salt solutions with different organic acid types and concentrations. Chemistry Research in Toxicology, 2024; 37(6):991-9. Available from: https://www.ncbi.nlm.nih.gov/pubmed/38778043
26. 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
27. Harris T. Physical and chemical characterization of aerosols produced from commercial nicotine salt-based e-liquids. Chemistry Research in Toxicology, 2025; 38(1):115-28. Available from: https://www.ncbi.nlm.nih.gov/pubmed/39654291
28. 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.
29. El Bouz M, Neto C, Mansuy T, Becher R, Valen H, et al. Health hazards of e-cigarettes and heated tobacco products: a comprehensive analysis of hazardous substances and regulatory gaps. Nicotine & Tobacco Research, 2026; 28(5):710-9. Available from: https://www.ncbi.nlm.nih.gov/pubmed/41351476
30. Kopa-Stojak PN and Pawliczak R. Disposable electronic cigarettes - chemical composition and health effects of their use. A systematic review. Toxicology Mechanisms and Methods, 2025; 35(3):250-61. Available from: https://www.ncbi.nlm.nih.gov/pubmed/39513380
31. Effah F, Sengupta M, Sun Y, Faizan MI, Kaur G, et al. A comparative toxicological evaluation of nicotine and its analog 6-methyl nicotine in E-cigarette aerosol utilizing a 3D in vitro human respiratory model. Toxicology, 2026; 522:154421. Available from: https://www.ncbi.nlm.nih.gov/pubmed/41628686
32. Krusemann EJZ, Havermans A, Pennings JLA, de Graaf K, Boesveldt S, et al. Comprehensive overview of common e-liquid ingredients and how they can be used to predict an e-liquid's flavour category. Tobacco Control, 2021; 30(2):185-91. Available from: https://www.ncbi.nlm.nih.gov/pubmed/32041831
33. Fazeli E, Martinez B, Son Y, and Khlystov A. Flavoring compound chemical class and vaping conditions determine toxic carbonyl emissions from e-cigarettes. Chemistry Research in Toxicology, 2026; 39(3):329-38. Available from: https://www.ncbi.nlm.nih.gov/pubmed/41649144
34. Tkach VV, Morozova TV, Gaivao I, Martins-Bessa A, Ivanushko YG, et al. Sweeteners in e-cigarettes: A minireview of flavoring and biological action. Journal of Xenobiotics, 2025; 15(6). Available from: https://www.ncbi.nlm.nih.gov/pubmed/41440756
35. Sun Y, Kaur G, Effah F, Friedman A, and Rahman I. Cytotoxic and oxidative effects of commercially available propylene glycol (PG) and vegetable glycerin (VG): Common humectants in electronic cigarettes. Toxicology Reports, 2025; 15:102171. Available from: https://www.ncbi.nlm.nih.gov/pubmed/41377029
36. Sun Y, Lin K, Effah F, Cartujano Barrera F, Li D, et al. Toxicity of humectants propylene glycol and vegetable glycerin in electronic nicotine delivery systems. Toxicology Letters, 2025; 413:111739. Available from: https://www.ncbi.nlm.nih.gov/pubmed/41043656
37. Jensen RP, Strongin RM, and Peyton DH. Solvent chemistry in the electronic cigarette reaction vessel. Scientific Reports, 2017; 7:42549. Available from: https://www.ncbi.nlm.nih.gov/pubmed/28195231
38. Ooi BG, Dutta D, Kazipeta K, and Chong NS. Influence of the e-cigarette emission profile by the ratio of glycerol to propylene glycol in e-liquid composition. ACS Omega, 2019; 4(8):13338-48. Available from: https://www.ncbi.nlm.nih.gov/pubmed/31460462
39. 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
40. 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
41. Kapiamba KF, Hao W, Adom S, Liu W, Huang YW, et al. Examining metal contents in primary and secondhand aerosols released by electronic cigarettes. Chemistry Research in Toxicology, 2022; 35(6):954-62. Available from: https://www.ncbi.nlm.nih.gov/pubmed/35385266
42. 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
43. 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
44. Queensland Government. Vaping: results are in. 2023. Available from: https://statements.qld.gov.au/statements/97806.
45. Besaratinia A. Electronic cigarette-derived metals: Exposure and health risks in vapers. Chemistry Research in Toxicology, 2025; 38(4):542-56. Available from: https://www.ncbi.nlm.nih.gov/pubmed/40094421
46. Granata S, Vivarelli F, Morosini C, Canistro D, Paolini M, et al. Toxicological aspects associated with consumption from electronic nicotine delivery system (ENDS): focus on heavy metals exposure and cancer risk. International Journal of Molecular Sciences, 2024; 25(5). Available from: https://www.ncbi.nlm.nih.gov/pubmed/38473984
47. Gajdosechova Z, Marleau-Gillette J, Polivchuk M, Kosarac I, Katuri GP, et al. Tracking metal presence in cannabis vaping products from source to inhalation. Scientific Reports, 2025; 15(1):31939. Available from: https://www.ncbi.nlm.nih.gov/pubmed/40883388
48. Chwal J, Filipowska A, Antonowicz M, Lisicki D, Kostka P, et al. Elemental composition of vaping and smoking aerosols: Influence of liquid type and tank conditions. PLoS One, 2025; 20(12):e0338087. Available from: https://www.ncbi.nlm.nih.gov/pubmed/41364684
49. Duan S, Yang J, Zhou Z, Xiao Y, Li S, et al. Quantitative relationship between paddy soil properties and cadmium content in tobacco leaves. Bulletin of Environmental Contamination and Toxicology, 2021; 106(5):878-83. Available from: https://www.ncbi.nlm.nih.gov/pubmed/33811509
50. Salazar MR, Saini L, Nguyen TB, Pinkerton KE, Madl AK, et al. Elevated toxic element emissions from popular disposable e‑cigarettes: sources, life cycle, and health risks. ACS Central Science, 2025; 11(8):1345-54. Available from: https://www.ncbi.nlm.nih.gov/pubmed/40893954
51. Lawson D and Coulson J. Influence of e-liquid ph on heavy metal emissions in open-system electronic cigarette aerosols and associated health risks. Nicotine & Tobacco Research, 2026. Available from: https://www.ncbi.nlm.nih.gov/pubmed/41894250
52. Omaiye EE and Talbot P. Quantification of 16 metals in fluids and aerosols from ultrasonic pod-style cigarettes and comparison to electronic cigarettes. Environmental Health Perspectives, 2025; 133(5):57020. Available from: https://www.ncbi.nlm.nih.gov/pubmed/40207990
53. 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
54. 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
55. Britannica. Formaldehyde. Available from: https://www.britannica.com/science/formaldehyde.
56. WHO Study Group on Tobacco Product Regulation. 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.
57. 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/.
58. 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. Chemistry Research in Toxicology, 2016; 29(10):1662-78. Available from: https://www.ncbi.nlm.nih.gov/pubmed/27641760
59. 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
60. El Hajj Moussa F, Hayeck N, Hajir S, El Hage R, Salman R, et al. Enhancement of benzene emissions in special combinations of electronic nicotine delivery system liquid mixtures. Chemistry Research in Toxicology, 2024; 37(2):227-33. Available from: https://www.ncbi.nlm.nih.gov/pubmed/38241642
61. Venugopal PD, Addo Ntim S, Goel R, Reilly SM, Brenner W, et al. Environmental persistence, bioaccumulation, and hazards of chemicals in e-cigarette e-liquids: short-listing chemicals for risk assessments. Tobacco Control, 2024; 33(6):781-9. Available from: https://www.ncbi.nlm.nih.gov/pubmed/37845042
62. Kubica P, Majchrzak T, and Vakh C. Unveiling per- and polyfluoroalkyl substances contamination in e-cigarette refill liquids: A comprehensive analytical assessment. Science of The Total Environment, 2025; 960:178297. Available from: https://www.ncbi.nlm.nih.gov/pubmed/39765165
63. Kapiamba KF, Achterberg S, Lin TC, Whitefield PD, Huang YW, et al. Characterizing the transient emission of particles and gases from a single puff of electronic cigarette smoke. Chemical Research in Toxicology, 2025; 38(2):270-80. Available from: https://www.ncbi.nlm.nih.gov/pubmed/39818726
64. Jaegers NR, Hu W, Weber TJ, and Hu JZ. Low-temperature (< 200 degrees C) degradation of electronic nicotine delivery system liquids generates toxic aldehydes. Scientific Reports, 2021; 11(1):7800. Available from: https://www.ncbi.nlm.nih.gov/pubmed/33833273
65. International Agency for Research on Cancer. Silica, some silicates, coal dust and para-aramid fibrils. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, 68: IARC, 1997. Available from: https://publications.iarc.who.int/Book-And-Report-Series/Iarc-Monographs-On-The-Identification-Of-Carcinogenic-Hazards-To-Humans/Silica-Some-Silicates-Coal-Dust-And-Em-Para-Em--Aramid-Fibrils-1997.
66. Chattopadhyay S, Malayil L, and Sapkota AR. Viable, multi-drug-resistant bacteria recovered from e-liquids used with commercial electronic cigarettes. International Journal of Environmental Research and Public Health, 2025; 22(11). Available from: https://www.ncbi.nlm.nih.gov/pubmed/41302671
67. Klupinski TP, Adetona A, Ivanov A, Richardson A, Strozier ED, et al. Chemical constituents and particle size distribution of mainstream emission from electronic waterpipe. Nicotine & Tobacco Research, 2025; 27(7):1265-73. Available from: https://www.ncbi.nlm.nih.gov/pubmed/39887006