Electronic cigarette-derived metals: exposure and health risks in vapers

Abstract

Despite the popularity of electronic cigarettes (e-cigs) among adolescent and youth never-smokers and adult smokers seeking a less harmful substitute for tobacco cigarettes, the long-term health impact of vaping is largely unknown. Biochemical, molecular, and toxicological analyses of biospecimens from e-cig users as well as assays in relevant in vitro models and in silico studies can identify chemical constituents of e-cig emissions that may contribute to the disease-causing potential of vaping. E-cig aerosol inhaled by vapers contains a wide range of toxic and carcinogenic compounds, of which metals are of particular concern. This is due to the known or suspected role of various metals in the pathogenesis of numerous diseases. Many metals and metalloids (herein referred to as “metals”) have been detected in e-cig liquid (e-liquid) and aerosol, and/or in cells, tissues, biofluids, or other specimens from e-cig users. Metals can contaminate the ingredients of e-liquid or corrode from the e-cig device internal components. Metals may also be directly aerosolized from the surface of the heating element or other parts of the device. Inhalation of e-cig metal emissions in habitual vapers and non-users through secondary exposure may add significantly to the body burden of toxic and carcinogenic chemicals. This review summarizes the state of research on the e-cig-derived metals and their contributions to the estimated health risks of vaping. Highlighting the chemical composition of e-cig liquid and aerosol, it focuses on the metal contents of the inhaled aerosol and the health risks associated with this exposure. Emphasis is placed on adolescents and youth who are vulnerable populations and bear a disproportionate burden of risk and harm from tobacco products. The gaps in knowledge, methodological challenges, and opportunities ahead are discussed. The importance of translating research findings into actionable information that can be used for regulation of the manufacturing of tobacco products is underscored.

Graphical Abstract

1. Introduction

The ongoing epidemic of youth vaping in the U.S. and the growing popularity of electronic cigarettes (e-cigs) among adolescents and youth in many parts of the world are unsettling and evolving public health problems^{1, 2}. Added to these concerns is the widespread use of e-cigs by adult smokers seeking a less harmful alternative to tobacco cigarettes^{1, 3}. While the short-term adverse effects of e-cig use have and continue to be investigated, the long-term effects of vaping on health remain mostly unknown^{4, 5}.

Understanding how e-cigs may cause adverse health consequences in users and non-users through secondary exposure is a high-priority research area^{4, 6, 7}. Towards this goal, evaluation of toxicity from exposure to complex mixture of chemicals present in-cig aerosol is an important avenue for research^8–12. Biochemical, molecular, and toxicological analyses of biospecimens from e-cig users as well as assays in in vitro models that are physiologically relevant to humans and in silico studies have the promise of identifying chemical constituents of e-cig aerosol that may contribute to the disease-causing potential of vaping^13–18. Furthermore, these investigations can uncover the role of product characteristics in modulating the toxicity profile of e-cigs. The term product characteristics encompasses materials, ingredients (additives, nicotine formulations, nicotine concentration, and flavors), design, composition, heating source, nicotine flux, and other features of e-cigs. The above research is expected to generate evidence-based data and actionable information that can be used by the U.S. Food and Drug Administration (FDA), which has the authority to regulate the manufacturing, marketing, and distribution of tobacco products to protect public health¹⁹.

E-cig aerosol inhaled by vapers contains a multitude of toxic and carcinogenic compounds, of which metals are of special concern^{4, 7, 20, 21}. This is due to due to the known or suspected role of various metals in the pathogenesis of a multitude of diseases. Diverse metals and metalloids (hereafter referred to as “metals”) have been detected in e-cig liquid (e-liquid) and aerosol, and/or in biospecimens from e-cig users, including blood, urine, saliva, hair, and exhaled breath^22–28. Metals are present in trace amounts in e-liquid as contaminants of its constituents, which include humectants, flavors, and/or nicotine^29–34. Metals can also leach from the internal device components into the e-liquid^35–38. Device components that contain leachable metals include heating filament (coil or mesh), solder joints, brass clamps, insulating sheaths, wicks, fluid reservoir, and vapor path^35–38. Upon heating and vaporization, e-liquid chemicals, including its contaminant and leachable metal contents, are inhaled by the users^39–43. Metals may also be directly aerosolized from the surface of the resistance filament or other parts of the device due to excessive heating during operation^43–46. Inhalation of metals in humans is known to cause numerous adverse health effects, including various diseases and conditions^47–49. These include, but are not limited to, damage to vital organs, such as the lungs, liver, kidney, and brain, and neurologic, respiratory, cardiovascular, developmental, immunologic, and carcinogenic effects^47–50.

This review article summarizes the current knowledge on the e-cig-derived metals and their contributions to the estimated cancer and non-cancer health risks of vaping. It briefly discusses the invention and evolution of modern e-cigs, trends in use, and controversies surrounding the health risks or potential benefits of e-cigs relative to conventional cigarettes. Highlighting the chemical composition of e-cig liquid and aerosol, it focuses on the e-cig-derived toxic and carcinogenic metals and their contributions to the disease-causing potential of vaping. A special emphasis is placed on adolescent and youth e-cig users and the underlying reasons why these populations are of special interest and high priority for vaping research. The knowledge gaps, methodological challenges, and directions of future research are outlined. The opportunities for translation of research findings to actionable information, which can be used for regulation of the manufacturing of tobacco products, are discussed.

2. Invention and evolution of e-cig technology

In 2003, the first modern e-cigs were developed by the Chinese pharmacist and inventor, Hon Lik, a former smoker who quit smoking after his father, also a heavy smoker, died of lung cancer⁵¹. E-cigs entered the U.S. market in 2007 as a novel nicotine delivery system^{4, 52, 53}. E-cigs are handheld battery-powered devices that mimic the feel and/or look of tobacco cigarettes^{7, 54}. E-cig use is commonly referred to as “vaping” and e-cig users are colloquially known as “vapers”^{4, 55}. E-cigs heat a solution, called e-liquid/e-juice, that contains a base solvent (propylene glycol (PG) and glycerin/vegetable glycerin (VG) at different proportions), nicotine, and a vast assortment of flavors and additives at varying concentrations^{7, 54, 55}. Heating and subsequent vaporization of e-liquid render an aerosol (‘vapor’) that users inhale through a mouthpiece^{7, 56}. While the ingredients of e-liquid are generally recognized as safe (GRAS) for ingestion, inhalation of the aerosolized ingredients cannot be considered risk free^{5, 7, 55}. Since their introduction into the market, e-cigs have evolved rapidly and significantly, from the first-generation “cig-a-likes” that were designed to look like conventional cigarettes, to the second-generation vape pens, third-generation box mods, and fourth-generation pod-based devices; the pod-based e-cig devices have and continue to be most popular among teens and youth^{7, 55}. With the exception of the COVID-19 era, e-cig devices have changed drastically in design and features every 2–4 years^{2, 7}. The substantial changes in feature and design of e-cigs have been accompanied by tremendous diversification of the e-liquids^57–59. Numerous chemicals have been added to e-liquids to create products that appease virtually every user’s desire^{5, 11, 59, 60}. Currently, there are about 20,000 e-liquids in the market^{11, 59, 61}. It is inevitable that the ever-growing number of e-liquids, containing countless combination of chemicals, exposes e-cig users to a broad range of compounds with known and unknown toxicity profiles^{13, 56, 62–68}.

3. Trends in e-cig use

Since the introduction of e-cigs into the U.S. market, these alternative nicotine delivery systems have gained popularity at a tremendous pace^{7, 53, 69}. In recent years, e-cigs have become the second most popular tobacco product among U.S. adults, with combustible cigarettes being the top product⁷⁰. From 2011 and onward, e-cigs have been vastly embraced by teens and youth who are known to experiment with tobacco products⁷¹. In 2021, 4.5% of American adults aged 18 and over were current e-cig users, with those between the ages of 18 and 24 having the highest rate of use (11.0%)⁷². Since 2014, e-cigs have been the most commonly used tobacco product U.S. youth⁷¹. In 2023, 2.1 million American middle and high school students (7.7%) were current e-cig users⁷³. Of these, 550,000 (4.6%) were middle school students (6^th to 8^th grades) and 1.56 million (10.0%) were high school students (9^th to 12^th grades). Among students who currently used e-cigarettes, the great majority reported using disposable e-cigs (60.7%) and e-cigs with pre-filled or refillable pods or cartridges (16.1%). Furthermore, 89.4% reported using flavored e-cig products, while fruit and candy were the most popular flavors⁷³. Survey data from the fall of 2023 showed that the most used tobacco products among American college students were e-cigs or other vape products. Approximately 75 percent of the college students who used tobacco products in the past three months, reported using e-cigs or other vape products⁷⁴.

4. Health risks or potential benefits of e-cig use relative to cigarette smoking

E-cigs have been marketed and advertised originally as safe, and subsequently as a less harmful alternative to combustible tobacco cigarettes^53–55. Unlike conventional cigarettes that burn tobacco leaves to generate smoke, e-cigs heat a solution (e-liquid/e-juice) to produce inhalable aerosol^{5, 7}. Heat-based vaporization of e-liquid results in fewer and generally lower levels of harmful and potentially harmful chemicals when compared to combustible tobacco cigarettes^{7, 55}. This has led, in part, to the perception that e-cigs are a reduced-harm alternative to tobacco cigarettes^{1, 53}. However, cumulative exposure of habitual e-cig users to the toxicants and carcinogens present in e-cig aerosol, irrespective of their quantity and concentrations, makes vaping a definite health risk^{7, 75}. Currently, there is a contentious debate over the health risks or potential benefits of e-cig use relative to cigarette smoking^{1, 6, 76–78}. On the one hand, vaping advocates claim that e-cig use, especially when combined with behavioral counseling, aids in smoking cessation^{1, 53}. Meta-analyses of randomized clinical trials have lent support to this claim^{3, 79}. An important caveat, however, is that e-cigs have been shown to help adult smokers quit only when they have been used as a medical intervention within the context of clinical trials^{3, 53, 80–83}. On the other hand, many population-based studies have documented that e-cigs, as a consumer product, are not effective in smoking cessation^{3, 79, 84–87}. Currently, nowhere in the world are e-cigs approved as a medical intervention. Instead, e-cigs are consumer products that can be purchased by anyone over a certain age, and used as much or as often as one wishes^{86, 88}. Another claim is that the remarkable reduction in youth smoking rates in recent years is driven by vaping^{53, 89}. The counterargument is that vaping has facilitated experimentation of teens and youth with tobacco products^{77, 88}. Consequently, a new generation of adolescents, youth, and young adults, who would never have smoked in the first place, has become addicted to nicotine^{4, 52, 60}. It is also argued that vaping may reverse the decades-long successful public health campaign against smoking, thus “renormalizing” this unhealthy habit^{82, 90, 91}. Whilst proponents of harm reduction argue that e-cigs are a viable substitute for the irrefutably dangerous tobacco cigarettes^{1, 53, 76}, evidence is accumulating on the pernicious effects of vaping^{4, 5, 7, 15, 16, 92–95}. Elucidating the biological effects of e-cig use can help determine the health risks or potential benefits of vaping relative to smoking^{7, 15}. This will, in turn, help resolve a pressing public health problem, concerning millions of adolescent and youth never-smokers and adult former smokers who have taken up this controvertible habit^{1, 3, 88}.

5. Complexity of research in adult vapers versus adolescent and youth vapers

Adult vapers are likely to be former smokers or ‘dual’ users of e-cigs and combustible cigarettes, i.e., those who alternate between vaping and smoking^{1, 3, 53, 56}. The published research on the effects of vaping on health is often contested by advocates of e-cigs and harm reduction who argue that the study subjects in many reports consist of ‘adult’ vapers with ‘past’ or ‘current’ smoking record, namely, former smokers current vapers or dual users, respectively^{6, 55}. The reported findings are refuted and the observed results are attributed, wholly or in part, to the persistent effects of past smoking (in ex-smokers) or current smoking (in dual users)^{1, 53, 55}. Unlike adult vapers^{75, 96, 97}, adolescent and youth e-cig users have little or no history of cigarette smoking^{56, 98–100}. Therefore, while it is important to investigate the health consequences of e-cig use in the general population, it is advantageous to tease out the influence of ‘exclusive’ vaping in adolescents and youth who have minimal or no confounding exposure to cigarette smoking^{4, 73, 95}.

6. Chemical constituents of e-cig liquid and vapor

Analytical chemistry has shown that many of the same toxic and carcinogenic compounds present in cigarette smoke are also detectable in e-cig liquid and aerosol, although in fewer numbers and at generally substantially lower concentrations^{4, 7, 55}. These harmful and potentially harmful chemicals include, but are not limited to, carbonyl compounds, volatile organic compounds (VOCs), free radicals, and metals^{5, 7, 9, 54}. There are also chemicals that are exclusively detectable in e-cig aerosol but not cigarette smoke^{10, 11}. The latter chemicals likely originate from mixing of the e-liquid ingredients and/or vaporization of the base solvent (PG/VG), flavors, or leachable elements from the internal device components^{63, 101}. There are, at least, seven groups of (potentially) harmful compounds in e-cig liquid and aerosol, including carbonyl compounds, VOCs, nicotine, nanoparticles, trace metals, bacterial endotoxins, and β-glucans⁵. The lower concentrations of toxicants and carcinogens in e-cig aerosol accord with the mode of operation of these devices as e-cigs, unlike traditional cigarettes, do not ‘burn’ tobacco to produce inhalable materials^{54, 55}. This has led, in part, to the perception that e-cig use is safe or less-harmful than tobacco smoking^{1, 53}. Although the reduced levels of toxic and carcinogenic compounds in e-cig aerosol are in tune with the concept of risk mitigation, they cannot equate to an absence of risk⁵⁶. A large body of evidence shows that exposure to many of the same constituents of e-cig aerosol, at varying concentrations, is associated with cardiovascular, immune-related (inflammatory), and respiratory diseases, and cancer, among other diseases^{5, 7, 54, 102, 103}.

7. E-cig derived metals

E-cig aerosol inhaled by vapers contains a wide variety of toxic and carcinogenic compounds, of which metals are of special concern^{4, 7, 20, 21}. This is due to the known or suspected role of many metals in the pathogenesis of a wide host of diseases^47–49. Various metals and metalloids (herein referred to as “metals”) have been detected in e-liquid and e-cig aerosol, and/or in biospecimens from e-cig users, including blood, urine, saliva, hair, and exhaled breath^22–28. The detected metals include aluminum (Al), arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb), antimony (Sb), selenium (Se), silicon (Si), tin (Sn), and zinc (Zn), among others^{17, 18, 104–109}. Metals are present in trace amounts in e-liquid as contaminants of its constituents, i.e., humectants, flavors, and/or nicotine^29–34. Metals can also leach from the internal device components, including heating filament (coil or mesh), solder joints, brass clamps, insulating sheaths, wicks, fluid reservoir, and vapor path, into the e-liquid^35–38. Upon heating and vaporization, e-liquid chemicals, including its contaminant or leachable metal contents, are inhaled by the users^39–43. Metals may also be directly aerosolized from the surface of the resistance filament or other parts of the device due to excessive heating during operation^43–46.

The propensity of the numerous metallic components of e-cigs for corrosion over time, especially when they come in contact with e-liquid constituents, together with cyclic temperature changes occurring within the devices can determine the physico-chemical and toxicological properties of e-cig emissions^{39, 46, 109, 110}. Table 1 summarizes the concentrations of select metals in e-cig aerosols. For comparison purposes, the corresponding metal concentrations in mainstream cigarette smoke are shown in Table 2. Overall, there are marked variabilities in concentration of metals within and between e-cig devices and brands^{21, 28, 41, 111}. These variations have mainly been attributed to differences in e-cig design and features, and methods of core assembly construction^{22, 32, 43, 112–114}. The metal content of e-cig aerosols have also been shown to depend on the device power settings, e-liquid flavor(s) and nicotine concentrations, techniques for sample collection, and vaping topography^{22, 40, 42, 105, 107, 108, 114–117}. The concentration of metals, like chromium, manganese, and nickel has been reported to be generally higher in e-cig aerosol than cigarette smoke, while cadmium levels tend to be lower^{21, 28, 111}. Furthermore, analysis of biospecimens from e-cig users and cigarette smokers has shown higher or similar levels of most metals/metalloids, with the exception of Cd, in vapers as compared to smokers^{20, 23–26}. The sheer number of parameters that influence the metal emissions of e-cigs^39–43 makes safety testing of these alternative tobacco products highly challenging for both manufacturers and regulatory agencies.

Table 1.

Concentrations of metals/metalloids in e-cig aerosols.

Device type	Al	As	Cd	Cr	Cu	Fe	Mn	Ni	Pb	Sb	Se	Sn	Zn	Ref.
Cig-a-like	39.4	*	*	0.7	20.3	52	0.2	0.5	1.7	*	*	3.7	5.8	240
Cig-a-like	*	<0.10 LOD	0.6 ± 0.6	<0.01 LOD	<0.06 LOD	*	<0.03 LOD	1.2 ± 0.8	0.6 ± 1.1	*	<0.04 LOD		<0.18 LOD	241
Cig-a-like	*	<1.4 LOQ	<1.4 LOQ	1.4	*	*	<1.3 LOQ	<1.4 LOQ	<1.4 LOQ	*	<1.4 LOQ	<1.4 LOQ	*	242
Cig-a-like	*	*	*	*	117 ± 83.6	*	*	*	*	*	*	*	*	243
Cig-a-like	*	*	*	0.6 ± 0.5	8.9 ± 10.2	*	*	2.0 ± 3.7	*	*	*	88.6 ± 322	3.8 ± 6.2	244
Cig-a-like	*	NQ	<0.2 LOD	0.4	1.9	4.2	*	NQ	<0.5 LOD	*	<0.08 LOD	<0.6 LOD	12.3	245
Cig-a-like	*	0.14	*	4.0	1.2	*	*	0.3	*	0.3	*	0.1	6.2	39
Cig-a-like	1.3	0.6	*	*	8	0.8	*	0.4	*	0.7	5.3	2.9	3.6	35
Tank	*	<0.7 LOQ	0.1	7	*	*	*	*	2.7	0.7	*	*	*	246
Tank	290	0.13	<0.01 LOD	*	<0.01 LOD	0.07	<0.01 LOD	14.5	<0.01 LOD	*	*	*	61.9	247
Tank	0.02 ± 0.05	0.004 ± 0.01	0.0001 ± 0.003	0.07 ± 0.27	0.05 ± 0.12	0.39 ± 1.33	0.01 ± 0.02	0.32 ± 1.06	0.08 ± 0.27	0.002 ± 0.004	*	0.02 ± 0.06	0.54 ± 0.88	30
Cig-a-like, Tank	0.98	*	*	*	0.98	0.44	0.01	0.05	0.21	*	*	*	0.65	110

Concentrations of all metals/metalloids are expressed as nanograms per puff of e-cig aerosol.

Results are shown as means ± standard deviation (if reported). The following studies did not report standard deviation or any other indicator of variability: Refs.^{35, 110, 240, 245, 247}.

Data were compiled by Zhao et al.²⁰.

^*Not measured

LOD = Limit of detection; LOQ = Limit of quantification; NQ = Not quantifiable.

Table 2.

Concentrations of metals/metalloids in cigarette smoke.

Cigarette type	Al	As	Cd	Cr	Cu	Fe	Mn	Ni	Pb	Se	Zn	Ref.
Marlboro Red ISO	4.0				5.8			0.4		0.8		35
Marlboro Red CS								0.2	0.7	1.5		35
American brands (N=12)								0.000007	0.01		1.2	248
Marlboro *	533.3	4.0	5.3		3.3	--	133.3	>UDL	0.7		66.7	247
NA†	22.8	0.8 – 1.5	70.0		1.2	28.0		42.1	19.3		21.5	249
Local and imported brands (N=12)	14.7		1.1					0 – 34.0	6.3			250
Marlboro and local brands (N=20)		0.3	4.3		0.9	1.1		0.1	2.1		8.5	251
Local brands (N=25)			0.5 – 23.3		12.7		0.2		1.1 – 65.3		8.0 – 80.7	252
Local brand							0.2					253

Concentrations of all metals/metalloids are expressed as nanograms per puff of mainstream cigarette smoke.

Results are shown as means (Refs.^{35, 247–253}) and/or range (Refs.^{249, 250, 252}).

^*Regular Marlboro (full flavor)

^†NA = Not available; no information was available.

UDL = Upper detection limit; Non-conclusive as the results were above the detection limit of 1389 μg/45 puffs.

Cannabis is known to contain trace amounts of toxic metals as environmental factors, such as polluted soil and water can contaminate the cannabis plant, also known as Cannabis sativa or hemp^{118, 119}. While cannabis (marijuana) vaping is on the rise, especially among youth^{120, 121}, scant data are available on the metal contents of cannabis vape liquid or aerosol^122–125. Vaping cannabis is a non-combustion process, whereby a cannabis liquid concentrate is aerosolized upon contact with a resistance-heated element, and inhaled through a mouthpiece^{125, 126}. Cannabis flower/bud may also be used for vaping^{125, 126}. Cannabis vaping can result in exposure of the users to metals originating not only from the cannabis itself but also from the vaping devices packaged with liquid cannabis cartridges. In case of nicotine vaping, e-liquid is known to be contaminated by the metal components of e-cig devices^39–43, so, it is highly likely that the same happens to the cannabis e-liquid after it comes into contact with the device components, both during storage and operation of the vaping devices. The nonpolar nature of the aerosol generated from cannabis e-liquid makes aqueous-based capture methods, which are conventionally used for analysis of metals in e-cig aerosol or cigarette smoke, largely inadequate for detection and quantification of metals in aerosols from cannabis vapes¹²⁴. Future methodological advances should facilitate measurement of the metal contents of cannabis vs. nicotine vape emissions. The rising popularity of medicinal and recreational cannabis vaping, in view of its legalization in many states in the U.S., Canada, and other countries^{127, 128}, underscores the importance of investigating the health risks of exposure to metals in cannabis vapers.

8. Disease risk of exposure to e-cig derived metals

Inhalation of metals in humans is known to cause a multitude of adverse health effects, including diverse diseases and conditions^47–49. Whereas nickel, cadmium, and chromium (VI) are known human carcinogens (Group 1, International Agency for Research on Cancer (IARC))^129–132, lead is classified as a probable human carcinogen (IARC Group 2A)¹³³. Furthermore, chromium can cause respiratory irritation, gastrointestinal problems, and lung function impairment^134–136. Nickel inhalation can damage the nasal cavity and produce lung irritation, lung inflammation, pulmonary cell hyperplasia, and fibrosis^137–139. Cadmium exposure can lead to kidney damage, emphysema, achalasia (swallowing disorder affecting the esophagus muscles), and cardiovascular diseases^140–144. Lead is neurotoxic, disrupts blood production, damages the digestive and urinary systems, and causes other cardiovascular effects^145–147. Cobalt is classified as a possible human carcinogen (IARC Group 2B)¹⁴⁸; inhalation of cobalt can cause reduced pulmonary function, asthma, interstitial lung disease, wheezing, and dyspnea, myocardial and thyroid disorders, and allergic dermatitis manifesting as eczema and erythema^{139, 149}. Iron inhalation can result in respiratory irritation, metal fume fever (a flu-like illness), siderosis, and fibrosis^{150, 151}. Exposure to manganese can cause lung irritation, coughing, bronchitis and pneumonitis, reduced lung function, pneumonia, manganism (a Parkinson-like disease), and other adverse neurological effects, including tremors, difficulty walking, and facial muscle spasms^{152, 153}. Inhalation of copper nanoparticles is shown to cause eye and respiratory irritation, coughing, sneezing, headache, dizziness, nausea, chest pain, and damage to the kidney, liver, and spleen^{154, 155}. Excess exposure to zinc can result in metal fume fever, reduced lung function, chest pain, coughing, dyspnea, and shortness of breath^{156, 157}. Tin inhalation can cause respiratory irritation, and prolonged exposure can result in stannosis (non-fibrotic pneumoconiosis)¹⁵⁸. Inhalation of selenium dust irritates mucous membranes in the nose and throat, and produces coughing, nosebleed, loss of olfaction; long-term exposure to selenium can lead to dyspnea, bronchial spasms, bronchitis, and chemical pneumonia^{159, 160}. Arsenic is highly toxic in various organs and body systems and can cause inflammation¹⁶¹; arsenic is carcinogenic in its inorganic form^162–164. Prolonged exposure to aluminum has been associated with development or progression of neurodegenerative disorders, including Alzheimer’s disease, bone disorders, kidney damage, hormonal imbalances, and cancers^165–168. Inhalation of antimony can cause inflammation of the lungs, chronic bronchitis, chronic emphysema, irritation of eye, skin, nose, and throat, and gastrointestinal and heart problems¹⁶⁹. Crystalline silica is a known human carcinogen (IARC Group 1)¹⁷⁰; inhalation of silicon dust can lead to silicosis (a progressive, debilitating, and often fatal lung disease), chronic obstructive pulmonary disease (COPD), kidney disease, cardiovascular impairment, and autoimmune disorders^{171, 172}.

Fowles et al.⁴⁸ have evaluated the reported concentration of metals in e-cig liquids and aerosols to estimate the corresponding cancer and non-cancer health risks. They have concluded that exposure to metals in e-liquids and aerosols may pose unacceptably high cancer and non-cancer health risks, at the mid and upper end of the reported ranges. Specifically, concentrations of nickel and chromium were found to be high enough in e-cig liquids and aerosols to present a cancer risk. The non-cancer health risks of e-cig metal emissions were primarily from nickel, with contributions from chromium and manganese⁴⁸. Zhao et al.⁴⁹ have recently reported that exposure to heavy metals in e-cig aerosols and e-liquids can cause varying levels of health risks in humans through different routes, with the inhalation route posing a higher overall risk than dermal or oral routes of exposure. The highest average cancer risks were for chromium 4.39 × 10^–4, nickel 3.11 × 10^–5, and arsenic 6.74 × 10^–6. For comparison, the average cancer risks for carcinogenic VOCs, such as formaldehyde (IARC Group 1) and acetaldehyde (IARC Group 2B) from e-cig emissions were 4.22 × 10^–5 and 1.57 × 10^–5, respectively. The average carcinogenic risks of chromium and nickel from e-cig use exceeded the acceptable ranges, and the risks would increase if the carcinogenic risks of other chemicals present in e-cig liquids and aerosols were added together to estimate the actual harm to humans. Moreover, hazard quotient (HQ) was calculated to assess the potential for non-cancer health hazards to occur as a result of exposure to e-cig derived metals⁴⁹. HQ is commonly used to estimate the non-cancer health risk of exposure to a contaminant with available non-cancer health guidelines (Minimal risk level (MRL), Reference dose (RfD), and Reference concentration (RfC))¹⁷³. A HQ is computed as the ratio of the potential exposure to a substance and the level at which no adverse effects are expected. If the HQ is calculated to be less than 1.0, then no adverse health effects are expected as a result of exposure. The HQs for nickel, copper, and manganese from e-cig emissions were 8.55, 7.04, and 1.10, respectively, and exceeded the acceptable ranges for inhalation exposure, indicating that exposure to these metals in regular e-cig users may present non-cancer health risks. Table 3 summarizes the estimated cancer and non-cancer health risks of e-cig-derived metals via inhalation exposure from the Zhao et al.⁴⁹ study. The authors concluded that excessive e-cig use can indeed bring about many health hazards, which refutes the popularly believed consumer notion that vaping e-cigs does not affect user’s health⁴⁹.

Table 3.

Cancer and non-cancer health risks of e-cig-derived metals via inhalation exposure.

	Cancer risk		Non-cancer risk |HQ*
Metal	Range	Average	Range	Average
As	0 – 4.87 × 10−5	6.74 × 10−6	0 – 9.47 × 10−1	1.31 × 10−1
Cd	0 – 3.49 × 10−6	1.97 × 10−7	0 – 4.08 × 10−2	2.29 × 10−3
Cr	0 – 1.88 × 10−2	4.39 × 10−4	0 – 1.61 × 101	3.77 × 10−1
Cu	NP	NP	4.06 × 10−3 – 7.79 × 101	7.04 × 100
Mn	NP	NP	8.15 × 10−3 – 5.24 × 100	1.10 × 100
Ni	3.17 × 10−12 – 1.56 × 10−3	3.11 × 10−5	8.73 × 10−7 – 4.29 × 102	8.55 × 100
Pb	0 – 2.30 × 10−5	3.67 × 10−7	0 – 3.84 × 100	6.59 × 10−2

^*Hazard quotient = HQ; HQ is calculated to evaluate the potential for non-cancer health hazards to occur from exposure to a contaminant with available non-cancer health guidelines (Minimal risk level (MRL), Reference dose (RfD), and Reference concentration (RfC)). A HQ is the ratio of the potential exposure to a substance and the level at which no adverse effects are expected. If the HQ is calculated to be less than 1.0, then no adverse health effects are expected as a result of exposure.

NP = Not performed; it indicates that risk assessment was not performed due to lack of corresponding reference data.

Data are from Ref.⁴⁹.

9. Vulnerability of adolescents and youth to the adverse health effects of e-cig derived metals

Adolescents and youth comprise a major portion of e-cig users in the general population^{73, 174, 175}. The presence of toxic and carcinogenic compounds, including metals, in e-cig liquid and aerosol is of special concern for children and teens who are a vulnerable population with developing nervous, endocrine, hematologic, and immune systems that may be more sensitive to the effects of toxicants and carcinogens^176–179. Several metals present in e-cig liquid and/or aerosol, e.g., copper, iron, lead, manganese, selenium, and zinc, are known to cause motor and cognitive deficits, and are linked to adult-onset neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis^180–182. Chronic exposure to e-cig-derived neurotoxic metals during adolescence and youth may accelerate adult-onset neurodegenerative diseases, like Alzheimer’s and Parkinson’s diseases, both of which are associated with metal dyshomeostasis^{181, 183, 184}. Furthermore, early life exposure to carcinogens can increase susceptibility to developing cancer later in life^185–187. Thus, exposure of adolescents and youth to the e-cig-derived carcinogenic metals may pose a lifetime cancer risk^{55, 56}. Moreover, metal ions for iron, copper, arsenic, chromium, and nickel can participate in Fenton and/or Haber-Weiss reactions to produce reactive oxygen species (ROS) that can affect mitochondrial function and redox reactions in biological systems^{117, 188–195}. This can lead to oxidative stress and disruption of cellular signaling pathways involved in cell cycle control and inflammation, which underlie various diseases and conditions, including cardiovascular diseases, respiratory diseases, metabolic disorders, neuroinflammatory diseases, and cancer, among others^{14, 50, 102, 196–199}.

10. Investigating the disease-causing potential of e-cig derived metals in vapers

Many e-cig-derived metals, particularly heavy metals are primarily deposited in specific tissues and organs, such as the bones, liver, kidney, brain, and adipose tissues, and gradually released into the body over time^{189, 200, 201}. The comparatively low doses of metals that e-cig users receive during each vaping session are unlikely to result in acute manifestation of a disease. However, cumulative doses of e-cig-derived metals in habitual vapers can be sufficient to induce cellular and molecular changes that underlie various diseases. Prospective epidemiologic studies allow examination of the relationship between exposure to e-cig-derived metals and risk of disease in human populations²⁰². However, this study design is often prohibitive due to the long follow up periods needed for clinical manifestation of most diseases^{4, 53, 79}. An alternative approach is to measure biomarkers of exposure and potential harm for e-cig-derived metals in biospecimens from vapers. Ideally, biomarkers of exposure should inform the extent and/or duration of exposure to e-cig-derived metals, whereas biomarkers of potential harm should represent onset of the cellular and molecular alterations that underlie disease development in (apparently) healthy vapers. The detectability of these biomarkers in non- or minimally invasively obtainable specimens (e.g., urine, hair, blood, saliva, or sputum) would be further advantageous for population-based studies^203–206. Towards this end, the interdisciplinary field of metallomics, involving multi-layers of transcriptomics, proteomics, metabolomics, and genomics^207–210 can be exploited for the discovery and validation of biomarkers of exposure and potential harm in cells and tissues of vapers and non-users involuntarily exposed to e-cig emissions (Fig. 1).

Figure 1. Development of biomarkers of exposure and potential harm for e-cig-derived metals in humans.

The proposed approach for discovery and validation of biomarkers of exposure and potential harm for vaping utilizes the interdisciplinary filed of metallomics whereby comprehensive analysis of metals and metalloids species within a cell or tissue type is integrated with transcriptomic, proteomic, metabolomic, and genomic analyses to arrive at the metallo-interactome.

As many biomarkers of potential harm are altered at early stages of diseases – often prior to clinical manifestation of the diseases^211–213 – it is likely that these biomarkers will be detected in (apparently) healthy vapers. The extent of the modulation of biomarkers of potential harm in disease-free vapers is expected to be lower than that in patients diagnosed with a disease. While long-term exposure to the toxic and carcinogenic metals present in e-cig emissions can lead to significant modulation of biomarkers of potential harm in healthy vapers, changes in levels of these biomarkers are expected to be more pronounced in patients diagnosed with a disease. Functional enrichment analysis of these biomarkers may reveal overrepresentation of the molecular pathways and gene networks that are disrupted in respiratory diseases, neurological diseases, cardiovascular diseases, immune (inflammatory) diseases, metabolic diseases, and cancer. This is consistent with the fact that these diseases are most commonly associated with- or caused by exposure to metals and other pernicious substances present in e-cig liquids and aerosols^{4, 5, 7, 52, 54, 92, 102, 214–216}. Presumably, most biomarkers of potential harm in vapers will be associated with multiple diseases. Of note, a biological pathway is rarely, if ever, affected in one disease only. To minimize noise and facilitate data interpretation, priority should be given to biomarker-disease pairs, which show the highest association specificity and sensitivity in vapers. While association studies of biomarkers of potential harm and disease risk are widely used in molecular epidemiology^{8, 211, 217–220}, it is prudent that the identified biomarkers of potential harm in vapers be validated in follow up functional studies in cell lines or animal models^{221, 222}. This would allow verification of the ‘causal’ role of e-cig-derived metals in disease pathogenesis in chronic vapers.

11. Concluding remarks: challenges and opportunities

A growing body of research shows that vapers are exposed to toxic and carcinogenic metals originating from e-cig liquid or various components of the e-cig device^{30, 92}. The contaminant or leached metals are transferred to e-cig aerosol, which is inhaled by the users^39–43. While accumulating data show significant quantities of e-cig-derived metals in cells and tissues of vapers, little is known about their long-term health effects on users^22–28. Biochemical, molecular, and toxicological analyses of biospecimens from e-cig users as well as assays in relevant in vitro models and in silico studies can identify metals and other chemical constituents of e-cig emissions that may contribute to the disease-causing potential of vaping. Biomarkers of exposure and potential harm hold great promise for investigating the health consequences of exposure to e-cig-derived metals in regular vapers and non-users involuntarily exposed to e-cig emissions. While comparing these biomarkers in cells and tissues of vapers vs. non-users, it is important to account for other sources of metal exposure, including environment, diet, lifestyle, and occupation^{50, 136, 193, 223}. Environmental exposure to metals can be unavoidable among people who live near natural deposits or industrial sites, or breath secondhand smoke at home and/or workplace^{136, 193}. As a lifestyle factor, tobacco smoking is a major source of exposure to metals²²⁴. This is due to the fact that tobacco plants readily absorb metals from the soil where they grow^{225, 226}. Dietary sources of metals include seafood (shellfish) and vegetables, such as roots, tubers, beet, spinach, parsley, and carrot^227–229. Occupational exposure to metals occurs mainly among welders, miners, smelters, construction workers, shipyard workers, battery manufacturers, machinists, and auto mechanics^{136, 200, 223}. When measuring the concentrations of certain e-cig-derived metals in cells and tissues of vapers vs. non-users, as biomarkers of exposure, an important consideration is speciation because the valence state of metals, like chromium (III/VI), manganese (II/III), and copper (I/II/III) can significantly influence their toxic or carcinogenic effects^230–232. However, speciation analysis of e-cig-derived metals may prove challenging as the valence state of these metals can rapidly change once they cross biological membranes, enter cells, and undergo biotransformation. This is of paramount importance in the oxygen-rich environment of the lungs wherein e-cig aerosol is inhaled. Future studies should utilize advanced analytical chemistry techniques capable of measuring trace-metal stable isotopes²³³ in sequentially collected biosamples from vapers. When assessing biomarkers of potential harm in vapers, one should be cognizant of the competitive demand for metabolism of metals and other e-cig-derived chemicals in human cells and tissues^{30, 92}. Equally significant is the possibility of additive or multiplicative effects of the e-cig-derived chemicals in the human body^{65, 234, 235}. Therefore, teasing out the effects of e-cig-derived metals on biomarkers of potential harm in vapers may not be straightforward. However, many of these challenges can be overcome by carefully designing and executing large-scale studies in which detailed information on history and patterns of e-cig use and other relevant covariates is collected from the study subjects. The fact that many heavy metals accumulate in the body and have a long half-life^{136, 200, 236} underscores the importance of querying the study population for detailed data on exposure history. Also, critically important is to gather comprehensive data on product characteristics, including specifications of the e-cig devices and e-liquids used. To identify sources of metals that may modulate biomarkers of exposure and potential harm in vapers, investigators must know the specifications of the e-cig device components and the type of e-liquids used. Identifying device components and/or e-liquid ingredients that are responsible for the modulation of biomarkers of exposure and potential harm will have tremendous translational potential^{26, 37, 38, 237}. Research on vaping product characteristics can produce actionable information for the FDA’s regulation of the manufacturing of tobacco products¹⁹. Lastly, as the significance and impact of research on health disparities are increasingly recognized, vulnerable populations for vaping research should be prioritized^{56, 238, 239}. Within this context, the popularity of e-cigs among adolescents and youth⁷³, susceptibility of this segment of the population to the harmful effects of e-cig-derived metals and other toxicants^176–179, and the relative lack of exposure of teens and youth to smoking as a key confounder^{4, 73, 95}, should be taken into account. These characteristics and tendencies make adolescents and youth a population of special interest and high priority for vaping research^{56, 238, 239}.

Abbreviations

Al

aluminum

As

arsenic

Cd

cadmium

Co

cobalt

COPD

chronic obstructive pulmonary disease

Cr

chromium

Cu

copper

e-cigs

electronic cigarettes

e-liquid

electronic cigarette liquid

FDA

Food and Drug Administration

Fe

iron

GRAS

generally recognized as safe

HQ

hazard quotient

IARC

International Agency for Research on Cancer

Mn

manganese

MRL

Minimal risk level

Ni

nickel

Pb

lead

PG

propylene glycol

RfC

Reference concentration

RfD

Reference dose

ROS

reactive oxygen species

Sb

antimony

Se

selenium

Si

silicon

Sn

tin

VG

vegetable glycerin

VOC

volatile organic compounds

Zn

zinc

Biography

Ahmad Besaratinia earned his Ph.D. in molecular epidemiology & genetic toxicology from Maastricht University in The Netherlands. Following a postdoctoral training in cancer biology, he became a faculty, and quickly rose to the rank of associate professor of research at Beckman Research Institute of the City of Hope, Duarte, CA. In 2013, he joined the faculty at the USC Keck School of Medicine, Los Angeles, CA; currently, he is a professor at the department of Population & Public Health Sciences, teaching graduate courses in genetic epidemiology and public health. His research explores the underlying causes of human disease, focusing on the genetic and epigenetic regulation of genes in health and disease.

Data Availability Statement

All data are contained within the article.