E-Cigarette Toxicology

Abstract

Since the spread of tobacco from the Americas hundreds of years ago, tobacco cigarettes and, more recently, alternative tobacco products have become global products of nicotine addiction. Within the evolving alternative tobacco product space, electronic cigarette (e-cigarette) vaping has surpassed conventional cigarette smoking among adolescents and young adults in the United States and beyond. This review describes the experimental and clinical evidence of e-cigarette toxicity and deleterious health effects. Adverse health effects related to e-cigarette aerosols are influenced by several factors, including e-liquid components, physical device factors, chemical changes related to heating, and health of the e-cigarette user (e.g., asthmatic). Federal, state, and local regulations have attempted to govern e-cigarette flavors, manufacturing, distribution, and availability, particularly to underaged youths. However, the evolving e-cigarette landscape continues to impede timely toxicological studies and hinder progress made toward our understanding of the long-term health consequence of e-cigarettes.

INTRODUCTION

The rapidly diversifying number of commercially available tobacco products on the global market has dramatically influenced tobacco use patterns (1). For many years, US consumption of conventional cigarettes has steadily declined (2, 3). Conversely, the use of alternative tobacco products (ATPs)—including electronic cigarettes (e-cigarettes)—has increased at an alarming pace (1, 4). In 2019, more than 10 million US adults reported using an e-cigarette (approximately one-third of the number of adult smokers) (5). More concerning still are estimates of adolescent vapers: 22% of high school and 9.4% of middle school students report daily e-cigarette use (6). These trends are worldwide and demonstrate the emergence and potential threat of a new tobacco epidemic (7–9). The popularity of e-cigarettes continues to outpace our medical knowledge about their safety, and contradictory scientific evidence has sparked a debate among tobacco control advocates, policy makers, and regulatory agencies.

Invented in China in 2003, e-cigarettes were introduced as nicotine delivery devices and developed with the intent of providing smokers the satisfaction of conventional tobacco cigarettes without deleterious health effects (10). Introduced to the United States in 2007, the sales revenues of e-cigarettes have surged worldwide. In response, the American Lung Association, American Association of Pediatrics, World Health Organization, and the US Centers for Disease Control and Prevention have all expressed concerns over e-cigarette health risks. In 2016, the US Food and Drug Administration (FDA) amended the Family Smoking Prevention and Tobacco Act to include deemed tobacco products, thus enabling federal oversight and regulation of the manufacture, marketing, and sale of e-cigarettes (the Deeming Rule is described in greater detail below).

Currently, we lack a comprehensive understanding of the health consequences associated with e-cigarette use. Assessing the full spectrum of health effects should consider not only e-cigarettes’ intrinsic toxicity but also their toxicity relative to that of the conventional cigarettes they were designed to replace. To date, most studies have evaluated the acute effects of vaping, and the long-term consequences of e-cigarette use are unlikely to be fully understood for years. Complicating this further is the potential additive (or synergistic) toxicity resulting from the use of two or more unique tobacco products. This review presents the current data regarding the cardiopulmonary toxicity of e-cigarettes.

E-CIGARETTES: HISTORICAL CONTEXT AND KEY REGULATIONS

In 2003, Hon Lik, a Chinese pharmacist, invented the first modern e-cigarette (11). Unsatisfied with extant nicotine-replacement therapies, and motivated by the loss of his father, a lifelong smoker, to lung cancer, Lik set out to create a safer nicotine delivery device. Following a successful launch in China, e-cigarettes were first marketed in the United States beginning in 2007 (Figure 1). In 2008, the company Smoking Everywhere was prevented from importing e-cigarettes into the United States, with the FDA citing that e-cigarettes were both a drug and a device. The FDA claimed these products fell under its jurisdiction and required FDA approval before they could be sold commercially (12).

Figure 1.

E-cigarettes: a historical timeline. Abbreviations: CDC, US Centers for Disease Control and Prevention; e-cigarette, electronic cigarette; EVALI, e-cigarette-or vaping-associated lung injury; FDA, US Food and Drug Administration; PMTA, premarket tobacco application.

In 2009, the FDA amended import regulations to include e-cigarette manufacturers, thus preventing e-cigarette imports into the United States. As a result, Smoking Everywhere sought an injunction to stop this ban based on precedent from the Supreme Court decision over FDA v. Brown and Williamson Tobacco Corporation, which did not grant the FDA the ability to regulate tobacco products (13). That same year, together with broad bipartisan support from Congress, President Obama signed the Family Smoking Prevention and Tobacco Control Act. Despite the authority of the FDA to regulate tobacco products, antiregulation industry lawsuits have been successfully argued in court (12, 14).

In 2011, the FDA drafted a guidance document (15) that was intended to help e-cigarette manufacturers submit (a) documentation of substantial equivalence (SE) if the product was similar enough to a tobacco product already on the market, or (b) a premarket tobacco product application (PMTA) if the product was substantially different. An SE or PMTA was required of any company that was planning or had already brought an e-cigarette to market after February 15, 2007.

By 2013, the year-over-year increase in the number of US vapers alarmed scientists, clinicians, and lawmakers. In response, US senators urged the FDA to exert regulatory oversight of new tobacco products (including e-cigarettes). In 2014, the FDA published a proposed new rule that would deem whether products met the definition of a tobacco product. Those that were deemed a tobacco product would then fall under the agency’s jurisdiction and thus grant the FDA authority to apply further regulations (including warning labels).

The final version of this guidance, known as the Deeming Rule, was published in 2016. Not only did this further restrict the sale of tobacco products by requiring age verification (even online), but it also required manufacturers to register new products, submit PMTAs, report ingredients, and place health warnings on packaging. While earlier drafts of the Deeming Rule hinted at a flavor ban, the final version did not grant the FDA the authority to regulate e-cigarette flavorings. ATP manufacturers were given until 2018 to submit PMTAs, plus an additional year for application review before needing to comply with FDA regulations. During those three years, manufacturers were permitted to continue selling e-cigarettes without FDA oversight.

In 2017, the FDA postponed the PMTA submission deadline until 2022, citing a desire to strike “an appropriate balance between regulation and encouraging development of innovative tobacco products” (16). Already overwhelmed with an emerging youth vaping crisis and citing record numbers of vaping adolescents, the American Academy of Pediatrics filed a lawsuit, objecting to the deferment of the PMTA submission deadline by an additional four years. In response, the FDA commissioner proposed the banning of flavorings (other than tobacco, menthol, and mint). A year later, more than 2,800 people in the United States, more than 60% of who were between the ages of 18 and 34, were hospitalized because of e-cigarette- or vaping-associated lung injury (EVALI) (17). The large numbers of youth and young adults diagnosed with EVALI prompted President Trump to sign an amendment to the Federal Food, Drug, and Cosmetic Act to raise the minimum age of tobacco product purchases to 21. Accompanying this legislation was new FDA guidance regulating e-cigarette flavors: While this guidance stated that the FDA would prioritize the prohibition of flavored cartridges or pod-based e-cigarettes to reduce their appeal to minors, there were several notable policy exceptions (e.g., tobacco- and menthol-flavored cartridge/pod-based e-cigarettes, flavored disposable e-cigarettes, and flavored e-liquids to be used in refillable e-cigarettes). Thus, many flavored e-cigarette products remain on the market, which has led to a spike in the use of disposable e-cigarettes by middle and high school students from 2019 to 2020 (18).

E-CIGARETTE DEVICES

E-cigarettes are battery-powered devices that heat a nicotine-containing liquid to the point of vaporization, creating an aerosol (19). The toxicity of e-cigarette aerosols is governed by both the intrinsic toxicity of the e-liquid serving as the aerosol source and the toxicity of chemicals produced when the e-liquid is vaporized by contact with the heating coil.

E-cigarettes were designed with two primary goals: (a) to deliver inhalable nicotine that mimicked the use and satisfaction associated with conventional cigarettes and (b) minimize the adverse health risks associated with smoking (10). The first-generation devices (cig-a-likes) not only looked like conventional cigarettes but were also disposable (or had disposable cartridges). Importantly, this disposable model gave manufacturers more control of these products by limiting user modifications. Second-generation devices (vape pens) were larger than the cig-a-likes and contained a clear cartridge that housed both the heating coil and e-liquid. This cartridge could hold significantly more e-liquid than earlier devices, and users could select (or change) the e-liquid added to the device (including changing the flavor and nicotine content) at their discretion; some vape pens had variable power outputs. Third-generation devices (box mods) permitted the greatest degree of device customization. These devices had replaceable heating coils, wicking materials, e-liquids, and batteries. Additionally, the heating coil in some box mod devices had a resistance under 1 ohm (subohm), which allowed for a greater power output (19).

In 2015, a fourth-generation pod-based device, the JUUL e-cigarette, rapidly overtook the market (20). JUUL looked less like a cigarette and more like a USB drive, and its appeal and unparalleled success spurred countless brands with a similar aesthetic (21). Thanks to its sleek look, popular fruit flavors, and aggressive marketing, JUUL is exceptionally popular among adolescents and young adults and considered the main culprit of a youth vaping epidemic (22, 23). To curb the appeal of e-cigarettes to youths, JUUL stopped production of flavored pods, with the exception of tobacco and menthol (24, 25). Increased FDA scrutiny of JUUL, together with an inability to market their most popular flavors, and the aforementioned loophole in the 2020 flavor ban ushered in a new wave of completely disposable flavored e-cigarettes (26, 27). The rise in popularity of flavored disposable e-cigarettes was captured in the 2020 National Youth Tobacco Survey: Despite a dramatic decrease in the number of youth reporting vaping (1.8 million fewer), flavored disposable e-cigarette use among high schoolers increased by 11-fold that same year (6).

ANATOMY OF AN E-CIGARETTE

Flavorings

The myriad chemicals used to flavor e-liquids present an added layer of complexity when evaluating the toxicity of e-cigarettes. In 2014, over 7,000 unique e-liquid flavors were available online (28). Many of the chemicals used as flavorants have received a designation as generally recognized as safe (GRAS), based upon their safety profile for ingestion. However, this does not reflect their potential to induce inhalation toxicity (29). One study detected more than 140 unique flavoring compounds across 28 e-liquids (e.g., vanillin, ethyl vanillin, ethyl maltol, and menthol), with the total concentration of flavorings ranging from 2.3 to 43 mg/mL (30). Another found more than 150 chemicals in 277 refill fluids; approximately 85% contained flavors exceeding 1 mg/mL (31). In some samples, common flavorants were detected in concentrations greater than those needed to elicit cytotoxicity. There is a great deal of in vitro research on the effects of flavored e-liquids (32–35), popular flavorings (36–38), and flavorings with unique effects on cells (34, 39). Together, these flavoring studies highlight the need for regulations that restrict their concentration in e-cigarettes and ban flavors that are known toxicants.

Flavoring chemicals exert a range of adverse pulmonary effects if present in high enough concentrations, and some common flavoring agents are chemically similar to irritants and sensitizers known to cause occupational asthma (40). Early research pinpointed diacetyl and cinnamaldehyde as particularly toxic (41), which led to efforts by manufacturers to remove both from e-liquids.

In vitro studies have compared the relative toxicity of flavoring chemicals and identified both flavor-dependent and flavor-independent [i.e., nicotine, propylene glycol (PG), and glycerin (GLY)] effects on indices of cytotoxicity (42) and inflammatory response (43). Cell-based assays have also found that flavorants can impair innate immune defenses that contribute to respiratory illnesses (44). Currently, few in vivo experiments have been conducted that evaluate potential adverse pulmonary consequences associated with e-liquid flavors.

Because the FDA’s flavor ban does not apply to all e-cigarette devices and e-liquids, research is still needed to address the changing flavor landscape. This work should consider not only the intrinsic toxicity of individual chemicals and mixtures but also how flavors modify or improve user experience (e.g., masking the harsh feel of inhaled nicotine aerosols), as this could be important in the development and reinforcement of nicotine dependence (45, 46).

Nicotine

Nicotine, a highly addictive chemical present in tobacco leaves, was initially added to e-liquids at 3–36 mg/mL (47–49). However, the creators of JUUL found that ionizing nicotine increased its solubility and permitted e-liquids with higher concentrations of dissolved nicotine (50). Dubbed nicotine salts, this form of nicotine is generated when an acid (i.e., benzoic, salicylic, lactic, or tartaric) is added to freebase nicotine (51), thereby lowering its pH (52). The resulting aerosol has been described as less harsh and shifts the pharmacokinetic profile of nicotine (52, 53). This reduction in negative aerosol sensations, coupled with increased nicotine bioavailability, has contributed to the popularity of nicotine salt products, which can have a nicotine content of over 50 mg/mL (50).

Humectants

The bulk of e-liquids (i.e., PG and GLY) are carriers for nicotine and flavors in the generation of the vaping aerosol (54). Although these chemicals, used in food and pharmaceuticals due to their hygroscopic and solvent properties (55, 56), have received a GRAS designation for ingestion (57, 58), research has shown that PG can alter physiological processes (59) and produce acute toxicity (60) and dermal (61) and airway irritation (62).

Thermal Degradation

Although PG and GLY are generally nontoxic at room temperature, they form thermal degradation products when heated (63) (Figure 2). PG heated to 527°C remained 99.9% intact, but traces of acetaldehyde, acetone, propylene oxide, and allyl alcohol were detectable (64). Importantly, PG undergoes oxidation to form acetone, acetaldehyde, and formaldehyde at temperatures as low as 157°C (65). Degradation of PG can occur at lower temperatures when PG contacts metals used to make e-cigarette coils (e.g., stainless steel and nichrome) (66). Although relatively safe when inhaled (67), GLY can also form thermal degradation products (68).

Figure 2.

Observed breakdown products of propylene glycol and glycerol.

Aldehydes, including carcinogens such as formaldehyde, acetaldehyde, and acrolein, have been detected in emissions from all generations of e-cigarettes (69, 70). The aldehydes generated by second-generation e-cigarettes were higher than those of their predecessors (69), likely because of the ability to increase the voltage to the heating coil (71). There have been concerns that third-generation subohm e-cigarettes would produce more aldehydes, because of the ability to increase the device’s power output well beyond that of previous generations. While these e-cigarettes do produce a large quantity of aldehydes, ranging from 131.5 to 293.6 mg/m³ (wattage of 50 and 85 ohms, respectively), other generations of devices yielded even more aldehydes at lower wattages (422.1 mg/m³). These data suggest that power output is not the sole determinant of aldehyde production and that the combination of power output and coil surface area should be considered (72). Thus, no single device/liquid component can be implicated in the production of toxic compounds resulting from the thermal degradation of e-liquids, and researchers should consider the cumulative effect when evaluating e-cigarette toxicity.

The clinical effects of GLY and PG themselves have been studied relative to the toxicity of nicotine and flavoring chemicals. A randomized, crossover, placebo-controlled study in healthy and asthmatic volunteers investigated the acute effects of vaping nicotine- and flavor-free e-liquids (70% PG, 30% GLY) (73). Although vaping elicited increased cough, chest tightness, and mucous in a subset of subjects (including healthy volunteers), there were no significant differences in group responses and no decreases in lung function, exhaled nitric oxide (FeNO), or C-reactive protein (CRP), a serum marker of inflammation. Another study found significant changes in lung function but not FeNO in nonsmokers after vaping, and the effect was greater in asthmatics (74). Thus, while acute exposure to PG- and GLY-containing e-cigarette aerosols may not cause significant pulmonary function changes, chronic exposure scenarios warrant investigation. A handful of in vivo animal studies have evaluated the chronic effects of e-cigarettes and shown minimal respiratory effects from PG and GLY aerosols without nicotine or flavor chemicals, although 90-day rat studies have demonstrated increased mucin production and nasal hemorrhaging from PG alone (75), while high concentrations of PG/VG altered pulmonary function in male rats (76).

Metals

The heating element within an e-cigarette is the largest potential source of toxic metal exposure. Several metals are routinely detected in popular e-cigarette cartomizers and tank models (e.g., chromium, nickel, selenium, and aluminum). Iron and lead have been found in some but not all products. Occasionally, other trace metals (i.e., manganese, cobalt, molybdenum, and titanium) have been detected (77). Generally, e-cigarette coil temperatures remain below 300°C if the wick remains saturated with e-liquid (78). Under conditions where the wicking material dries out, coils can exceed 1000°C (79), which promotes coil degradation and facilitates the release of metals. Analyses have also found that 150 heat cycles result in the loss of chromium and iron from Kanthal coils (up to 19% and 58%, respectively) and iron and nickel from nichrome coils (up to 14% and 43%, respectively) (78). Thus, e-liquid contact with the heating coil appears to facilitate the release of metals found in e-cigarette aerosols (80).

RESPIRATORY TOXICITY

Adverse respiratory effects have been documented in clinical studies, as well as in animal and cellular models of e-cigarette aerosol exposures. These effects have been observed in all regions of the respiratory tract, including the oral cavity, nasal passages, and lower airways.

Oral Toxicity

As previously mentioned, carcinogens, including metals, formaldehyde, and acrolein, have been identified in e-cigarette aerosols. The oral cavity is the first tissue exposed to e-cigarette aerosols during vaping, and it is not surprising that biological changes occur in oral tissues challenged with e-cigarettes. While data on oral cancers and e-cigarette use are very limited (81), emergent evidence of procarcinogenic changes associated with e-cigarettes, including DNA damage, has been described (82). Similar to cigarette smoking, RNA sequencing and functional pathway analyses performed on oral cells from vapers have identified cancer as the leading disease pathway (83). Also, a pilot study found that buccal cells from e-cigarette users had altered expression levels of tumor suppressor and DNA repair genes after a 20-puff vaping session (84). Because most vapers are dual-/polytobacco consumers, it is essential to understand the health consequences of tobacco product interactions, such as the enhanced benzopyrene metabolism in oral keratinocytes treated with e-cigarette condensates (85).

The oral microbiome, the second most populated tissue after the gut microbiome, contributes to one’s health status and as with cigarette smoking, vaping adversely affects the oral microbiome (86–88) and increases inflammatory and oxidative stress biomarkers in saliva (89). Moreover, e-cigarettes are also associated with dental caries, periodontal disease, and oral mucosal lesions (90–92), demonstrating a continuum of adverse effects in the oral cavity.

Nasal Toxicity

Although e-cigarette aerosols are inhaled orally, nasal tissues have been studied, in part because many e-cigarette users exhale through their nose. A significant decrease in immune-related gene expression has been reported in the nasal epithelium, suggesting a suppressed immune response (93) and rendering e-cigarette users more susceptible to viral or bacterial infections. In support of this hypothesis, exposure of nasal epithelial cells to e-cigarettes increased the expression of platelet-activating factor receptor, which is used by pneumococci to adhere to host cells (94). Furthermore, nasal mucosa from vapers who received the nasal flu vaccine [i.e., live-attenuated influenza virus (LAIV)] had blunted immune responses (95); the sex of the participant also mediated the extent of nasal immune response to LAIV. These data, together with previously published transcriptomic differences between male and female smokers (96), suggest that sex may be a modifier of e-cigarette-mediated gene expression.

Pulmonary Toxicity

In this review of the pulmonary toxicity of e-cigarettes, the lung injury associated with the EVALI epidemic (97, 98) should be evaluated separately from the adverse effects associated with vaping e-liquids containing nicotine and flavors. Also, while nicotine is the most biologically active e-liquid constituent, the toxicological profile of inhaled nicotine is well characterized (99, 100), and its pharmacological effects are beyond the scope of this review.

Lung Diseases: Asthma and Chronic Obstructive Pulmonary Disease

Cigarettes are the major risk factor for chronic obstructive pulmonary disease (COPD), and both mainstream and secondhand cigarette smoke are linked with asthma (101). Similarly, epidemiological studies have found a positive association between vaping and asthma in adolescents and adults (102, 103). A meta-analysis of 11 cross-sectional studies corroborated these associations even after controlling for confounders such as former and/or current cigarette smoking status (104). Separately, an analysis of a nationally representative database [Population Assessment of Tobacco and Health (PATH)] found that current e-cigarette users are more likely to have respiratory disorders (e.g., COPD and asthma) compared to nonsmokers and that odds of respiratory disease were even greater in dual users (105, 106). Other PATH analyses have reproduced these findings, with some reporting dose-dependent relationships between e-cigarette use and lung diseases (107).

Classic inhalation toxicology can determine the biological basis underlying causal relationships and explain how airborne exposures lead to adverse respiratory outcomes. Such mechanistic data strengthen epidemiological findings and identify pathways susceptible to airborne toxicants. For example, impairments in pulmonary function in mice exposed to e-cigarette aerosols support the epidemiological evidence linking asthma and e-cigarettes (108). This is particularly important because of numerous e-cigarette variables that can cumulatively contribute to adverse health effects in epidemiology studies. In vivo and in vitro toxicology studies also contribute to our knowledge of the toxicity of specific e-cigarette components such as PG/GLY. For example, aerosols containing only PG/GLY altered immune regulation in the lungs of mice, although pulmonary function reductions were observed only with the addition of vanillin (109). One study did observe altered pulmonary function in mice exposed to PG/VG alone, although these effects were less severe than those seen with aerosols also containing nicotine and flavoring (110). Separately, another study found that e-cigarettes elicited different flavor-dependent pulmonary effects in a mouse model of house dust mite–induced allergic asthma (111). Thus, rodent studies provide a biological explanation to support the epidemiological link between adverse respiratory effects and e-cigarettes.

While a growing body of experimental and population-based evidence links vaping and e-cigarette components with adverse respiratory effects (i.e., COPD and asthma) (112), questions remain regarding the pulmonary toxicity profile of e-cigarettes relative to conventional cigarettes. Experimental models of dual use are also needed to reveal whether tobacco products interact to worsen pulmonary health outcomes.

Lung Immune Responses

Because vaping impairs lung responses to infection, e-cigarette users are more likely to contract pulmonary infections. Importantly, there is a greater incidence of a positive coronavirus disease 2019 (COVID-19) diagnosis among vapers: Individuals who had ever vaped were five times more likely to contract COVID-19, while risk to dual users was higher still (113). Another study found a positive association between vaping and number of COVID cases and deaths (114). Together, these data suggest that vaping increases susceptibility to viral infection.

Impaired host defense responses against pathogens have further demonstrated that e-cigarettes impair host responses against bacterial pathogens. For example, diversity in taxonomic abundance profiles was significantly shifted in the oral epithelial microbiome of e-cigarette users toward bacteria implicated in periodontal destruction (115). Interestingly, these bacteria were not altered in the oral epithelium of cigarette smokers. When exposed to e-cigarette aerosols and then challenged with a bacterial infection in vitro, cells launched a larger inflammatory response, suggesting that vaping may serve to enhance pathobiont infection (115). Additional in vitro models found that e-cigarettes increase pathogen virulence and promote inflammation, as evidenced by increased biofilm and cytokine secretion (116). These findings were recapitulated in a mouse model, where a Staphylococcus aureus infection increased biofilm formation and bacterial invasion of epithelial cells following e-cigarette exposure. Further, e-cigarettes impaired host defense processes by reducing the antimicrobial activity of epithelial cells, alveolar macrophages, and neutrophils (117). E-cigarette aerosols impede neutrophilic activity by reducing their ability to recognize, attack, and destroy bacteria (118). Similarly, impaired phagocytosis is observed in macrophages exposed to e-liquids (119). Thus, innate immune cells critical for pathogenic host responses are susceptible to e-cigarettes, supporting the epidemiological association between vaping and susceptibility to infection.

Cancer

Although e-cigarette aerosols contain fewer carcinogens than does cigarette smoke, there are still concerns regarding the potential for e-cigarette-induced cancer. Given the relatively recent advent of e-cigarettes (circa 2007), the latency period for cancer likely exceeds the length of time that people have been using e-cigarettes. Carbonyls in e-cigarette aerosols (i.e., acrolein and formaldehyde) have carcinogenic potential, but few animal studies have examined cancer.

Notably, in vitro studies using breast cancer cells have shown that e-cigarette condensate promotes cell growth and metastasis (120). E-cigarette aerosols also induce DNA adducts and impair DNA repair activity in the lung, bladder, and heart in mice (121). Perhaps the strongest evidence is a year-long vaping study where exposed mice developed lung adenocarcinomas and bladder urothelial hyperplasia (122). As this is still a nascent field of study, epidemiological observations will be essential in determining the strength of associations between vaping and cancer, particularly in comparison to conventional cigarettes.

CARDIOVASCULAR TOXICITY

Although the cardiovascular toxicity of combustible cigarettes and nicotine has been studied extensively, the long-term consequences of vaping on cardiovascular function and regulation are comparatively unknown. Moreover, although authorities have placed flavor restrictions, numerous flavors are still available on the e-cigarette market, and their cardiovascular toxicity is unclear. Similarly, e-cigarettes produce acrolein, formaldehyde, and acetaldehyde, which can be toxicologically active in the cardiovascular system (123). The following sections review the cardiovascular effects of e-cigarettes in human and animal studies.

Epidemiological Studies

A small number of epidemiological studies have examined the association between e-cigarettes and cardiovascular disease. In general, these studies report a greater incidence of adverse cardiovascular outcomes, including chest pain, coronary heart disease, arrhythmias, and myocardial infarctions, associated with vaping (124, 125). Notably, many of these associations are dependent upon vaping frequency and lose statistical significance as e-cigarette use decreases (126). Also, such studies should be interpreted with caution because the exclusion of former smokers or current polytobacco users is complicated if appropriate tobacco-use details are not obtained at the time of data collection. Interestingly, studies have found that dual users of e-cigarettes and cigarettes are at greater risk of cardiovascular disease than are exclusive e-cigarette users (127, 128). Thus, large retrospective epidemiological studies suggest adverse cardiovascular effects in e-cigarette users, although the findings are complicated by polytobacco use.

Clinical Studies

Compared to secondary analyses of large epidemiologic data sets, clinical experiments allow control of experimental conditions and subject monitoring and can tease out contributions of individual e-liquid constituents, including nicotine, to cardiovascular responses such as blood pressure and heart rate (129, 130). For example, ten e-cigarette puffs were sufficient to increase endothelial progenitor cell numbers to the same magnitude as a cigarette (131). Intriguingly, in a double-blind cross-over study, acute exposure to both nicotine and nicotine-free aerosols elevated blood pressure in healthy subjects, whereas only the aerosol containing nicotine significantly increased heart rate and arterial stiffness (132). Such nicotine-independent effects have been demonstrated where nicotine-free aerosols blunt flow-mediated dilation in healthy nonsmokers (133). Vaping also shifts autonomic regulation toward sympathetic dominance as indexed by heart rate variability (HRV). Thus, nicotine and other e-liquid components affect cardiovascular regulation (134). These changes in HRV have been accompanied by increased oxidative damage, oxidative stress, and inflammation in both habitual vapers (135) and healthy nonvapers acutely exposed to e-cigarette aerosols (136).

To further evaluate how cardiovascular responses are impacted by e-cigarettes, clinical studies have assessed cardiac outcomes before and after acute vaping/smoking sessions. Whereas cigarette smoking produced cardiac and vascular changes, vaping elicited little to no effect on heart function (137) or arterial stiffness (138) in habitual smokers, suggesting that cigarette smokers suffer fewer adverse cardiovascular outcomes from acute vaping (139).

In a crossover switching study, both smoking and vaping phases significantly increased iso-prostaglandin F2α and decreased FeNO bioavailability, vitamin E levels, and flow-mediated dilation. For some outcomes, e-cigarettes produced effects to a lesser extent than did cigarettes (140). Other studies have found that vaping promotes platelet activation (141) and increases blood pressure and aortic stiffness but not to the same degree as smoking (142).

Thus, multiple studies document short-term cardiovascular consequences of vaping. Both nicotine and other e-liquid constituents independently produce cardiotoxic outcomes, at least transiently. However, the chronic cardiovascular effects of e-cigarettes are a crucial knowledge gap. Moreover, differences in e-cigarette brands, device generations, e-liquid formulations, nicotine concentrations, and subject demographics restrict interstudy comparisons and impede a comprehensive characterization of the cardiovascular risk of vaping.

In Vivo Animal Studies

Animal inhalation studies permit controlled examination of responses after single or repeated exposures to explore causal relationships and mechanisms. For example, e-cigarette aerosols increased aortic stiffness and impaired vascular relaxation in mice, although these effects were less severe than in mice exposed to cigarette smoke (143). Separately, atherosclerosis-prone ApoE^−/− mice developed atherosclerotic plaques after 6 months of exposure to cigarette smoke but not to e-cigarette aerosols (with or without flavors). Furthermore, increases in oxidative stress markers and proinflammatory cytokines occurred in cigarette smoke- but not e-cigarette-exposed animals (144). These results replicate the findings from human studies, suggesting that the cardiovascular effects of e-cigarettes are less than those associated with conventional cigarettes (145).

Acute animal exposure studies have reproduced other findings from human studies, including vaping-induced platelet activation and shortened thrombosis occlusion (146, 147). Consistent with human responses, HRV was attenuated in mice exposed to e-cigarette aerosols (148), suggesting that e-cigarettes can impair cardiac autonomic regulation in multiple species (135, 149). Another study demonstrated a decrease in left ventricular ejection fraction in mice exposed to 3 months of e-cigarette aerosols. In addition, these mice had more atherosclerotic lesions than did control mice, and altered gene expression and cardiomyocyte histology accompanied this increase in atherosclerotic lesions (150). These murine data, while provocative, are not comprehensive and underscore the need to gain a more complete understanding of the risk that e-cigarettes pose to the cardiovascular system.

ADDITIONAL FACTORS INFLUENCING TOXICITY

Biological Sex

It is well known that the spectrum of adverse health effects of inhaled toxicants varies greatly within human populations and animal models. A multitude of factors, including genetic background, age, sex, weight, and preexisting disease status, interact to contribute to adverse health effects. The influence of biological sex, for example, is an important and often significant driver of outcomes. Data from the 2017 Global Youth Tobacco Survey revealed differences in vaping prevalence by sex (higher in males) and in motivations for vaping (females commonly cited e-cigarettes as less harmful to others and helpful in cutting down on cigarettes) (151). Females are also less likely to use e-liquids with high nicotine contents or third-generation devices (152), and recent e-cigarette use predicts future cigarette smoking among males (153).

Vaping Emissions: Risk of Secondhand Exposure?

Another concern regarding the adverse health effects of e-cigarettes is whether secondhand e-cigarette aerosols pose a risk to bystanders. Indoor air quality is a significant predictor of health, and estimates suggest the average person spends approximately 75% of their day indoors (154). Approximately 25% of the inhaled e-cigarette aerosols are deposited in the lung, suggesting that most inhaled particles are exhaled and available for passive indoor exposure (155). Intriguingly, airborne particles at an e-cigarette vaping convention exceeded 10,000 μg/m³ (156). While not representative of most real-world secondhand risk scenarios, this example suggests e-cigarettes can affect indoor air quality. A recent cross-sectional analysis revealed that an estimated 16% of European nonvapers were exposed to secondhand e-cigarette aerosols in some type of indoor setting and that the results varied by country, with a median exposure duration of 43 min/day (157). Separately, youths living with someone that vaped reported passive exposure to e-cigarette aerosols that were threefold more than in tobacco-free homes (158). Indeed, although the level of nicotine captured by silicone wristbands worn by children in vaping households was less than that for cigarette homes, it was still several-fold higher than for children residing in nonsmoking/nonvaping homes (159, 160). A clinical study found that tobacco-free individuals exposed to secondhand e-cigarette aerosols for 30 min reported respiratory and ocular symptoms and that throat irritation persisted beyond the exposure window (161). Importantly, the nontobacco smell associated with e-cigarette aerosols reduces harm perception, which may result in increased tolerance to secondhand e-cigarette exposures (162, 163).

What’s Worse? Vaping Versus Smoking

Most vaping studies test the null hypothesis that e-cigarettes are as toxic as cigarettes. This premise assumes these products have the potential to pose equivalent harm with regard to relative risk, target tissue toxicity, and mechanism, yet studies using established combustible tobacco end points have found less evidence of vaping-related health effects than with combustible cigarettes (164, 165). While e-cigarette aerosols contain fewer toxic chemicals than combustible smoke (70), e-liquid constituents and device features are far more diverse and customizable than cigarettes (166, 167). Complicating this further are consumer behaviors that dictate exposure frequency, aerosol mass, inhalation volume, and particle deposition (168, 169). Importantly, studies comparing the effects of e-cigarettes to tobacco-free individuals have yielded mixed results: Some studies suggest that vaping causes adverse molecular and physiological changes (170, 171), while others report no change in health risk (127, 172). A notable example of a health outcome unique to vaping was the EVALI outbreak in 2018–2019, and most cases were attributed to the presence of vitamin E acetate or other additives in the e-liquids (173). It is increasingly clear that established tobacco end points may not capture the range of potential adverse health outcomes of vaping. For example, flavor additives in e-cigarettes contribute to the preference for nasal exhalation in vapers. Thus, it is useful to consider not only the context in which vaping and smoking risks overlap but also the potential for disparate health consequences associated with product-specific behaviors and features.

Dual- and Polytobacco Product Use

Most vapers do not exclusively use e-cigarettes; rather, these individuals vape and use one or more additional tobacco products (dubbed dual- and polyusers, respectively) (174). Thus, evaluating the health risks associated with exclusive e-cigarette use has been challenging, as the preference in choosing a secondary tobacco product can vary with location, access to tobacco products, and nicotine craving (175, 176). Often, vaping individuals who use additional tobacco products are analyzed as one homogenous group, irrespective of tobacco product type or frequency of use (177). Studies that have attempted to profile dual users have identified unique use patterns. For example, some long-time smokers vape to facilitate smoking cessation, whereas some exclusive e-cigarette users transition to cigarettes on their way to becoming exclusive or dual-use smokers (178, 179). Both intention to quit smoking and product-specific harm perception are strong predictors of an individual’s likely success in product switching (180). Intriguingly, a retrospective study of former smokers who started vaping and became dual-users found that the number of cigarettes consumed declined, despite participants reporting higher nicotine intake and dependence, which may point to compensatory behavior (i.e., more frequent use or e-liquids with higher nicotine content) (181). Increasingly, studies are finding evidence of worsened cardiopulmonary outcomes in dual users than in those who use combustible tobacco alone (124, 128, 182). A possible explanation for this may be the potential for vaping to enhance the metabolism of tobacco-specific carcinogens (85).

CONCLUSIONS

Since their introduction to market just over a decade ago, evidence of e-cigarette toxicity continues to unfold. Importantly, controlled human, animal, and cell studies have provided biological underpinnings in support of epidemiologic associations between vaping and cardiopulmonary illnesses. This broad investigative approach is necessary because of the multitude of factors influencing e-cigarette toxicity. An important unanswered question is whether e-cigarettes are a safer alternative to conventional cigarettes. Although e-cigarette aerosols contain fewer toxic chemicals, e-liquid constituents and device features are more diverse and customizable than cigarettes (Figure 3). In addition, consumer behaviors that drive exposure frequency, inhalation volume, and particle deposition vary greatly. Studies evaluating the independent risk of toxicity associated with e-cigarettes have yielded mixed results. One explanation for this may simply be that traditional tobacco smoke end points may not capture the range of adverse health outcomes associated with vaping. For example, exhalation patterns between smokers and vapers differ, with the latter preferring nasal over oral exhalation. Thus, vaping toxicity should be contextualized: The health risks associated with e-cigarettes should be evaluated relative to those of cigarettes and also independently to identify any unique health risks posed by vaping device features and consumer behaviors.

Figure 3.

Separate factors influencing the toxicity of cigarettes and e-cigarettes as nicotine-delivery devices.

Assessing the relative risk of vaping is further complicated by myriad confounding issues: Currently, the extent to which polytobacco use augments risk is unknown. Epidemiology studies tend to indiscriminately group polytobacco users into a homogenous group of vapers, regardless of tobacco product type or frequency of use. Despite these limitations, a growing body of literature has identified associations between dual use and worsened cardiopulmonary outcomes that exceed the risk associated with exclusive combustible smoking. While epidemiological studies are likely to lead e-cigarette research by identifying associations between use and health outcomes, controlled experimental studies are necessary to generate data that support epidemiological trends. A more comprehensive understanding of the mechanisms by which e-cigarettes lead to deleterious health outcomes, coupled with behavioral studies, is essential to generate the scientific evidence needed to hone regulatory priorities and justify policy changes to better protect public health.