Gurjot Kaur1
Anshuman Gaurav- 1School of Pharmaceutical Sciences, Shoolini University, Solan, India
- 2Department of Environmental Medicine, University of Rochester Medical Center, Rochester, NY, United States
Electronic nicotine delivery systems/devices (ENDS) such as electronic cigarettes (e-cigarettes) have been made available globally, with the intent to reduce tobacco smoking. To make these products more appealing to young adults, many brands have added flavoring agents. However, these flavoring agents are shown to progressively result in lung toxicity when inhaled via e-cigarettes. While recent federal regulations have banned the sale of flavored e-cigarettes other than tobacco or menthol flavors, concerns have been raised about the health effects of even these flavors. In this review, we evaluate the current toxicological data with regard to effects upon exposure in animal models and in vitro cell culture for these popular flavorants. We have tabulated the current e-cigarette products containing these most common flavors (menthol, mint, and tobacco) in the market. We have also indicated the prevalence of tobacco and menthol-flavor use among e-cigarette users and highlighted the possible challenges and benefits that will result from new federal regulations.
Introduction
E-cigarettes are a diverse group of products which allow for the inhalation of nicotine. Popular examples of these devices include cig-a-likes, vape pens, and mods. In addition to nicotine, e-cigarette aerosols contain many other chemicals. These include, but are not limited to, flavors, humectants, such as propylene glycol, formaldehyde, acrolein, and specific nitrosamines.
These devices can deliver various concentrations of nicotine, dependent on the various constituents of the e-cigarette (Kaur et al., 2018). As of January 2014, there were 466 unique brands of electronic nicotine products and this number increased, on average, by 10.5 brands per month from August 2013 to January 2014 (Barrington-Trimis et al., 2014). There is an extremely diverse range of e-cigarette flavors available in the US market; with over 8,000 flavors available from mint to fruit to dessert flavors, brands have established a broad appeal to both adults and children (Kaur et al., 2018). Although adolescents have been made aware of the risks of e-cigarettes, many continue to hold relatively favorable attitudes toward e-cigarettes (Gorukanti et al., 2017). According to the Health Information National Trends Survey (HINTS) data reported in 2015, Americans who believed that e-cigarettes were less addictive than tobacco cigarettes were almost 2.5 times more likely to try e-cigarettes than those who believed e-cigarettes were equally or more addictive than tobacco cigarettes (Lewis-Thames et al., 2020). In addition, e-cigarette users often assume that it is more acceptable to use e-cigarettes both indoors and outdoors in contrast to conventional cigarettes that can be used only outdoors. Common misconceptions among adolescents also include the belief that e-cigarettes are safer than conventional cigarettes, that they help people quit smoking, and that they contain little or no nicotine (Gorukanti et al., 2017).
Of particular concern is the widespread use of e-cigarettes among high school and middle school students. The 2011–2018 National Youth Tobacco Survey (NYTS) showed an increase in e-cigarette use in both high school and middle school students, 20.8 and 4.9%, respectively. Specifically, between 2017 and 2018, there was a 78 and 48% increase in e-cigarette use by high school students and middle school students, respectively (Cullen et al., 2018).
Young e-cigarette users may be influenced into adopting this harmful habit by the marketing of e-cigarette manufacturing companies. These companies often use harmful marketing strategies to increase sales, i.e., displaying e-cigarettes as safer alternatives to other forms of smoking while also promoting appealing flavors (Bhalerao et al., 2019). According to the NYTS held jointly by the U.S. Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention, around 3.6 million students were using e-cigarettes in 2018 (Bhalerao et al., 2019). In the 2019 NYTS, e-cigarette usage in high school students reportedly increased to 27.5% and in middle school students to 10.5%. Approximately 59% of high school students and 54% of middle school students used JUUL as their usual device, with both groups preferring fruit flavors (Cullen et al., 2019).
Due to this increase in e-cigarette usage among adolescents and the high preference for flavored e-cigarettes, the FDA took action to limit access to these devices. In January 2020 the administration ruled that the sale of any flavored, cartridge-based electronic nicotine systems (ENDS), other than tobacco and menthol flavors, would be prohibited (Enforcement Priorities for Electronic Nicotine Delivery System (ENDS) and Other Deemed Products on the Market Without Premarket Authorization). Due to the ban on flavored, cartridge-based ENDS, concern has now largely shifted to the currently available flavors, menthol and tobacco.
Menthol and Tobacco Flavor Usage
E-cigarettes that contained 3.5% menthol have been shown to have a greater likelihood of usage compared to e-cigarettes without menthol. Menthol usage (0.5–3.5%) resulted in a significant improvement in taste and thus, higher nicotine concentrations (12 mg/ml) could be used (Krishnan-Sarin et al., 2017). Interestingly, unit sales of menthol e-cigarettes as a percent of all units sold remained stable from 2012 (39.9%) to 2016 (36.6%) (Kuiper et al., 2018). But first flavor purchases have altered over time. Tobacco and menthol flavors have been the highest and second-highest purchased flavors approximately 5 years ago. Fruit flavors ranked as the top choice for the last 3 years and even more prominently in the last year. Tobacco and menthol preference has decreased over time, with menthol ranked fourth and tobacco as the second (Russell et al., 2018). Among adults, the most common flavor used within the past 30 days of the survey was menthol, while in youth, menthol was the fourth most common flavor (Schneller et al., 2018). Currently, there is great diversity in the e-cigarette flavors within menthol and tobacco categories.
To evaluate the current market share and usage of the most common flavors, i.e., tobacco, menthol and mint, we performed a market investigation of brands that sell ENDS products with these three flavors (Supplementary Table 1). As stated, there is a considerable portion of marketed ENDS products that have either of the three flavors. Out of the three flavors, tobacco flavoring captured the most ENDS products, leading in e-liquid, e-liquid with salts, pods, and cartridges categories. It may be assumed that this preference and thus availability is due to public preference for tobacco flavor. This may arise from the desire to replace the sensation of tobacco in the absence of conventional cigarettes. Nevertheless, menthol and mint are also common flavors and closely followed tobacco in various categories.
Current Safety Status of the Most Common Flavors in E-Cigarettes
In e-liquids that had at least one flavoring chemical with a concentration greater than 10 mg/ml, menthol was present in 50% of the samples. Menthol concentration has been shown to be cytotoxic in 34% of refill fluids (Omaiye et al., 2019). In addition, mint and menthol ENDS are shown to contain pulegone. However, the FDA has already banned synthetic pulegone as a food additive as it is a known carcinogen (Jabba and Jordt, 2019). In traditional cigarettes, studies have shown that menthol increased the reinforcing nature of nicotine on smoking behavior (Ahijevych and Garrett, 2010). Along with this reinforced nature, menthol in traditional cigarettes can result in an increase in nicotine dependence compared to non-menthol cigarette smokers (Villanti et al., 2017). Menthol cigarette smoking was found to be most prevalent in adolescent smokers between 2008 and 2010. There was generally a more rapid decline in non-mentholated cigarette smoking than in mentholated cigarette smoking (Giovino et al., 2015). A similar scenario is expected in menthol containing e-cigarettes.
a. Tobacco Flavors
As with many other e-cigarette flavors, tobacco flavors are often marketing with enticing names such as King Pin, Havana Cigar, Classic Tobacco, Renegade, Wizard’s Leaf, and Cowboy. An extensive list of various brands’ tobacco flavors in given in Supplementary Table 1.
Several studies have sought to investigate the cellular toxicity of e-cigarette tobacco flavors. Some have found that epithelial cells exposed to tobacco flavor vapor showed increased levels of cell death over a period of several hours to several weeks (Yu et al., 2016). Others have shown that exposure to the tobacco flavoring can cause inflammatory responses in cells such as fibroblasts (Sundar et al., 2016; Table 1). The general results reported among available studies looking at tobacco flavors often included decreased cell viability, decreased numbers of cells, and increased inflammation after exposure (Table 2).
Table 1. Current literature on mouse inhalation toxicology after flavorant exposure.
Table 2. Current literature on in vitro inhalation toxicology after flavorant exposure.
b. Menthol/mint Flavors
Menthol and mint flavors are likewise marketed to appeal to both adolescents and adults. Examples of brand-specific flavor names in this category include Arctic Blast, Mountain Chill, Polar Bear, Kringle’s Curse, Blue Slushie Iced, and Candy Cane. Additional examples of mint and menthol flavors are listed in Supplementary Table 1.
The chemical menthol is often used to impart a mint flavor and may be used in combination with other flavoring chemicals. One study investigating the cellular effects of exposure to menthol flavors found that lung epithelial cells exposed to a pod menthol flavor showed decreased mitochondria function with resulting decreased respiration in the mitochondria (Lamb et al., 2020). Another showed that exposure to these flavors resulted in a decrease in the amount of ATP in a sample of fibroblasts (Willershausen et al., 2014). Another study observed increased inflammation in bronchial epithelial cells after exposure to menthol (Leigh et al., 2016). Most studies utilizing menthol and mint flavors showed general trends such as higher levels of DNA damage and increased cell death following exposure (Table 2).
c. E-cigarette or Vaping Product Use-Associated Lung Injury (EVALI) chemicals and toxicity based on forensic chemistry and biology
Additional chemicals have been implicated in the e-cigarette or vaping product use associated lung injury, or EVALI, observed in some e-cigarette users. Some of these include vitamin E acetate, THC, various hydrocarbons, terpenes, pesticides, plasticizers, and assorted metals (Muthumalage et al., 2020a). Among these components, vitamin E acetate has become a popular subject of research. This chemical especially is under scrutiny for its role in lung injury as it has been observed in sampled patients. Studies into the toxic effects of inhaled acetate have shown that this chemical may play a role in inducing inflammation responses in cells (Muthumalage et al., 2020b). Other vaping chemicals, such as medium-chain triglycerides, have also been the subject of study in relation to their possible role in this severe form of lung injury. One study investigated the effects of both this group of chemicals and vitamin E acetate on human cells. The exposure of pulmonary epithelial cells and immune cells to these chemicals resulted in harmful effects to the cells via lipid mediators. Decreased barrier function among the epithelial cells was observed, as well as a general increased immune response via activation of Toll-like receptor (TLR) or transient receptor potential (TRP)-like channels (Muthumalage et al., 2020b; Figures 1, 2).
Figure 1. Cellular mechanisms of toxicity for menthol and tobacco flavors. The figure summarizes the current cellular pathways and pathogenesis mechanisms involved in cellular toxicity for menthol and tobacco flavorings.
Figure 2. Cellular dysfunction by inhaled flavoring agents. Flavoring chemicals induced oxidative stress and inflammatory responses are associated with immune-responses via alterations in barrier tight junction dysfunction in the lung. Mitochondrial dysfunction and other cellular alterations can lead to susceptibility to infections. Tobacco flavor (nicotine) activate Nicotinic acetylcholine receptors (nAChRs) and menthol activate cold and menthol receptor 1 (i.e. via TRPM8) leading to various downstream cellular signaling events.
Toxicological Evaluation of ENDS
To assess the toxicology associated with the usage of flavors added to e-cigarettes, we compiled and exhaustively analyzed original research articles associated with the topic. We divided the original data into animal exposure studies (Table 1) and in vitro cell culture studies (Table 2).
Current Status on Flavor Induced Toxicology in Mice
We could identify four original articles that observed flavor (tobacco, menthol, or mint) associated toxicity in mice (Table 1). Lerner et al. (2015) tested six tobacco flavored e-liquids and one menthol flavored e-liquid in mice through whole-body inhalation exposure and observed increased OX/ROS reactivity along with oxidative stress in the presence of flavoring agents in the absence of nicotine. Zelikoff et al. (2018) demonstrated a sans nicotine effect on the developing central nervous system (CNS) in C57BL/6 mice and hypothesized the possible role of flavoring agents in the stunted CNS growth. Although the direct experimental evidence for the involvement of flavoring agents could not be provided by the other two studies (Chen et al., 2018; Glynos et al., 2018), it was shown that the presence of flavoring agents might have a role in increased IL-1β, IL-6, and TNF-α levels and other oxidative stress markers in lungs. We did find many previous studies that used flavored e-liquids to assess the effect of nicotine as part of ENDS product in mice; however, these earlier studies only compared nicotine treatments and generally, did not mention the exact concentrations or composition of flavoring agents (Table 3). Interestingly, recent studies also lack in the measurement of flavorants (Table 1). In general, the use of a 1–5% tobacco flavor did not yield any marked differences in measurable outcomes of testis toxicity (El Golli et al., 2016b; Rahali et al., 2018) or hepatic function (El Golli et al., 2016a). Similar results were obtained in DNA damage and mitochondrial dysfunction with tobacco flavor (concentration not mentioned) (Espinoza-Derout et al., 2019), and pro-senescence phenotypes (Sundar et al., 2016; Jabba and Jordt, 2019; Lucas et al., 2020) leading to transformational changes of normal cells by e-cig derived mint/menthol flavor toxicants.
Table 3. Current literature on flavor induced inhalation toxicology.
Additional studies into the effects of e-cigarette exposure with developing mice found that prenatal exposure may increase chances of those mice later developing pulmonary diseases (Wang et al., 2020). A study into the effects of exposure to both e-cigarette vapor and traditional cigarette smoke showed that mice exposed to both showed altered lung function, differing from even the effects of cigarette smoke alone (Lechasseur et al., 2020). An analysis of the relationship between e-cigarette exposure and cancer genesis has also been performed in mice. This study concluded that e-cigarette vapor potentially produced carcinogenic effects in the lung and bladder tissue of exposed mice, including lung adenocarcinomas (Tang et al., 2019).
Current Status on Flavor Induced Toxicology in Human in vitro
We could identify numerous studies wherein flavorant effects were observed after exposure to human cells in culture (Table 2). Cells treated with 10 μg/ml menthol e-liquid displayed a reduction in ATP levels in fibroblasts. A significant reduction in cell proliferation was observed between 24 and 96 h and cell migration at 72 h (Willershausen et al., 2014). A recent study measured menthol concentrations in all 68 e-liquids and showed a dose-dependent association with cytotoxicity (Al-Saleh et al., 2020). Lamb et al. (2020) reported menthol flavoring dependent mitochondrial dysfunction in BEAS-2B cells and successfully identified and measured the individual chemical constituents in the flavors through mass spectrometry that may be responsible for the effect. Menthol flavoring chemicals have been shown to cause endothelial cell dysfunction. In human aortic endothelial cells (HAECs) treated with the highest dose, 100 mmol/L of menthol resulted in a significant increase in cell death and IL-6 secretion. HAECs treated with concentrations of 0.001, 0.01, and 0.1 mmol/L menthol resulted in a significant decrease in nitric oxide production when stimulated with A23187, an endothelial nitric oxide synthase agonist. This indicates endothelial dysfunction since the increase in nitric oxide results in vasodilation and is an indication of cardiovascular health (Fetterman et al., 2018).
Menthol was previously known to inhibit the liver microsomal oxidation of nicotine to its metabolite cotinine, which can potentially lead to an increase in plasma levels of nicotine (MacDougall et al., 2003). In one study, an alveolar blood barrier consisting of a co-culture of epithelial lung cells on the apical compartment and endothelial cells on the basal compartment, was treated on the apical compartment with condensed e-cigarette aerosols of menthol and tobacco flavors with nicotine. Exposure for 24 h with the condensed aerosol identified as ‘Menthol 2’ resulted in a significant barrier dysfunction due to a reduction in transepithelial electrical resistance compared to both control and condensed aerosol base, composed of propylene glycol, vegetable glycerin, and nicotine. This reduction was not seen in the two tobacco condensed aerosols or the other menthol condensed aerosol, potentially indicating that interaction of menthol flavoring chemicals with other chemicals such as carvone (terpenoid) can increase cytotoxicity (Bengalli et al., 2017).
Neuronal stem cells (NSCs) treated with either e-liquids or aerosols in cell culture media of either tobacco or menthol flavors (1%) showed a significant increase in total autophagosome area compared to the control at both 4- and 24-h time points. Treatments with 0.5% menthol and tobacco e-liquids and menthol and tobacco six-total-puff equivalents increased percent lysosome co-localization. In turn, this potentially indicates a decrease in degradation and potential contribution to increased autophagic load (Zahedi et al., 2019). In HPdLF fibroblasts, exposure to Blu Classic Tobacco with 16 mg nicotine aerosol resulted in a significant increase in protein carbonylation while Blu Magnificent Menthol with 0 mg nicotine aerosol increased protein carbonylation but was not significant compared to the control. Meanwhile, IL-8 secretion and phosphorylated γH2A.X was increased in both Blu Classic Tobacco and Blu Magnificent Menthol aerosols (Sundar et al., 2016). Figures 1, 2 describe an overview of the mechanism of toxicity after menthol, mint and tobacco flavoring exposure.
In an additional study into the effects of exposure to general vapor from ENDS, damage to both lung epithelial cells and macrophages was noted. Following exposure, increased apoptosis and necrosis of the epithelial cells was observed, as well as increased cell death in the macrophages (Serpa et al., 2020).
Most studies reported reduced cell viability and/or increased pro-inflammatory mediators, although the study design did not include measurement of flavoring agent and therefore, more work may be needed to prove an association (Leigh et al., 2016; Yu et al., 2016; Leslie et al., 2017; Rowell et al., 2017; Behar et al., 2018; Otero et al., 2019; Go et al., 2020). Platelet-activating factor receptor (PAFR) mediated adhesion has been reported to increase in nicotine-free samples, although the study did not provide an experimental design assessing the tested tobacco flavor concentrations and exposure on the lung epithelial cell lines (Miyashita et al., 2018). No tobacco or menthol flavor associated effects were observed in human bronchial airway epithelial cells and human fetal lung fibroblasts (Lerner et al., 2015) and human monocytes (Muthumalage et al., 2017), though the concentration of flavoring agent in the aerosol were not measured. Flavors other than tobacco, mint, and menthol also displayed cytotoxicity in the tested cells. Cinnamaldehyde is the only chemical that consistently demonstrated cytotoxicity and increased cytokine release consistently (Table 2). We identified a few studies that demonstrated contrasting results and have tabulated them in Table 3.
Challenges and Advantages Associated With E-Cigarette Flavor Ban
Despite pushing for the ban of flavored e-cigarettes and an FDA ban on the majority of flavors in e-cigarettes, concerns regarding e-cigarette users switching to combustion cigarettes has arisen. A longitudinal study looking at a group of adult e-cigarette users found that roughly 50% of participants reported that in light of the flavor ban they would attempt to “find a way to buy my flavor” or “add flavoring agents myself.” In addition, 9.6% of participants reported that “I would return to smoking traditional tobacco cigarettes” if there was a ban on all non-tobacco flavors (Du et al., 2020). In a discrete choice experiment using a population of adult smokers or recent quitters, it was observed that banning flavors in e-cigarettes while continuing to allow menthol in traditional cigarettes would result in an increase in 8.3% in traditional cigarette smoker, a decrease in 11.1% of e-cigarette use, and only 3% of participants would abandon both cigarettes and e-cigarettes (Buckell et al., 2018).
However, most young adults have reported that the first e-cigarette they used was flavored to taste like something other than tobacco. The most popular flavor reported was fruit and the second most common flavor reported was candy or dessert. In adults, tobacco-flavored e-cigarettes were more common compared to young adults and youth (Harrell et al., 2017). Despite the potential benefit of banning flavored e-cigarettes to reduce usage in youth and young adults, the usage may not decrease but rather a shift may be observed toward menthol or tobacco-flavored e-cigarettes.
It is also worth noting that regulating flavors of conventional cigarettes may also contribute to shifts in e-cigarette use. One online study found that among surveyed menthol cigarette smokers, approximately 15% reported that if a ban were placed on menthol cigarettes, they would most likely switch to using e-cigarettes (Wackowski et al., 2015).
Despite the challenges, studies have indicated that a governmental ban remains the most effective path to reducing the use of certain products or flavors. One study into an attempted “self-regulation” by one brand, where certain flavors were intentionally removed from the market, found that this strategy merely lead to consumers switching or other brands or increasing use of alternative flavors (Liber et al., 2020).
Conclusion
Despite attempts by the United States government to curb the appeal of e-cigarettes to young people, the availability of menthol/mint-flavored e-cigarettes poses a potential issue. Menthol cigarettes are popular in adolescents as a result of reinforcement and thus nicotine dependence and e-cigarette use in youth and young adults continues. Despite the potential risk of adolescent use, a ban that would extend to menthol-flavored e-cigarettes would run a risk of pushing e-cigarette users back to traditional cigarette smoke. Future regulation of e-cigarettes needs to take into consideration the health effects of both tobacco and menthol flavors and any ban that would include menthol flavors with specific injurious chemicals would need to be in combination with a ban of menthol/mint cigars and cigarettes.
WHO document on assessment of menthol usage in tobacco products (including traditional cigarettes and e-cigarettes), published in 2018, emphasizes that restrictions should be imposed to other flavors in addition to menthol. A complete ban would reduce the potential shift to another flavor including menthol/mint. Although implementation of such a ban may vary dependent on a country’s economy, i.e., low-income, middle-income or high income. The ban on all tobacco products and flavoring additives will limit the likelihood that tobacco and menthol/mint use will simply shift to other products categories. However, flavor capsules (tobacco, menthol/mint and other flavors) are now gaining popularity due to lack of regulations. Canada, a high-income country, has implemented incremental restrictions on menthol usage in tobacco products and new product categories, i.e., ENDS have been clearly mentioned in the ban.
Tobacco, menthol, and mint have captured the ENDS market due to their aesthetic appeal, although guidelines for their use are largely lacking. Unfortunately, recent studies have reproducibly demonstrated cytotoxic effects in laboratory-based experimentation for these flavors. In addition, human reports studying the flavoring-based effects of ENDS products are yet to be conducted. We recommend that such human studies are required and should be conducted at the earliest opportunity to understand chronic exposure. In addition, earlier studies did not measure the flavoring agent concentrations or identify individual chemical constituents, possibly due to lack of proper detection methods. Newer studies are utilizing GC/MS based methods that may help in delineating any aerosol dose dependency based toxic effects. For example, flavors, especially cinnamaldehyde, are toxic in mice and in vitro human cell experiments, and warrant further investigation. Finally, we recommend that more studies are conducted with an experimental design based on the effect of individual flavor concentrations to make an accurate assessment.
Author Contributions
GK and IR conceptualized the review. AG, TM, and MP prepared the figures and tables with the help from GK and IR. GK, AG, TL, MP, TM, and IR prepared the first draft of the manuscript. GK and IR finalized the manuscript. All authors contributed to the article and approved the submitted version.
Funding
This study was supported by WNY Center for Research on Flavored Tobacco Products (CRoFT) Nos. U54CA228110 and NIH 1R01HL135613. Research reported in this publication was supported by NCI/NIH and FDA Center for Tobacco Products (CTP). This content is the sole property of the authors and does not represent the official views of the NIH or the Food and Drug Administration.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Supplementary Material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fphys.2020.613948/full#supplementary-material
References
Ahijevych, K., and Garrett, B. E. (2010). The role of menthol in cigarettes as a reinforcer of smoking behavior. Nicotine Tob. Res. 12(Suppl. 2), S110–S116. doi: 10.1093/ntr/ntq203
Al-Saleh, I., Elkhatib, R., Al-Rajoudi, T., Al-Qudaihi, G., Manogarannogaran, P., Eltabache, C., et al. (2020). Cytotoxic and genotoxic effects of e-liquids and their potential associations with nicotine, menthol and phthalate esters. Chemosphere 249:126153. doi: 10.1016/j.chemosphere.2020.126153
Barrington-Trimis, J. L., Samet, J. M., and Mcconnell, R. (2014). Flavorings in electronic cigarettes: an unrecognized respiratory health hazard? JAMA 312, 2493–2494. doi: 10.1001/jama.2014.14830
Behar, R. Z., Wang, Y., and Talbot, P. (2018). Comparing the cytotoxicity of electronic cigarette fluids, aerosols and solvents. Tob. Control 27, 325–333. doi: 10.1136/tobaccocontrol-2016-053472
Bengalli, R., Ferri, E., Labra, M., and Mantecca, P. (2017). Lung toxicity of condensed aerosol from E-CIG liquids: influence of the flavor and the in vitro model used. Int. J. Environ. Res. Public Health 14:1254. doi: 10.3390/ijerph14101254
Bhalerao, A., Sivandzade, F., Archie, S. R., and Cucullo, L. (2019). Public health policies on E-cigarettes. Curr. Cardiol. Rep. 21:111. doi: 10.1007/s11886-019-1204-y
Buckell, J., Marti, J., and Sindelar, J. L. (2018). Should flavours be banned in cigarettes and e-cigarettes? Evidence on adult smokers and recent quitters from a discrete choice experiment. Tob. Control 28, 168–175. doi: 10.3386/w23865
Chen, H., Li, G., Chan, Y. L., Chapman, D. G., Sukjamnong, S., Nguyen, T., et al. (2018). Maternal E-Cigarette exposure in mice alters DNA methylation and lung cytokine expression in offspring. Am. J. Respir. Cell Mol. Biol. 58, 366–377.
Cullen, K. A., Ambrose, B. K., Gentzke, A. S., Apelberg, B. J., Jamal, A., and King, B. A. (2018). Notes from the field: use of electronic cigarettes and any tobacco product among middle and high school students - United States, 2011-2018. MMWR Morb. Mortal Wkly. Rep. 67, 1276–1277. doi: 10.15585/mmwr.mm6745a5
Cullen, K. A., Gentzke, A. S., Sawdey, M. D., Chang, J. T., Anic, G. M., Wang, T. W., et al. (2019). e-Cigarette use among youth in the United States, 2019. JAMA 322, 2095–2103.
Du, P., Bascom, R., Fan, T., Sinharoy, A., Yingst, J., Mondal, P., et al. (2020). Changes in flavor preference in a cohort of long-term electronic cigarette users. Ann. Am. Thorac. Soc. 17, 573–581.
El Golli, N., Jrad-Lamine, A., Neffati, H., Rahali, D., Dallagi, Y., Dkhili, H., et al. (2016a). Impact of e-cigarette refill liquid with or without nicotine on liver function in adult rats. Toxicol. Mech. Methods 26, 419–426.
El Golli, N., Rahali, D., Jrad-Lamine, A., Dallagi, Y., Jallouli, M., Bdiri, Y., et al. (2016b). Impact of electronic-cigarette refill liquid on rat testis. Toxicol. Mech. Methods 26, 427–434.
Espinoza-Derout, J., Shao, X. M., Bankole, E., Hasan, K. M., Mtume, N., Liu, Y., et al. (2019). Hepatic DNA damage induced by electronic cigarette exposure is associated with the modulation of NAD+/PARP1/SIRT1 axis. Front. Endocrinol. 10:320. doi: 10.3389/fendo.2019.00320
Fetterman, J. L., Weisbrod, R. M., Feng, B., Bastin, R., Tuttle, S. T., Holbrook, M., et al. (2018). Flavorings in tobacco products induce endothelial cell dysfunction. Arterioscler. Thromb. Vasc. Biol. 38, 1607–1615.
Giovino, G. A., Villanti, A. C., Mowery, P. D., Sevilimedu, V., Niaura, R. S., Vallone, D. M., et al. (2015). Differential trends in cigarette smoking in the USA: is menthol slowing progress? Tob. Control 24, 28–37.
Glynos, C., Bibli, S. I., Katsaounou, P., Pavlidou, A., Magkou, C., Karavana, V., et al. (2018). Comparison of the effects of e-cigarette vapor with cigarette smoke on lung function and inflammation in mice. Am. J. Physiol. Lung Cell. Mol. Physiol. 315, L662–L672.
Go, Y. Y., Mun, J. Y., Chae, S. W., Chang, J., and Song, J. J. (2020). Comparison between in vitro toxicities of tobacco- and menthol-flavored electronic cigarette liquids on human middle ear epithelial cells. Sci. Rep. 10:2544.
Gorukanti, A., Delucchi, K., Ling, P., Fisher-Travis, R., and Halpern-Felsher, B. (2017). Adolescents’ attitudes towards e-cigarette ingredients, safety, addictive properties, social norms, and regulation. Prev. Med. 94, 65–71. doi: 10.1016/j.ypmed.2016.10.019
Harrell, M. B., Weaver, S. R., Loukas, A., Creamer, M., Marti, C. N., Jackson, C. D., et al. (2017). Flavored e-cigarette use: characterizing youth, young adult, and adult users. Prev. Med. Rep. 5, 33–40.
Jabba, S. V., and Jordt, S. E. (2019). Risk analysis for the carcinogen pulegone in mint- and menthol-flavored e-cigarettes and smokeless tobacco products. JAMA Intern. Med. 179, 1721–1723. doi: 10.1001/jamainternmed.2019.3649
Kaur, G., Muthumalage, T., and Rahman, I. (2018). Mechanisms of toxicity and biomarkers of flavoring and flavor enhancing chemicals in emerging tobacco and non-tobacco products. Toxicol. Lett. 288, 143–155. doi: 10.1016/j.toxlet.2018.02.025
Kerr, D. M. I., Brooksbank, K. J. M., Taylor, R. G., Pinel, K., Rios, F. J., Touyz, R. M., et al. (2019). Acute effects of electronic and tobacco cigarettes on vascular and respiratory function in healthy volunteers: a cross-over study. J. Hypertens. 37, 154–166.
Krishnan-Sarin, S., Green, B. G., Kong, G., Cavallo, D. A., Jatlow, P., Gueorguieva, R., et al. (2017). Studying the interactive effects of menthol and nicotine among youth: an examination using e-cigarettes. Drug Alcohol Depend 180, 193–199.
Kuiper, N. M., Loomis, B. R., Falvey, K. T., Gammon, D. G., King, B. A., Wang, T. W., et al. (2018). Trends in unit sales of flavored and menthol electronic cigarettes in the United States, 2012-2016. Prev. Chronic Dis. 15:E105.
Lamb, T., Muthumalage, T., and Rahman, I. (2020). Pod-based menthol and tobacco flavored e-cigarettes cause mitochondrial dysfunction in lung epithelial cells. Toxicol. Lett. 333, 303–311. doi: 10.1016/j.toxlet.2020.08.003
Larcombe, A. N., Janka, M. A., Mullins, B. J., Berry, L. J., Bredin, A., and Franklin, P. J. (2017). The effects of electronic cigarette aerosol exposure on inflammation and lung function in mice. Am. J. Physiol. Lung Cell. Mol. Physiol. 313, L67–L79.
Lauterstein, D. E., Tijerina, P. B., Corbett, K., Akgol Oksuz, B., Shen, S. S., Gordon, T., et al. (2016). Frontal cortex transcriptome analysis of mice exposed to electronic cigarettes during early life stages. Int. J. Environ. Res. Public Health 13:417.
Lechasseur, A., Huppé, C., Talbot, M., Routhier, J., Aubin, S., Beaulieu, M., et al. (2020). Exposure to nicotine-free and flavor-free e-cigarette vapors modifies the pulmonary response to tobacco cigarette smoke in female mice. Am. J. Physiol. Lung Cell Mol. Physiol. 319, L717–L727. doi: 10.1152/ajplung.00037.2020
Leigh, N. J., Lawton, R. I., Hershberger, P. A., and Goniewicz, M. L. (2016). Flavourings significantly affect inhalation toxicity of aerosol generated from electronic nicotine delivery systems (ENDS). Tob. Control 25, ii81–ii87.
Lerner, C. A., Rutagarama, P., Ahmad, T., Sundar, I. K., Elder, A., and Rahman, I. (2016). Electronic cigarette aerosols and copper nanoparticles induce mitochondrial stress and promote DNA fragmentation in lung fibroblasts. Biochem. Biophys. Res. Commun. 477, 620–625.
Lerner, C. A., Sundar, I. K., Yao, H., Gerloff, J., Ossip, D. J., Mcintosh, S., et al. (2015). Vapors produced by electronic cigarettes and e-juices with flavorings induce toxicity, oxidative stress, and inflammatory response in lung epithelial cells and in mouse lung. PLoS One 10:e0116732. doi: 10.1371/journal.pone.0116732
Leslie, L. J., Vasanthi Bathrinarayanan, P., Jackson, P., Mabiala, Ma Muanda, J. A., Pallett, R., et al. (2017). A comparative study of electronic cigarette vapor extracts on airway-related cell lines in vitro. Inhal. Toxicol. 29, 126–136. doi: 10.1080/08958378.2017.1318193
Lewis-Thames, M. W., Langston, M. E., Fuzzell, L., Khan, S., Moore, J. X., and Han, Y. (2020). Rural-urban differences e-cigarette ever use, the perception of harm, and e-cigarette information seeking behaviors among U.S. adults in a nationally representative study. Prev. Med. 130:105898. doi: 10.1016/j.ypmed.2019.105898
Li, G., Chan, Y. L., Nguyen, L. T., Mak, C., Zaky, A., Anwer, A. G., et al. (2019). Impact of maternal e-cigarette vapor exposure on renal health in the offspring. Ann. N. Y. Acad. Sci. 1452, 65–77.
Liber, A., Cahn, Z., Larsen, A., and Drope, J. (2020). Flavored E-Cigarette sales in the United States under self-regulation from January 2015 Through October 2019. Am. J. Public Health 110, 785–787. doi: 10.2105/AJPH.2020.305667
Lucas, J. H., Muthumalage, T., Wang, Q., Friedman, M. R., Friedman, A. E., and Rahman, I. (2020). E-Liquid containing a mixture of coconut, vanilla, and cookie flavors causes cellular senescence and dysregulated repair in pulmonary fibroblasts: implications on premature aging. Front. Physiol. 11:924. doi: 10.3389/fphys.2020.00924
MacDougall, J. M., Fandrick, K., Zhang, X., Serafin, S. V., and Cashman, J. R. (2003). Inhibition of human liver microsomal (S)-nicotine oxidation by (-)-menthol and analogues. Chem. Res. Toxicol. 16, 988–993. doi: 10.1021/tx0340551
Miyashita, L., Suri, R., Dearing, E., Mudway, I., Dove, R. E., Neill, D. R., et al. (2018). E-cigarette vapour enhances pneumococcal adherence to airway epithelial cells. Eur. Respir. J. 51, 1701592.
Moheimani, R. S., Bhetraratana, M., Peters, K. M., Yang, B. K., Yin, F., Gornbein, J., et al. (2017). Sympathomimetic effects of acute E-Cigarette use: role of nicotine and non-nicotine constituents. J. Am. Heart Assoc. 6:e006579.
Muthumalage, T., Friedman, M., McGraw, M., Ginsberg, G., Friedman, A., and Rahman, I. (2020a). Chemical constituents involved in e-cigarette, or vaping product use-associated lung injury (EVALI). Toxics 8:25. doi: 10.3390/toxics8020025
Muthumalage, T., Lucas, J., Wang, Q., Lamb, T., McGraw, M., and Rahman, I. (2020b). Pulmonary toxicity and inflammatory response of e-cigarette vape cartridges containing medium-chain triglycerides oil and Vitamin E acetate: implications in the pathogenesis of EVALI. Toxics 8:46. doi: 10.3390/toxics8030046
Muthumalage, T., Prinz, M., Ansah, K. O., Gerloff, J., Sundar, I. K., and Rahman, I. (2017). Inflammatory and oxidative responses induced by exposure to commonly used e-Cigarette flavoring chemicals and flavored e-Liquids without nicotine. Front. Physiol. 8:1130. doi: 10.3389/fphys.2017.01130
Nguyen, T., Li, G. E., Chen, H., Cranfield, C. G., Mcgrath, K. C., and Gorrie, C. A. (2019). Neurological effects in the offspring after switching from tobacco cigarettes to e-cigarettes during pregnancy in a mouse model. Toxicol. Sci. [Epub ahead of print] doi: 10.1093/toxsci/kfz194
Omaiye, E. E., Mcwhirter, K. J., Luo, W., Tierney, P. A., Pankow, J. F., and Talbot, P. (2019). High concentrations of flavor chemicals are present in electronic cigarette refill fluids. Sci. Rep. 9:2468. doi: 10.1038/s41598-019-39550-2
Otero, C. E., Noeker, J. A., Brown, M. M., Wavreil, F. D. M., Harvey, W. A., Mitchell, K. A., et al. (2019). Electronic cigarette liquid exposure induces flavor-dependent osteotoxicity and increases expression of a key bone marker, collagen type I. J. Appl. Toxicol. 39, 888–898. doi: 10.1002/jat.3777
Putzhammer, R., Doppler, C., Jakschitz, T., Heinz, K., Forste, J., Danzl, K., et al. (2016). Vapours of US and EU market leader electronic cigarette brands and liquids are cytotoxic for human vascular endothelial cells. PLoS One 11:e0157337. doi: 10.1371/journal.pone.0157337
Qasim, H., Karim, Z. A., Silva-Espinoza, J. C., Khasawneh, F. T., Rivera, J. O., Ellis, C. C., et al. (2018). Short-Term E-Cigarette exposure increases the risk of thrombogenesis and enhances platelet function in mice. J. Am. Heart Assoc. 7, e009264.
Rahali, D., Jrad-Lamine, A., Dallagi, Y., Bdiri, Y., Ba, N., El May, M., et al. (2018). Semen parameter alteration, histological changes and role of oxidative stress in adult rat epididymis on exposure to electronic cigarette refill liquid. Chin. J. Physiol. 61, 75–84. doi: 10.4077/CJP.2018.BAG521
Ramirez, J. E. M., Karim, Z. A., Alarabi, A. B., Hernandez, K. R., Taleb, Z. B., Rivera, J. O., et al. (2020). The JUUL E-Cigarette elevates the risk of thrombosis and potentiates platelet activation. J. Cardiovasc. Pharmacol. Ther. 25, 578–586.
Rau, A. S., Reinikovaite, V., Schmidt, E. P., Taraseviciene-Stewart, L., and Deleyiannis, F. W. (2017). Electronic cigarettes are as toxic to skin flap survival as tobacco cigarettes. Ann. Plast. Surg. 79, 86–91. doi: 10.1097/SAP.0000000000000998
Rowell, T. R., Reeber, S. L., Lee, S. L., Harris, R. A., Nethery, R. C., Herring, A. H., et al. (2017). Flavored e-cigarette liquids reduce proliferation and viability in the CALU3 airway epithelial cell line. Am. J. Physiol. Lung Cell Mol. Physiol. 313, L52–L66.
Russell, C., Mckeganey, N., Dickson, T., and Nides, M. (2018). Changing patterns of first e-cigarette flavor used and current flavors used by 20,836 adult frequent e-cigarette users in the USA. Harm. Reduct. J. 15:33. doi: 10.1186/s12954-018-0238-6
Schneller, L. M., Bansal-Travers, M., Goniewicz, M. L., Mcintosh, S., Ossip, D., and O’connor, R. J. (2018). Use of flavored electronic cigarette refill liquids among adults and youth in the US-Results from Wave 2 of the population assessment of tobacco and health study (2014-2015). PLoS One 13:e0202744. doi: 10.1371/journal.pone.0202744
Serpa, G. L., Renton, N. D., Lee, N., Crane, M. J., and Jamieson, A. M. (2020). Electronic nicotine delivery system aerosol-induced cell death and dysfunction in macrophages and lung epithelial cells. Am. J. Respir. Cell. Mol. Biol. 63, 306–316. doi: 10.1165/rcmb.2019-0200OC
Sundar, I. K., Javed, F., Romanos, G. E., and Rahman, I. (2016). E-cigarettes and flavorings induce inflammatory and pro-senescence responses in oral epithelial cells and periodontal fibroblasts. Oncotarget 7, 77196–77204. doi: 10.18632/oncotarget.12857
Sussan, T. E., Gajghate, S., Thimmulappa, R. K., Ma, J., Kim, J. H., Sudini, K., et al. (2015). Exposure to electronic cigarettes impairs pulmonary anti-bacterial and anti-viral defenses in a mouse model. PLoS One 10:e0116861. doi: 10.1371/journal.pone.0116861
Tang, M., Wu, X., Lee, H., Xia, Y., Deng, F., Moreirac, A. L., et al. (2019). Electronic-cigarette smoke induces lung adenocarcinoma and bladder urothelial hyperplasia in mice. Proc. Natl. Acad. Sci. U.S.A. 116, 21727–21731.
Vardavas, C. I., Anagnostopoulos, N., Kougias, M., Evangelopoulou, V., Connolly, G. N., and Behrakis, P. K. (2012). Short-term pulmonary effects of using an electronic cigarette: impact on respiratory flow resistance, impedance, and exhaled nitric oxide. Chest 141, 1400–1406. doi: 10.1378/chest.11-2443
Villanti, A. C., Collins, L. K., Niaura, R. S., Gagosian, S. Y., and Abrams, D. B. (2017). Menthol cigarettes and the public health standard: a systematic review. BMC Public Health 17:983. doi: 10.1186/s12889-017-4987-z
Wackowski, O. A., Delnevo, C. D., and Pearson, J. L. (2015). Switching to E-Cigarettes in the event of a menthol cigarette ban. Nicotine Tob. Res. 17, 1286–1287. doi: 10.1093/ntr/ntv021
Wang, Q., Sundar, I. K., Blum, J. L., Ratner, J. R., Lucas, J. H., Chuang, T., et al. (2020). Prenatal exposure to E-Cigarette aerosols leads to sex-dependent pulmonary extracellular matrix remodeling and myogenesis in offspring mice. Am. J. Respir. Cell. Mol. Biol. [Epub ahead of print] doi: 10.1165/rcmb.2020-0036OC
Willershausen, I., Wolf, T., Weyer, V., Sader, R., Ghanaati, S., and Willershausen, B. (2014). Influence of E-smoking liquids on human periodontal ligament fibroblasts. Head Face Med. 10, 39. doi: 10.1186/1746-160X-10-39
Wu, Q., Jiang, D., Minor, M., and Chu, H. W. (2014). Electronic cigarette liquid increases inflammation and virus infection in primary human airway epithelial cells. PLoS One 9:e108342. doi: 10.1371/journal.pone.0108342
Yu, V., Rahimy, M., Korrapati, A., Xuan, Y., Zou, A. E., Krishnan, A. R., et al. (2016). Electronic cigarettes induce DNA strand breaks and cell death independently of nicotine in cell lines. Oral Oncol. 52, 58–65.
Zahedi, A., Phandthong, R., Chaili, A., Leung, S., Omaiye, E., and Talbot, P. (2019). Mitochondrial stress response in neural stem cells exposed to electronic cigarettes. iScience 16, 250–269. doi: 10.1016/j.isci.2019.05.034
Zelikoff, J. T., Parmalee, N. L., Corbett, K., Gordon, T., Klein, C. B., and Aschner, M. (2018). Microglia activation and gene expression alteration of neurotrophins in the hippocampus following early-life exposure to E-Cigarette aerosols in a murine model. Toxicol. Sci. 162, 276–286. doi: 10.1093/toxsci/kfx257
Keywords: e-cigarettes, menthol, mint, tobacco, toxicity
Citation: Kaur G, Gaurav A, Lamb T, Perkins M, Muthumalage T and Rahman I (2020) Current Perspectives on Characteristics, Compositions, and Toxicological Effects of E-Cigarettes Containing Tobacco and Menthol/Mint Flavors. Front. Physiol. 11:613948. doi: 10.3389/fphys.2020.613948
Received: 04 October 2020; Accepted: 23 October 2020;
Published: 19 November 2020.
Edited by:
Dominic L. Palazzolo, Lincoln Memorial University, United StatesReviewed by:
Jagjit S. Yadav, University of Cincinnati, United StatesChendil Damodaran, University of Louisville, United States
Dhyan Chandra, University at Buffalo, United States
Copyright © 2020 Kaur, Gaurav, Lamb, Perkins, Muthumalage and Rahman. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Irfan Rahman, irfan_rahman@urmc.rochester.edu