A Summary of In Vitro and In Vivo Studies Evaluating the Impact of E-Cigarette Exposure on Living Organisms and the Environment

Cell signaling and epigenetic regulation of nicotine-induced carcinogenesis

Nicotine, a naturally occurring tobacco alkaloid responsible for tobacco addiction, has long been considered non-carcinogenic. However, emerging evidence suggests that nicotine may possess carcinogenic properties in mice and could be a potential carcinogen in humans. This review aims to summarize the potential molecular mechanisms underlying nicotine-induced carcinogenesis, with a specific focus on epigenetic regulation and the activation of nicotinic acetylcholine receptors (nAChRs) in addition to genotoxicity and excess reactive oxygen species (ROS). Additionally, we explore a novel hypothesis regarding nicotine's carcinogenicity involving the downregulation of stem-loop binding protein (SLBP), a critical regulator of canonical histone mRNA, and the polyadenylation of canonical histone mRNA. By shedding light on these mechanisms, this review underscores the need for further research to elucidate the carcinogenic potential of nicotine and its implications for human health.

[Harmful health effects of flavors in e-cigarettes]

BACKGROUND

 Almost all e-cigarettes contain flavorings that make the product more attractive. In the evaluation of e-cigarettes on health, flavors have so far played a subordinate role.

METHOD

 Selective literature search in PubMed, supplemented by legal regulations on the use of flavors in e-cigarettes.

RESULTS

 Flavors make it easier to start using e-cigarettes and have a consumption-promoting effect. Deeper inhalation increases nicotine uptake and the absorption of toxic substances from the e-cigarette liquid. For some flavors, pathological effects have been demonstrated in addition to other toxic components of the e-cigarette. To date, no toxicological analyses are available for the vast majority of flavors contained in e-cigarettes.

CONCLUSIONS

 The proven consumption-promoting effect and the health risks that can be extrapolated from preclinical data are significant for the political discussion of a ban on flavors for e-cigarettes, analogous to the ban on flavors in tobacco products already in force.

Toxicological Aspects Associated with Consumption from Electronic Nicotine Delivery System (ENDS): Focus on Heavy Metals Exposure and Cancer Risk

Tobacco smoking remains one of the leading causes of premature death worldwide. Electronic Nicotine Delivery Systems (ENDSs) are proposed as a tool for smoking cessation. In the last few years, a growing number of different types of ENDSs were launched onto the market. Despite the manufacturing differences, ENDSs can be classified as "liquid e-cigarettes" (e-cigs) equipped with an atomizer that vaporizes a liquid composed of vegetable glycerin (VG), polypropylene glycol (PG), and nicotine, with the possible addition of flavorings; otherwise, the "heated tobacco products" (HTPs) heat tobacco sticks through contact with an electronic heating metal element. The presence of some metals in the heating systems, as well as in solder joints, involves the possibility that heavy metal ions can move from these components to the liquid, or they can be adsorbed into the tobacco stick from the heating blade in the case of HTPs. Recent evidence has indicated the presence of heavy metals in the refill liquids and in the mainstream such as arsenic (As), cadmium (Cd), chromium (Cr), nickel (Ni), copper (Cu), and lead (Pb). The present review discusses the toxicological aspects associated with the exposition of heavy metals by consumption from ENDSs, focusing on metal carcinogenesis risk.

Zebrafish (Danio rerio) as a Model for the Study of Developmental and Cardiovascular Toxicity of Electronic Cigarettes

The increasing popularity of electronic cigarettes (e-cigarettes) as an alternative to conventional tobacco products has raised concerns regarding their potential adverse effects. The cardiovascular system undergoes intricate processes forming the heart and blood vessels during fetal development. However, the precise impact of e-cigarette smoke and aerosols on these delicate developmental processes remains elusive. Previous studies have revealed changes in gene expression patterns, disruptions in cellular signaling pathways, and increased oxidative stress resulting from e-cigarette exposure. These findings indicate the potential for e-cigarettes to cause developmental and cardiovascular harm. This comprehensive review article discusses various aspects of electronic cigarette use, emphasizing the relevance of cardiovascular studies in Zebrafish for understanding the risks to human health. It also highlights novel experimental approaches and technologies while addressing their inherent challenges and limitations.

Emerging ENDS products and challenges in tobacco control toxicity research

Electronic nicotine delivery systems (ENDS) continue to rapidly evolve. Current products pose unique challenges and opportunities for researchers and regulators. This commentary aims to highlight research gaps, particularly in toxicity research, and provide guidance on priority research questions for the tobacco regulatory community. Disposable flavoured ENDS have become the most popular device class among youth and may contain higher nicotine levels than JUUL devices. They also exhibit enhanced harmful and potentially harmful constituents production, contain elevated levels of synthetic coolants and pose environmental concerns. Synthetic nicotine and flavour capsules are innovations that have recently enabled the circumvention of Food and Drug Administration oversight. Coil-less ENDS offer the promise of delivering fewer toxicants due to the absence of heating coils, but initial studies show that these products exhibit similar toxicological profiles compared with JUULs. Each of these topic areas requires further research to understand and mitigate their impact on human health, especially their risks to young users.

Nicotine, THC, and Dolutegravir Modulate E-Cigarette-Induced Changes in Addiction- and Inflammation-Associated Genes in Rat Brains and Astrocytes

E-cigarette use has been marketed as a safer alternative to traditional cigarettes, as a means of smoking cessation, and are used at a higher rate than the general population in people with HIV (PWH). Early growth receptor 2 (EGR2) and Activity-Regulated Cytoskeleton-Associated Protein (ARC) have a role in addiction, synaptic plasticity, inflammation, and neurodegeneration. This study showed that 10 days of exposure to e-cigarette vapor altered gene expression in the brains of 6-month-old, male, Sprague Dawley rats. Specifically, the e-cigarette solvent vapor propylene glycol (PG) downregulated EGR2 and ARC mRNA expression in frontal cortex, an effect which was reversed by nicotine (NIC) and THC, suggesting that PG could have a protective role against NIC and cannabis dependence. However, in vitro, PG upregulated EGR2 and ARC mRNA expression at 18 h in cultured C6 rat astrocytes suggesting that PG may have neuroinflammatory effects. PG-induced upregulation of EGR2 and ARC mRNA was reversed by NIC but not THC. The HIV antiretroviral DTG reversed the effect NIC had on decreasing PG-induced upregulation of EGR2, which is concerning because EGR2 has been implicated in HIV latency reversal, T-cell apoptosis, and neuroinflammation, a process that underlies the development of HIV-associated neurocognitive disorders.

Chemical analysis of selected harmful and potentially harmful constituents and in vitro toxicological evaluation of leading flavoured e-cigarette aerosols in the Chinese market

Use of electronic cigarettes (e-cigarettes) has increased significantly over the past decade due to consumer perception that these products represent a less risky alternative to combustible cigarettes. E-liquids generally contain a simple mix of vegetable glycerin, propylene glycerol, nicotine, organic acids, and flavourings. Regulators require that harmful and potentially harmful constituents (HPHCs) that might cause harm to the consumer must be monitored in the aerosol generated by e-cigarettes and in cigarette smoke (CS). To quantify HPHCs in aerosols from commercial flavoured e-cigarettes in Chinese market, this study has systematically compared levels of HPHCs, including eight carbonyls, five volatile organic compounds, four tobacco-specific nitrosamines, 16 polycyclic aromatic hydrocarbons, and seven heavy metals, in the aerosols of four market-leading flavoured e-cigarettes and mainstream CS, alongside in vitro cytotoxicity and mutagenicity assays. The vast majority of HPHCs were either undetected or significantly lower in the e-cigarette aerosols than in commercial CS or reference CS (3R4F). Where HPHCs were detected, there were small variations among the different flavoured e-cigarettes. In the neutral red uptake and Ames assays, aqueous extracts of the e-cigarette aerosols did not induce obvious cytotoxicity or mutagenicity, whereas CS aqueous extract showed dose-related cytotoxicity and mutagenicity. Collectively, these results indicate that use of e-cigarettes might potentially lead to a significant reduction in exposure to harmful substances, with fewer cytotoxic and mutagenic effects, as compared with conventional smoking. Further studies based on human puffing conditions and longer evaluation periods will be needed to substantiate this potential.

Not all vaping is the same: differential pulmonary effects of vaping cannabidiol versus nicotine

RATIONALE

Vaping has become a popular method of inhaling various psychoactive substances. While evaluating respiratory effects of vaping have primarily focused on nicotine-containing products, cannabidiol (CBD)-vaping is increasingly becoming popular. It currently remains unknown whether the health effects of vaping nicotine and cannabinoids are similar.

OBJECTIVES

This study compares side by side the pulmonary effects of acute inhalation of vaporised CBD versus nicotine.

METHODS

In vivo inhalation study in mice and in vitro cytotoxicity experiments with human cells were performed to assess the pulmonary damage-inducing effects of CBD or nicotine aerosols emitted from vaping devices.

MEASUREMENTS AND MAIN RESULTS

Pulmonary inflammation in mice was scored by histology, flow cytometry, and quantifying levels of proinflammatory cytokines and chemokines. Lung damage was assessed by histology, measurement of myeloperoxidase activity and neutrophil elastase levels in the bronchoalveolar lavage fluid and lung tissue. Lung epithelial/endothelial integrity was assessed by quantifying BAL protein levels, albumin leak and pulmonary FITC-dextran leak. Oxidative stress was determined by measuring the antioxidant potential in the BAL and lungs. The cytotoxic effects of CBD and nicotine aerosols on human neutrophils and human small airway epithelial cells were evaluated using in vitro air-liquid interface system. Inhalation of CBD aerosol resulted in greater inflammatory changes, more severe lung damage and higher oxidative stress compared with nicotine. CBD aerosol also showed higher toxicity to human cells compared with nicotine.

CONCLUSIONS

Vaping of CBD induces a potent inflammatory response and leads to more pathological changes associated with lung injury than vaping of nicotine.

Comparison of the toxicological potential of two JUUL ENDS products to reference cigarette 3R4F and filtered air in a 90-day nose-only inhalation toxicity study

Electronic nicotine delivery systems (ENDS) are generally recognized as less harmful alternatives for those who would otherwise continue to smoke cigarettes. The potential toxicity of aerosols generated from JUUL Device and Virginia Tobacco (VT3) or Menthol (ME3) JUULpods at 3.0% nicotine concentration was assessed in rats exposed at target aerosol concentrations of 1400 μg/L for up to 6 h/day on a 5 day/week basis for at least 90 days (general accordance with OECD 413). 3R4F reference cigarette smoke (250 μg/L) and Filtered Air were used as comparators. JUUL ENDS product aerosol exposures at >5x the 3R4F cigarette smoke level resulted in greater plasma nicotine and cotinine levels (up to 2x). Notable cigarette smoke related effects included pronounced body weight reductions in male rats, pulmonary inflammation evidenced by elevated lactate dehydrogenase, pro-inflammatory cytokines and neutrophils in bronchoalveolar lavage fluid, increased heart and lung weights, and minimal to marked respiratory tract histopathology. In contrast, ENDS aerosol exposed animals had minimal body weight changes, no measurable inflammatory changes and minimal to mild laryngeal squamous metaplasia. Despite the higher exposure levels, VT3 and ME3 did not result in significant toxicity or appreciable respiratory histopathology relative to 3R4F cigarette smoke following 90 days administration.

The controversial effect of smoking and nicotine in SARS-CoV-2 infection

The effects of nicotine and cigarette smoke in many diseases, notably COVID-19 infection, are being debated more frequently. The current basic data for COVID-19 is increasing and indicating the higher risk of COVID-19 infections in smokers due to the overexpression of corresponding host receptors to viral entry. However, current multi-national epidemiological reports indicate a lower incidence of COVID-19 disease in smokers. Current data indicates that smokers are more susceptible to some diseases and more protective of some other. Interestingly, nicotine is also reported to play a dual role, being both inflammatory and anti-inflammatory. In the present study, we tried to investigate the effect of pure nicotine on various cells involved in COVID-19 infection. We followed an organ-based systematic approach to decipher the effect of nicotine in damaged organs corresponding to COVID-19 pathogenesis (12 related diseases). Considering that the effects of nicotine and cigarette smoke are different from each other, it is necessary to be careful in generalizing the effects of nicotine and cigarette to each other in the conducted researches. The generalization and the undifferentiation of nicotine from smoke is a significant bias. Moreover, different doses of nicotine stimulate different effects (dose-dependent response). In addition to further assessing the role of nicotine in COVID-19 infection and any other cases, a clever assessment of underlying diseases should also be considered to achieve a guideline for health providers and a personalized approach to treatment.

Flavored E-cigarette product aerosols induce transformation of human bronchial epithelial cells

OBJECTIVES

E-cigarettes are the most commonly used nicotine containing products among youth. In vitro studies support the potential for e-cigarettes to cause cellular stress in vivo; however, there have been no studies to address whether exposure to e-liquid aerosols can induce cell transformation, a process strongly associated with pre-malignancy. We examined whether weekly exposure of human bronchial epithelial cell (HBEC) lines to e-cigarette aerosols would induce transformation and concomitant changes in gene expression and promoter hypermethylation.

MATERIALS AND METHODS

An aerosol delivery system exposed three HBEC lines to unflavored e-liquid with 1.2% nicotine, 3 flavored products with nicotine, or the Kentucky reference cigarette once a week for 12 weeks. Colony formation in soft agar, RNA-sequencing, and the EPIC Beadchip were used to evaluate transformation, genome-wide expression and methylation changes.

RESULTS

Jamestown e-liquid aerosol induced transformation of HBEC2 and HBEC26, while unflavored and Blue Pucker transformed HBEC26. Cigarette smoke aerosol transformed HBEC4 and HBEC26 at efficiencies up to 3-fold greater than e-liquids. Transformed clones exhibited extensive reprogramming of the transcriptome with common and distinct gene expression changes observed between the cigarette and e-liquids. Transformation by e-liquids induced alterations in canonical pathways implicated in lung cancer that included axonal guidance and NRF2. Gene methylation, while prominent in cigarette-induced transformed clones, also affected hundreds of genes in HBEC2 transformed by Jamestown. Many genes with altered expression or epigenetic-mediated silencing were also affected in lung tumors from smokers.

CONCLUSIONS

These studies show that exposure to e-liquid aerosols can induce a pre-malignant phenotype in lung epithelial cells. While the Food and Drug Administration banned the sale of flavored cartridge-based electric cigarettes, consumers switched to using flavored products through other devices. Our findings clearly support expanding studies to evaluate transformation potency for the major categories of e-liquid flavors to better inform risk from these complex mixtures.

Electronic Cigarette Use during Pregnancy: Is It Harmful?

Although combustible cigarette smoking rates have declined in recent years, alternative tobacco product use, particularly electronic cigarette use ("vaping"), has increased among young adults. Recent studies indicate that vaping during pregnancy is on the rise, possibly due to the perception that it is a safer alternative to combustible cigarette smoking. However, e-cigarette aerosols may contain several newer, potentially toxic compounds, including some known developmental toxicants that may adversely impact both the mother and the fetus. However, there is paucity of studies that have examined the effects of vaping during pregnancy. While the adverse perinatal outcomes of cigarette smoking during pregnancy are well established, the specific risks associated with inhaling vaping aerosols during pregnancy requires more research. In this article, we discuss the existing evidence and knowledge gaps on the risks of vaping during pregnancy. Studies that investigate vaping-associated systemic exposure and its effects (i.e., biomarker analyses) and maternal and neonatal clinical health outcomes are needed to reach more robust conclusions. We particularly emphasize the need to go beyond comparative studies with cigarettes, and advocate for research that objectively evaluates the safety of e-cigarettes and other alternative tobacco products.

Aerosolized nicotine from e-cigarettes alters gene expression, increases lung protein permeability, and impairs viral clearance in murine influenza infection

E-cigarette use has rapidly increased as an alternative means of nicotine delivery by heated aerosolization. Recent studies demonstrate nicotine-containing e-cigarette aerosols can have immunosuppressive and pro-inflammatory effects, but it remains unclear how e-cigarettes and the constituents of e-liquids may impact acute lung injury and the development of acute respiratory distress syndrome caused by viral pneumonia. Therefore, in these studies, mice were exposed one hour per day over nine consecutive days to aerosol generated by the clinically-relevant tank-style Aspire Nautilus aerosolizing e-liquid containing a mixture of vegetable glycerin and propylene glycol (VG/PG) with or without nicotine. Exposure to the nicotine-containing aerosol resulted in clinically-relevant levels of plasma cotinine, a nicotine-derived metabolite, and an increase in the pro-inflammatory cytokines IL-17A, CXCL1, and MCP-1 in the distal airspaces. Following the e-cigarette exposure, mice were intranasally inoculated with influenza A virus (H1N1 PR8 strain). Exposure to aerosols generated from VG/PG with and without nicotine caused greater influenza-induced production in the distal airspaces of the pro-inflammatory cytokines IFN-γ, TNFα, IL-1β, IL-6, IL-17A, and MCP-1 at 7 days post inoculation (dpi). Compared to the aerosolized carrier VG/PG, in mice exposed to aerosolized nicotine there was a significantly lower amount of Mucin 5 subtype AC (MUC5AC) in the distal airspaces and significantly higher lung permeability to protein and viral load in lungs at 7 dpi with influenza. Additionally, nicotine caused relative downregulation of genes associated with ciliary function and fluid clearance and an increased expression of pro-inflammatory pathways at 7 dpi. These results show that (1) the e-liquid carrier VG/PG increases the pro-inflammatory immune responses to viral pneumonia and that (2) nicotine in an e-cigarette aerosol alters the transcriptomic response to pathogens, blunts host defense mechanisms, increases lung barrier permeability, and reduces viral clearance during influenza infection. In conclusion, acute exposure to aerosolized nicotine can impair clearance of viral infection and exacerbate lung injury, findings that have implications for the regulation of e-cigarette products.

Comparative study of lung toxicity of E-cigarette ingredients to investigate E-cigarette or vaping product associated lung injury

No comparative study has yet been performed on the respiratory effects of individual E-cigarette ingredients. Here, lung toxicity of individual ingredients of E-cigarette products containing nicotine or tetrahydrocannabinol was investigated. Mice were intratracheally administered propylene glycol (PG), vegetable glycerin (VG), vitamin E acetate (VEA), or nicotine individually for two weeks. Cytological and histological changes were noticed in PG- and VEA-treated mice that exhibited pathophysiological changes which were associated with symptoms seen in patients with symptoms of E-cigarette or Vaping Use-Associated Lung Injuries (EVALI) or E-cigarette users. Compared to potential human exposure situations, while the VEA exposure condition was similar to the dose equivalent of VEA content in E-cigarettes, the PG condition was about 47-137 times higher than the dose equivalent of the daily PG intake of E-cigarette users. These results reveal that VEA exposure is much more likely to cause problems related to EVALI in humans than PG. Transcriptomic analysis revealed that PG exposure was associated with fibrotic lung injury via the AKT signaling pathway and M2 macrophage polarization, and VEA exposure was associated with asthmatic airway inflammation via the mitogen-activated protein kinase signaling pathway. This study provides novel insights into the pathophysiological effects of individual ingredients of E-cigarettes.

Influence of puff topographies on e-liquid heating temperature, emission characteristics and modeled lung deposition of Puff Bar™

Puff Bar™, one of the latest designs of e-cigarettes, heats a mixture of liquid using a battery-powered coil at certain temperatures to emit aerosol. This study presents a mass-based characterization of emissions from seven flavors of Puff Bar™ devices by aerosolizing with three puff topographies [(puff volume: 55 < 65 < 75-mL) within 4-seconds at 30-seconds interval]. We evaluated the effects of puff topographies on heating temperatures; characterized particles using a cascade impactor; and measured volatile carbonyl compounds (VCCs). Modeled dosimetry and calculated mass median aerodynamic diameters (MMADs) were used to estimate regional, total respiratory deposition of the inhaled aerosol and exhaled fractions that could pose secondhand exposure risk. Temperatures of Puff Bar™ e-liquids increased with increasing puff volumes: 55mL (116.6 °C), 65 mL (128.3 °C), and 75mL (168.9 °C). Flavor types significantly influenced MMADs, total mass of particles, and VCCs (μg/puff: 2.15-2.30) in Puff Bar™ emissions (p < 0.05). Increasing puff volume (mL:55 < 65 < 75) significantly increased total mass (mg/puff: 4.6 < 5.6 < 6.2) of particles without substantially changing MMADs (~1μm:1.02~0.99~0.98). Aerosol emissions were estimated to deposit in the pulmonary region of e-cigarette user (41-44%), which could have toxicological importance. More than 2/3 (67-77%) of inhaled particles were estimated to be exhaled by users, which could affect bystanders. The VCCs measured contained carcinogens-formaldehyde (29.6%) and acetaldehyde (16.4%)-as well as respiratory irritants: acetone (23.9%), isovaleraldehyde (14.5%), and acrolein (4.9%). As Puff Bar™ emissions contain respirable particles and harmful chemicals, efforts should be made to minimize exposures, especially in indoor settings where people (including vulnerable populations) spend most of their life-time.

Toxicological Assessment of Particulate and Metal Hazards Associated with Vaping Frequency and Device Age

Electronic nicotine delivery systems (ENDS) aerosols are complex mixtures of chemicals, metals, and particles that may present inhalation hazards and adverse respiratory health risks. Despite being considered a safer alternative to tobacco cigarettes, metal exposure levels and respiratory effects associated with device aging and vaping frequency have not been fully characterized. In this study, we utilize an automated multi-channel ENDS aerosol generation system (EAGS) to generate aerosols from JUUL pod-type ENDS using tobacco-flavored e-liquid. Aerosol puff fractions (1-50) and (101-150) are monitored and sampled using various collection media. Extracted aerosols are prepared for metal and toxicological analysis using human primary small airway epithelial cells (SAEC). ENDS aerosol-mediated cellular responses, including reactive oxygen species (ROS), oxidative stress, cell viability, and DNA damage, are evaluated after 24 h and 7-day exposures. Our results show higher particle concentrations in later puff fractions (0.135 mg/m3) than in initial puff fractions (0.00212 mg/m3). Later puff fraction aerosols contain higher toxic metal concentrations, including chromium, copper, and lead, which elicit increased levels of ROS followed by significant declines in total glutathione and cell viability. Notably, a 30% increase in DNA damage was observed after 7 days because of later puff fraction exposures. This work is consistent with ENDS aerosols becoming more hazardous across the use of pre-filled pod devices, which may threaten respiratory health.

Cytotoxicity and cell injuries of flavored electronic cigarette aerosol and mainstream cigarette smoke: A comprehensive in vitro evaluation

INTRODUCTION

Although electronic cigarettes (e-cigarettes) have attracted much attention due to their claimed harm-reduction effects compared with conventional cigarettes, the adverse effects of e-cigarette aerosol exposure on human health are still unclear. In this work we compared the cytotoxic effects of combustion cigarettes with four commercially available flavored electronic cigarettes and their main components on ten cell lines. Cell injury mechanism of e-cigarette aerosol and combustible cigarette smoke was also explored using cellular models.

METHODS

Eleven kinds of e-cigarettes aerosol condensates (ECSCs) and cigarette smoke constituent's condensates (CSC) were collected by Cambridge filter pad, and the nicotine contents were determined by UPLC to provide an equivalent nicotine dosage. The CCK-8 assay was used to measure the cell viability differences between ECSC and CSC. Based on RNA-seq results, we compared the effects of ECSC and CSC on various cell injury pathways. Oxidative stress and inflammatory responses were further tested by Western Blot, immunofluorescence, and qRT-PCR assays.

RESULTS

CSC was found to be more cytotoxic than flavored ECSC and their main components, and BEAS-2B cell line was the most sensitive cells by comparing the IC50 value. With prolonged exposure duration and higher doses, ECSC began to exhibit cytotoxicity at and above 72 µg/mL. The IC50 values of ECSC were 15-fold higher than that of CSC. Transcriptome analyses indicated that cell injury-related processes were enriched after the treatment of CSC. CSC could significantly induce more oxidative stress and inflammatory signals than ECSC.

CONCLUSION

ECSCs and their components induced significantly less cytotoxicity than CSC under the laboratory exposure conditions, and CSC caused much severe cell injuries. Our study adds to the body of scientific evidence for a more comprehensive safety evaluation of e-cigarette products as compared to cigarettes.

Effects of Electronic (E)-cigarette Vapor and Cigarette Smoke in Cultured Vocal Fold Fibroblasts

OBJECTIVE

The public use of electronic-cigarettes (e-cigs) is rapidly growing. When heated, e-cigs produce a vapor that is inhaled. The vocal folds are among the first tissues exposed to this insult. However, the impact of e-cigs on vocal fold health is almost entirely unknown. Our objective was to evaluate the effects of e-cig vapor on cultured human vocal fold fibroblasts (hVFFs), the primary cell type of the lamina propria. We compared the cellular effects of e-cig vapor without and with nicotine and conventional cigarette smoke.

STUDY DESIGN

In vitro.

METHODS

E-cig vapor extract (EVE) and cigarette smoke extract (CSE) were created by bubbling vapor and smoke, respectively, into the cell culture medium. hVFFs were exposed to EVE without or with nicotine or CSE for 24 hours. Untreated cells were used as a control group. Cells were harvested, and cytotoxicity, extracellular matrix and inflammatory gene expression, and DNA damage were assessed.

RESULTS

Undiluted EVE without and with nicotine reduced the viability of hVFFs to a cytotoxic level. CSE reduced hVFFs viability to a greater extent than EVE and induced DNA damage as measured by DNA double-strand breaks. No changes in gene expression were observed following EVE or CSE exposure.

CONCLUSION

EVE induces cytotoxicity in hVFFs. However, cellular responses were greater following exposure to CSE, suggesting cigarette smoke may induce more harm, at least in the short term. Findings from this investigation improve our understanding of responses of hVFFs to e-cigs and form the basis for an in vitro methodology to study the vocal fold responses to these products.

LEVEL OF EVIDENCE

NA Laryngoscope, 133:139-146, 2023.

In Vitro Biological Effects of E-Cigarette on the Cardiovascular System-Pro-Inflammatory Response Enhanced by the Presence of the Cinnamon Flavor

The potential cardiovascular effects of e-cigarettes remain largely unidentified and poorly understood. E-liquids contain numerous chemical compounds and can induce exposure to potentially toxic ingredients (e.g., nicotine, flavorings, etc.). Moreover, the heating process can also lead to the formation of new thermal decomposition compounds that may be also hazardous. Clinical as well as in vitro and in vivo studies on e-cigarette toxicity have reported potential cardiovascular damages; however, results remain conflicting. The aim of this study was to assess, in vitro, the toxicity of e-liquids and e-cigarette aerosols on human aortic smooth muscle cells. To that purpose, cells were exposed either to e-liquids or to aerosol condensates obtained using an e-cigarette device at different power levels (8 W or 25 W) to assess the impact of the presence of: (i) nicotine, (ii) cinnamon flavor, and (iii) thermal degradation products. We observed that while no cytotoxicity and no ROS production was induced, a pro-inflammatory response was reported. In particular, the production of IL-8 was significantly enhanced at a high power level of the e-cigarette device and in the presence of the cinnamon flavor (confirming the suspected toxic effect of this additive). Further investigations are required, but this study contributes to shedding light on the biological effects of vaping on the cardiovascular system.

E-cigarette aerosol impairs male mouse skeletal muscle force development and prevents recovery from injury

To date, there has been a lag between the rise in E-cigarette use and an understanding of the long-term health effects. Inhalation of E-cigarette aerosol delivers high doses of nicotine, raises systemic cytokine levels, and compromises cardiopulmonary function. The consequences for muscle function have not been thoroughly investigated. The present study tests the hypothesis that exposure to nicotine-containing aerosol impairs locomotor muscle function, limits exercise tolerance, and interferes with muscle repair in male mice. Nicotine-containing aerosol reduced the maximal force produced by the extensor digitorum longus (EDL) by 30%-40% and, the speed achieved in treadmill running by 8%. Nicotine aerosol exposure also decreased adrenal and increased plasma epinephrine and norepinephrine levels, and these changes in catecholamines manifested as increased muscle and liver glycogen stores. In nicotine aerosol exposed mice, muscle regenerating from overuse injury only recovered force to 80% of noninjured levels. However, the structure of neuromuscular junctions (NMJs) was not affected by e-cigarette aerosols. Interestingly, the vehicle used to dissolve nicotine in these vaping devices, polyethylene glycol (PG) and vegetable glycerin (VG), decreased running speed by 11% and prevented full recovery from a lengthening contraction protocol (LCP) injury. In both types of aerosol exposures, cardiac left ventricular systolic function was preserved, but left ventricular myocardial relaxation was altered. These data suggest that E-cigarette use may have a negative impact on muscle force and regeneration due to compromised glucose metabolism and contractile function in male mice.NEW & NOTEWORTHY In male mice, nicotine-containing E-cigarette aerosol compromises muscle contractile function, regeneration from injury, and whole body running speeds. The vehicle used to deliver nicotine, propylene glycol, and vegetable glycerin, also reduces running speed and impairs the restoration of muscle function in injured muscle. However, the predominant effects of nicotine in this inhaled aerosol are evident in altered catecholamine levels, increased glycogen content, decreased running capacity, and impaired recovery of force following an overuse injury.

A Review of Toxicity Mechanism Studies of Electronic Cigarettes on Respiratory System

Electronic cigarettes (e-cigarettes) have attracted much attention as a new substitute for conventional cigarettes. E-cigarettes are first exposed to the respiratory system after inhalation, and studies on the toxicity mechanisms of e-cigarettes have been reported. Current research shows that e-cigarette exposure may have potentially harmful effects on cells, animals, and humans, while the safety evaluation of the long-term effects of e-cigarette use is still unknown. Similar but not identical to conventional cigarettes, the toxicity mechanisms of e-cigarettes are mainly manifested in oxidative stress, inflammatory responses, and DNA damage. This review will summarize the toxicity mechanisms and signal pathways of conventional cigarettes and e-cigarettes concerning the respiratory system, which could give researchers a better understanding and direction on the effects of e-cigarettes on our health.

The Impact of the Storage Conditions and Type of Clearomizers on the Increase of Heavy Metal Levels in Electronic Cigarette Liquids Retailed in Romania

The growing popularity of electronic cigarettes has raised several public health concerns, including the risks associated with heavy metals exposure via e-liquids and vapors. The purpose of this study was to determine, using atomic absorption spectrometry, the concentrations of Pb, Ni, Zn, and Co in some commercially available e-liquid samples from Romania immediately after purchase and after storage in clearomizers. Lead and zinc were found in all investigated samples before storage. The initial concentrations of Pb ranged from 0.13 to 0.26 mg L⁻¹, while Zn concentrations were between 0.04 and 0.07 mg L⁻¹. Traces of nickel appeared in all investigated e-liquids before storage but in very small amounts (0.01–0.02 mg L⁻¹). Co was below the detection limits. We investigated the influence of the storage period (1, 3, and 5 days), storage temperature (22 °C and 40 °C), and type of clearomizer. In most cases, the temperature rise and storage period increase were associated with higher concentrations of heavy metals. This confirms that storage conditions can affect metal transfer and suggests that the temperature of storage is another parameter that can influence this phenomenon.

Comparison of the in vivo genotoxicity of electronic and conventional cigarettes aerosols after subacute, subchronic and chronic exposures

Tobacco smoking is classified as a human carcinogen. A wide variety of new products, in particular electronic cigarettes (e-cigs), have recently appeared on the market as an alternative to smoking. Although the in vitro toxicity of e-cigs is relatively well known, there is currently a lack of data on their long-term health effects. In this context, the aim of our study was to compare, on a mouse model and using a nose-only exposure system, the in vivo genotoxic and mutagenic potential of e-cig aerosols tested at two power settings (18 W and 30 W) and conventional cigarette (3R4F) smoke. The standard comet assay, micronucleus test and Pig-a gene mutation assay were performed after subacute (4 days), subchronic (3 months) and chronic (6 months) exposure. The generation of oxidative stress was also assessed by measuring the 8-hydroxy-2'-deoxyguanosine and by using the hOGG1-modified comet assay. Our results show that only the high-power e-cig and the 3R4F cigarette induced oxidative DNA damage in the lung and the liver of exposed mice. In return, no significant increase in chromosomal aberrations or gene mutations were noted whatever the type of product. This study demonstrates that e-cigs, at high-power setting, should be considered, contrary to popular belief, as hazardous products in terms of genotoxicity in mouse model.

Evaluation of Inhalation Exposures and Potential Health Impacts of Ingredient Mixtures Using in vitro to in vivo Extrapolation

In vitro methods offer opportunities to provide mechanistic insight into bioactivity as well as human-relevant toxicological assessments compared to animal testing. One of the challenges for this task is putting in vitro bioactivity data in an in vivo exposure context, for which in vitro to in vivo extrapolation (IVIVE) translates in vitro bioactivity to clinically relevant exposure metrics using reverse dosimetry. This study applies an IVIVE approach to the toxicity assessment of ingredients and their mixtures in e-cigarette (EC) aerosols as a case study. Reported in vitro cytotoxicity data of EC aerosols, as well as in vitro high-throughput screening (HTS) data for individual ingredients in EC liquids (e-liquids) are used. Open-source physiologically based pharmacokinetic (PBPK) models are used to calculate the plasma concentrations of individual ingredients, followed by reverse dosimetry to estimate the human equivalent administered doses (EADs) needed to obtain these plasma concentrations for the total e-liquids. Three approaches (single actor approach, additive effect approach, and outcome-oriented ingredient integration approach) are used to predict EADs of e-liquids considering differential contributions to the bioactivity from the ingredients (humectant carriers [propylene glycol and glycerol], flavors, benzoic acid, and nicotine). The results identified critical factors for the EAD estimation, including the ingredients of the mixture considered to be bioactive, in vitro assay selection, and the data integration approach for mixtures. Further, we introduced the outcome-oriented ingredient integration approach to consider e-liquid ingredients that may lead to a common toxicity outcome (e.g., cytotoxicity), facilitating a quantitative evaluation of in vitro toxicity data in support of human risk assessment.

From Tobacco Cigarettes to Electronic Cigarettes: The Two Sides of a Nicotine Coin

Tobacco smoking-related diseases, including cardiovascular disease, pulmonary disease, stroke, and cancer in multiple organ sites, are the leading causes of preventable death, worldwide. Youth electronic cigarette use (vaping) is an evolving public health problem in the United States and around the world. Many of the same toxicants and carcinogens present in tobacco smoke are also found in electronic cigarette vapor, although mostly at substantially lower levels. The reduced concentrations of these chemicals in electronic cigarette vapor may imply lower health risk; however, they cannot equate to no risk. To date, the long-term health consequences of vaping are largely unknown. This “Perspective” provides a concise chronology of events leading up to an unprecedented global challenge, namely the convergence of global tobacco epidemic and youth vaping epidemic. Current state of knowledge, outstanding questions in the field, present challenges, and future directions in research are highlighted. The existing data show a continued and dynamic evolution of the converged epidemics. The goal should be to prevent youth vaping while improving smoking cessation strategies. In smokers who are unable or unwilling to quit smoking, the objective should be to provide “provably” safe or less-harmful alternatives, which should “completely” or “substantially” substitute tobacco cigarettes.

A novel role for vaping in mitochondrial gene dysregulation and inflammation fundamental to disease development

We constructed and analyzed the whole transcriptome in leukocytes of healthy adult vapers (with/without a history of smoking), ‘exclusive’ cigarette smokers, and controls (non-users of any tobacco products). Furthermore, we performed single-gene validation of expression data, and biochemical validation of vaping/smoking status by plasma cotinine measurement. Computational modeling, combining primary analysis (age- and sex-adjusted limmaVoom) and sensitivity analysis (cumulative e-liquid- and pack-year modeling), revealed that ‘current’ vaping, but not ‘past’ smoking, is significantly associated with gene dysregulation in vapers. Comparative analysis of the gene networks and canonical pathways dysregulated in vapers and smokers showed strikingly similar patterns in the two groups, although the extent of transcriptomic changes was more pronounced in smokers than vapers. Of significance is the preferential targeting of mitochondrial genes in both vapers and smokers, concurrent with impaired functional networks, which drive mitochondrial DNA-related disorders. Equally significant is the dysregulation of immune response genes in vapers and smokers, modulated by upstream cytokines, including members of the interleukin and interferon family, which play a crucial role in inflammation. Our findings accord with the growing evidence on the central role of mitochondria as signaling organelles involved in immunity and inflammatory response, which are fundamental to disease development.

E-Cigarette Use: Device Market, Study Design, and Emerging Evidence of Biological Consequences

Electronic cigarettes are frequently viewed as a safer alternative to conventional cigarettes; however, evidence to support this perspective has not materialized. Indeed, the current literature reports that electronic cigarette use is associated with both acute lung injury and subclinical dysfunction to the lung and vasculature that may result in pathology following chronic use. E-cigarettes can alter vascular dynamics, polarize innate immune populations towards a proinflammatory state, compromise barrier function in the pulmonary endothelium and epithelium, and promote pre-oncogenic phenomena. This review will summarize the variety of e-cigarette products available to users, discuss current challenges in e-cigarette study design, outline the range of pathologies occurring in cases of e-cigarette associated acute lung injury, highlight disease supporting tissue- and cellular-level changes resulting from e-cigarette exposure, and briefly examine how these changes may promote tumorigenesis. Continued research of the mechanisms by which e-cigarettes induce pathology benefit users and clinicians by resulting in increased regulation of vaping devices, informing treatments for emerging diseases e-cigarettes produce, and increasing public awareness to reduce e-cigarette use and the onset of preventable disease.

Effects of E-Cigarette Refill Liquid Flavorings with and without Nicotine on Human Retinal Pigment Epithelial Cells: A Preliminary Study

Smoking is an etiologic factor for age-related macular degeneration (AMD). Although cigarette smoke has been extensively researched for retinal pigment epithelial (RPE) cell degeneration, the potential for adverse effects on the retinal epithelium following exposure to flavored e-cigarette refill liquid has never been explored. In this preliminary study, we have examined the effects of 20 e-liquids (10 different flavored nicotine-free and 10 nicotine-rich e-liquids) used in e-cigarettes on the metabolic activity, membrane integrity, and mitochondrial membrane potential of RPE cells. Our results showed that of the flavors studied over the concentration range: 0.5, 1, and 2% v/v for a duration of 48 h, cinnamon was the most toxic and menthol was the second most toxic, while other flavors showed lesser or no cytotoxicity. The presence of nicotine augmented cytotoxicity for cinnamon, menthol, strawberry, vanilla, and banana while for other flavors there was no synergism. Together, our results demonstrate that exposure of RPE to flavored e-cigarette refill liquids caused significant cytotoxicity and may be a risk factor for the development of retinal pathogenesis, although further in-depth studies are necessary.

A robotic system for real-time analysis of inhaled submicron and microparticles

Vitamin E acetate (VEA) has been strongly linked to outbreak of electronic cigarette (EC) or vaping product use-associated lung injury. How VEA leads to such an unexpected morbidity and mortality is currently unknown. To understand whether VEA impacts the disposition profile of inhaled particles, we created a biologically inspired robotic system that quantitatively analyzes submicron and microparticles generated from ECs in real-time while mimicking clinically relevant breathing and vaping topography exactly as happens in humans. We observed addition of even small quantities of VEA was sufficient to alter size distribution and significantly enhance total particles inhaled from ECs. Moreover, we demonstrated utility of our biomimetic robot for studying influence of nicotine and breathing profiles from obstructive and restrictive lung disorders. We anticipate our system will serve as a novel preclinical scientific research, decision-support tool when insight into toxicological impact of modifications in electronic nicotine delivery systems is desired.

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Vitamin E acetate (VEA) has been strongly linked to outbreak of EVALI

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A bio-inspired robot was created for real-time analysis of inhaled particles from ENDS

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VEA in e-liquid, even at small doses, was sufficient to enhance total inhaled particles

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This robotic system enables preclinical toxicity evaluation of ENDS and tobacco products

ECAM: A low-cost vaping device for generating and collecting electronic cigarette condensate for in vitro studies

The use of electronic cigarettes (ECs) has become widespread despite many unknowns around their long-term health impact. ECs work by vapourising a liquid, known as an e-liquid, typically consisting of propylene glycol, glycerol, flavourings and nicotine. The chemical constituents and resultant impact on cells and tissue are dependent on several factors, including the flavourings used, the vaping topography/use pattern, and the device used. ECAM (Electronic Cigarette Aerosol Machine) is an open source, portable device for creating EC aerosol - for condensate collection and in vitro studies - using a controlled methodology. ECAM was developed as a low cost, automated, and customisable alternative to commercial devices. ECAM consists of a micro diaphragm gas pump to draw air/EC aerosol through the system. The device is automated using an Arduino and solenoid pinch valves are used to alternate between air and EC vapour. Condensate is collected in a vial within a cold-water bath. Each ECAM unit uses a temperature/humidity sensor to measure ambient air conditions and a differential pressure sensor to determine the pressure within the system. ECAM is programmed to adhere to International Standards Organisation 20768:2018. The design files, source code, and build instructions for this device can be found at https://dx.doi.org/10.17605/OSF.IO/3NGU4.

Cytotoxicity and Genotoxicity of E-Cigarette Generated Aerosols Containing Diverse Flavoring Products and Nicotine in Oral Epithelial Cell Lines

Electronic cigarettes are the most commonly used nicotine containing product among teenagers. The oral epithelium is the first site of exposure and our recent work revealed considerable diversity among e-liquids for composition and level of chemical constituents that impact nicotine deposition in a human oral-trachea cast and affect the formation of reactive carbonyls. Here, we evaluate the dose response for cytotoxicity and genotoxicity of e-cigarette-generated aerosols from 10 diverse flavored e-liquid products with and without nicotine compared with unflavored in 3 immortalized oral epithelial cell lines. Three e-liquids, Blue Pucker, Love Potion, and Jamestown caused ≥20% cell toxicity assessed by the neutral red uptake assay. Nine products induced significant levels of oxidative stress up to 2.4-fold quantified by the ROS-Glo assay in at least 1 cell line, with dose response seen for Love Potion with and without nicotine across all cell lines. Lipid peroxidation detected by the thiobarbituric acid reactive substances assay was less common among products; however, dose response increases up to 12-fold were seen for individual cell lines. Micronuclei formation indicative of genotoxicity was increased up to 5-fold for some products. Blue Pucker was the most genotoxic e-liquid, inducing micronuclei across all cell lines irrespective of nicotine status. A potency score derived from all assays identified Blue Pucker and Love Potion as the most hazardous e-liquids. These in vitro acute exposure studies provide new insight about the potential for some flavored vaping products to induce significant levels of oxidative stress and genotoxicity.

Smoking and the patient with a complex lower limb injury

Smoking is known to increase the risk of peri-operative complications in Orthoplastic surgery by impairing bone and wound healing. The effects of nicotine replacement therapies (NRTs) and electronic cigarettes (e-cigarettes) has been less well established. Previous reviews have examined the relationship between smoking and bone and wound healing separately. This review provides surgeons with a comprehensive and contemporaneous account of how smoking in all forms interacts with all aspects of complex lower limb trauma. We provide a guide for surgeons to refer to during the consent process to enable them to tailor information towards smokers in such a way that the patient may understand the risks involved with their surgical treatment. We update the literature with recently discovered methods of monitoring and treating the troublesome complications that occur more commonly in smokers effected by trauma.

Long-term electronic cigarette exposure induces cardiovascular dysfunction similar to tobacco cigarettes: role of nicotine and exposure duration

Electronic cigarette (e-cig) vaping (ECV) has been proposed as a safer alternative to tobacco cigarette smoking (TCS); however, this remains controversial due to a lack of long-term comparative studies. Therefore, we developed a chronic mouse exposure model that mimics human vaping and allows comparison with TCS. Longitudinal studies were performed to evaluate alterations in cardiovascular function with TCS and ECV exposure durations of up to 60 wk. For ECV, e-cig liquid with box-mod were used and for TCS, 3R4F-cigarettes. C57/BL6 male mice were exposed 2 h/day, 5 days/wk to TCS, ECV, or air control. The role of vape nicotine levels was evaluated using e-cig-liquids with 0, 6, or 24 mg/mL nicotine. Following 16-wk exposure, increased constriction to phenylephrine and impaired endothelium-dependent and endothelium-independent vasodilation were observed in aortic segents, paralleling the onset of systemic hypertension, with elevations in systemic vascular resistance. Following 32 wk, TCS and ECV induced cardiac hypertrophy. All of these abnormalities further increased out to 60 wk of exposure, with elevated heart weight and aortic thickness along with increased superoxide production in vessels and cardiac tissues of both ECV and TCS mice. While ECV-induced abnormalities were seen in the absence of nicotine, these occurred earlier and were more severe with higher nicotine exposure. Thus, long-term vaping of e-cig can induce cardiovascular disease similar to TCS, and the severity of this toxicity increases with exposure duration and vape nicotine content.NEW & NOTEWORTHY A chronic mouse exposure model that mimics human e-cigarette vaping and allows comparison with tobacco cigarette smoking was developed and utilized to perform longitudinal studies of alterations in cardiovascular function. E-cigarette exposure led to the onset of cardiovascular disease similar to that with tobacco cigarette smoking. Impaired endothelium-dependent and endothelium-independent vasodilation with increased adrenergic vasoconstriction were observed, paralleling the onset of systemic hypertension and subsequent cardiac hypertrophy. This cardiovascular toxicity was dependent on exposure duration and nicotine dose.

Comparison of the chemical composition of aerosols from heated tobacco products, electronic cigarettes and tobacco cigarettes and their toxic impacts on the human bronchial epithelial BEAS-2B cells

The electronic cigarettes (e-cigs) and more recently the heated tobacco products (HTP) provide alternatives for smokers as they are generally perceived to be less harmful than conventional cigarettes. However, it is crucial to compare the health risks of these different emergent devices, in order to determine which product should be preferred to substitute cigarette. The present study aimed to compare the composition of emissions from HTP, e-cigs and conventional cigarettes, regarding selected harmful or potentially harmful compounds, and their toxic impacts on the human bronchial epithelial BEAS-2B cells. The HTP emitted less polycyclic aromatic hydrocarbons and carbonyls than the conventional cigarette. However, amounts of these compounds in HTP aerosols were still higher than in e-cig vapours. Concordantly, HTP aerosol showed reduced cytotoxicity compared to cigarette smoke but higher than e-cig vapours. HTP and e-cig had the potential to increase oxidative stress and inflammatory response, in a manner similar to that of cigarette smoke, but after more intensive exposures. In addition, increasing e-cig power impacted levels of certain toxic compounds and related oxidative stress. This study provides important data necessary for risk assessment by demonstrating that HTP might be less harmful than tobacco cigarette but considerably more harmful than e-cig.

Whole body electronic cigarette exposure system for efficient evaluation of diverse inhalation conditions and products

Objectives: To develop and test a new system for whole body exposure of small animals to support investigation of the biological effects of aerosol generated by electronic cigarette (e-cig) products under diverse inhalation conditions with improved control and monitoring of the e-cig vape exposure and nicotine delivered to the animal's systemic circulation. Methods: A computer-controlled design, with built-in sensors for real time monitoring of O₂, CO₂, relative humidity, and temperature within the exposure chambers and port for measuring total particulate matter (TPM) was developed, constructed and tested. This design accommodates a variety of commercial vaping devices, offers software flexibility to adjust exposure protocols to mimic different users' puffing patterns, enables variable nicotine delivery to the animal's systemic circulation; minimizes travel time and alterations of aerosol quality or quantity by delivering aerosol directly to the exposure chamber, offers local or remote operation of up to six distinct exposure chambers from a single control unit, and can simultaneously test different exposure conditions or products in diverse animal groups, which reduces inter-run variability, saves time, and increases productivity. Results: The time course pattern of TPM concentration during different phases of the exposure cycle was measured. With increased puffing duration or number of exposure cycles, higher TPM exposure and plasma cotinine levels were observed with plasma cotinine levels in the range reported in light or heavy smokers. Conclusion: Overall, this novel, versatile, and durable exposure system facilitates high-throughput evaluation of the relative safety and potential toxicity of a variety of e-cig devices and liquids.

Addressing the challenges of E-cigarette safety profiling by assessment of pulmonary toxicological response in bronchial and alveolar mucosa models

Limited toxicity data on electronic cigarette (ECIG) impede evidence-based policy recommendations. We compared two popular mixed fruit flavored ECIG-liquids with and without nicotine aerosolized at 40 W (E-smoke) with respect to particle number concentrations, chemical composition, and response on physiologically relevant human bronchial and alveolar lung mucosa models cultured at air–liquid interface. E-smoke was characterized by significantly increased particle number concentrations with increased wattage (25, 40, and 55 W) and nicotine presence. The chemical composition of E-smoke differed across the two tested flavors in terms of cytotoxic compounds including p-benzoquinone, nicotyrine, and flavoring agents (for example vanillin, ethyl vanillin). Significant differences in the expression of markers for pro-inflammation, oxidative stress, tissue injury/repair, alarm anti-protease, anti-microbial defense, epithelial barrier function, and epigenetic modification were observed between the flavors, nicotine content, and/ or lung models (bronchial or alveolar). Our findings indicate that ECIG toxicity is influenced by combination of multiple factors including flavor, nicotine content, vaping regime, and the region of respiratory tree (bronchial or alveolar). Toxic chemicals and flavoring agents detected in high concentrations in the E-smoke of each flavor warrant independent evaluation for their specific role in imparting toxicity. Therefore, multi-disciplinary approaches are warranted for comprehensive safety profiling of ECIG.

How effective are electronic cigarettes for reducing respiratory and cardiovascular risk in smokers? A systematic review

Background

Electronic cigarettes (e-cigarettes) are widely promoted as harm-reduction products for smokers, and smokers commonly perceive them as less harmful than combustible cigarettes. One of the key questions regarding public health consequences of e-cigarettes is the magnitude of harm reduction achievable by smokers who switch from combustible cigarettes to e-cigarettes. We conducted a systematic literature review of epidemiological studies that estimated odds of respiratory and cardiovascular outcomes among former smokers who use e-cigarettes compared to current smokers.

Methods

This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) statement. We searched the PubMed and Embase databases in September 2020 to identify epidemiological studies that compared odds of cardiovascular and respiratory outcomes among former smokers who transitioned to e-cigarettes relative to odds among current smokers not using e-cigarettes (current exclusive smokers). We included studies that provided direct estimates of relevant odds ratios (ORs). We also included studies where indirect estimates of relevant ORs could be calculated based on published results. Two reviewers independently extracted data and conducted quality appraisals.

Results

Six population-based studies with sample sizes ranging from 19,475 to 161,529 respondents met review inclusion criteria, five of which were cross-sectional and one longitudinal. Three studies reported respiratory outcomes and three reported cardiovascular outcomes. ORs of respiratory outcomes (including chronic obstructive pulmonary disease, chronic bronchitis, emphysema, asthma, and wheezing) in former smokers who transitioned to e-cigarettes versus current exclusive smokers were below 1.0, ranging from 0.58 (95%CI 0.36–0.94) to 0.66 (95%CI 0.50–0.87; all p < 0.05). All ORs for cardiovascular outcomes (including stroke, myocardial infarction, and coronary heart disease) did not differ significantly from 1.0.

Conclusion

Though our review included a small number of studies, it provided consistent results. Former smokers who transitioned to e-cigarettes showed ~ 40% lower odds of respiratory outcomes compared to current exclusive smokers. Switching from smoking to e-cigarette does not appear to significantly lower odds of cardiovascular outcomes. Since the utility of cross-sectional studies for causal inference remains limited, both randomized controlled trials and prospective cohort studies are needed to better evaluate contributions of e-cigarettes as harm reduction tools for smokers.

Persistent cognitive and affective alterations at late withdrawal stages after long-term intermittent exposure to tobacco smoke or electronic cigarette vapour: Behavioural changes and their neurochemical correlates

Smoking cessation induces a withdrawal syndrome associated with anxiety, depression, and impaired neurocognitive functions, but much less is known about the withdrawal of e-cigarettes (e-CIG). We investigated in Balb/c mice the behavioural and neurochemical effects of withdrawal for up to 90 days after seven weeks' intermittent exposure to e-CIG vapour or cigarette smoke (CIG). The withdrawal of e-CIG and CIG induced early behavioural alterations such as spatial memory deficits (spatial object recognition task), increased anxiety (elevated plus maze test) and compulsive-like behaviour (marble burying test) that persisted for 60-90 days. Notably, attention-related (virtual object recognition task) and depression-like behaviours (tail suspension and sucrose preference tests) appeared only 15-30 days after withdrawal and persisted for as long as up to 90 days. At hippocampal level, the withdrawal-induced changes in the levels of AMPA receptor GluA1 and GluA2/3 subunits, PSD 95 protein, corticotropin-releasing factor (Crf) and Crf receptor 1 (CrfR1) mRNA were biphasic: AMPA receptor subunit and PSD95 protein levels initially remained unchanged and decreased after 60-90 days, whereas Crf/CrfR1 mRNA levels initially increased and then markedly decreased after 60 days. These late reductions correlated with the behavioural impairments, particularly the appearance of depression-like behaviours. Our findings show that major behavioural and neurochemical alterations persist or even first appear late after the withdrawal of chronic CIG smoke or e-CIG vapour exposure, and underline importance of conducting similar studies of humans, including e-CIG vapers.