Chest
Volume 157, Issue 5, May 2020, Pages 1362-1390
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Thoracic Oncology: CHEST Reviews
The Evolving Landscape of e-Cigarettes: A Systematic Review of Recent Evidence

https://doi.org/10.1016/j.chest.2019.12.042Get rights and content

Smoking continues to be a burden to economies and health-care systems across the world. One proposed solution to the problem has been e-cigarettes; however, because they are a relatively new product in the market, little is known about their potential health impacts. Furthermore, e-cigarettes continue to evolve at a rapid rate, making it necessary to regularly review and summarize available studies. Although e-cigarettes are marketed as a smoking cessation tool by some manufacturers, the reality is that many nonsmokers, including youth, are using them. This review focuses on two major demographic groups (smokers and nonsmokers) and evaluates the most recent data (early 2017 to mid 2019) regarding the potential health effects of e-cigarettes. We assessed peer-reviewed studies on the health impacts of e-cigarettes, with a particular focus on common questions asked by policy makers, clinicians, and scientists: (1) What are the effects of e-cigarettes compared with air/not smoking?; (2) Is there any direct evidence of harm or benefit to humans?; (3) Is there a risk from secondhand exposure?; (4) What are the risks and/or benefits of e-cigarettes compared with tobacco cigarette use?; (5) Are there risks or benefits to specific populations (eg, people with COPD or asthma, pregnant women [and their offspring])?; (6) What are the effects of flavoring chemicals?; (7) What are the effects of including nicotine in e-liquids?; (8) How often is nicotine concentration labeling incorrect?; and (9) What are the risks when e-cigarettes explode?

Section snippets

Materials and Methods

The authors conducted a PubMed search in the title or abstract for “electronic cigarette” OR “e-cigarette” OR “electronic nicotine delivery system” OR “personal vaporiser” OR “personal vaporizer”’ published between February 2017 until end of May 2019 as well as “e-liquid” AND “nicotine content” until end of May 2019 as these were not covered in the NASEM report. This initial series of searches identified 2,687 unique results. Items were removed if they were a duplicate, not in English, or

The Harms of Vaping to Nonsmokers

Numerous studies have investigated the health effects of using e-cigarettes compared with breathing air/not smoking as well the effects of secondhand exposure to people, animals, or cells naive to tobacco smoke (e-Tables 1 and 2; Table 226, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66).

Harms and/or Benefits of Vaping for Smokers

e-Cigarettes are proposed as a harm reduction tool for tobacco smokers wishing to quit. Systematic reviews suggest a lack of clear efficacy of e-cigarettes in smoking cessation.23,24 Although a recent randomized controlled trial reported a higher quit rate with e-cigarettes compared with nicotine replacement in committed quitters, 80% of those in the e-cigarette group were still vaping at 12 months.19 It is therefore vital to determine the potential risks and/or benefits of transitioning to

Harms and/or Benefits of e-Cigarette Use in High-Risk Populations

Populations that are especially vulnerable to the effects of tobacco smoking include people with COPD and asthma, and pregnant women and their offspring. Recent studies on the effects of e-cigarettes in these vulnerable populations were therefore reviewed (e-Table 4).

Health Effects of Flavor Additives in e-Cigarettes

The “generally recognized as safe” classification of flavorings is based on ingestion into the GI system, not heating until vaporization and inhalation into the lungs.139 Fruit, candy/dessert, and tobacco-based flavorings are the most popular among e-cigarette users.140 Additional chemical compounds are generated during the vaporization process, and studies suggest adducts may form over time,141 complicating the issue further. Much of the flavoring research to date has focused on toxicity in a

Effects of Nicotine vs Nicotine-Free e-Cigarettes

Most commercial e-cigarettes/e-liquids include nicotine. The more recent e-cigarettes use nicotine salts to deliver high nicotine levels up to 59 mg/mL. It is therefore important to understand the contribution of nicotine to the health effects of e-cigarettes (e-Table 7). In a large-scale population-based sample, depressive symptoms were associated with e-cigarette use and nicotine concentration.149 Several human studies have shown that e-cigarettes containing nicotine have greater effects than

Incorrect Nicotine Concentration Labeling

Many studies have shown that nicotine concentrations in e-liquids are often considerably different from the concentrations listed on the labels151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163 (e-Table 6). There is no consistent trend in measured concentrations being higher or lower than on the label, yet variation beyond 10% is commonplace.164 These inaccuracies are unsurprising due to lack of quality control in the e-liquid manufacturing industry, which already suffers from the

Harm From Exploding e-Cigarettes

Another concern regarding e-cigarette use is the potential of these devices to spontaneously explode and cause harm to users, with one reported death.165 Currently, the frequency of e-cigarette explosions remains unclear, although many cases have been reported (e-Table 8). These explosions are largely attributed to the overheating of lithium ion batteries in e-cigarettes,166 which could be due to faulty batteries or user modification of batteries. Furthermore, storing the battery in contact

Relation Between e-Cigarette Use and Cancer Risk

Many questions exist about whether e-cigarettes pose a similar, lesser, or greater cancer risk than cigarette smoking. Known carcinogens, formaldehyde, and acrolein have been found in e-cigarette vapor179, 180, 181 at lower levels than cigarette smoke,182,183 but it remains unclear if the levels produced are enough to contribute to cancer development. Decreased levels of carcinogens were observed in e-cigarette users vs smokers in two studies; however, no healthy control subjects were assessed.

Conclusions

The findings in this review established via in vitro, ex vivo, and animal models that e-cigarette exposure/use leads to distinct immunologic alterations that may contribute to an increased susceptibility to infection. Although the presence of nicotine contributes to the detrimental effects of e-cigarettes, recent research has highlighted the potential toxicity of flavor additives. Furthermore, flavor-specific findings highlight the need for human studies to consider whether varied flavor use

Acknowledgments

Financial/nonfinancial disclosures: None declared.

Additional information: The e-Tables can be found in the Supplemental Materials section of the online article.

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    Mr Bozier and Ms Chivers contributed equally to this manuscript.

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