E-Cigarette Use Among Adolescents: An Overview of the Literature and Future Perspectives

Background Electronic cigarettes (e-cigarettes) are rapidly emerging into a new trend among adolescents, signaling a new époque, that of vapers. E-cigarettes are battery-powered nicotine delivery devices that heat a typically flavoring liquid solution into an aerosol mist that users inhale, allowing them to imitate the act of conventional smoking. There are concerns about the impact of e-cigarettes at both individual and public health level. Aim To discuss the characteristics of the most vulnerable, to become e-cigarette users, group of adolescents and to further highlight their behaviors and characteristics. Methods An electronic search in PubMed, EMBASE, and Google Scholar databases was conducted, using combinations of the following keywords: adolescents, teenagers, e-cigarettes, vaping. The search included all types of articles written in English until August 2017. A total of 100 articles were found, and 25 were finally included in the present review. Results Older age, male gender, conventional smokers, peer influence, daily smoking, and heavier smoking are the most common characteristics of adolescent e-cigarette users. Conclusion E-cigarette use is common, especially between certain subgroups in the adolescent population. Since e-cigarette use is increasing and considering that the long term health effects are still under investigation, targeted interventions towards more susceptible individuals may be an effective prevention strategy.


Rationale-Objectives
Tobacco purchase and usage have shifted to alternative products since the introduction of electronic nicotine delivery systems into the market in the mid-2000s, raising concerns due to increased public interest (1,2). Electronic cigarettes (e-cigarettes) are novel battery-operated hand-held devices designed to deliver smokeless doses of nicotine, through a vaporization process. E-cigarettes have been designed to simulate the sensory experience of smoking, although without combustion.
Nowadays, a wide variety of e-cigarette brands is easily accessible in retail and online shops (3). E-cigarette advertising expenditures increased sharply (4), while safety and long-term health effects are still vague based on the present scientific evidence. As a result of the large-scale marketing, e-cigarettes gained widespread pervasiveness among all age groups, including vulnerable adolescents and youths populations (5)(6)(7)(8). Indeed, recent reports from United States showed that 4.3% of middle school students and 11.3% of high-school students reported having used e-cigarettes in 2016 (9). In addition, reports from UK, comprising data from 60,000 young individuals, aged 11-16 years, showed regular e-cigarette use between 1 and 3% and ever-use between 7 and 18% (10). Furthermore, data from 24,658 individuals in the 2012 National Youth Tobacco Survey reported that almost one-third of adolescents in the United States consider e-cigarettes as less harmful than conventional cigarettes (11).
E-cigarette vapor contains many of the known harmful toxins of traditional cigarettes, such as formaldehyde, cadmium, and lead, even though usually at a reduced percentage (12). However, short-and long-term health implications on e-cigarette users remain foggy. E-cigarette marketing is of particular concern, because is creating an illusion that e-cigarettes are safer and healthier than conventional tobacco cigarettes, whereas their safety and their potential role in smoking cessation is still a matter of ongoing debate.
Diverse characteristics influence the vulnerability of adolescents toward e-cigarette usage. These can be intrapersonal, like adolescents' age, interpersonal, like conflict with family and peers, and contextual comprising community structures and district laws (13). Several marketing and design product features seem to be more attractive for young people. For example, flavorings or lack of age regulation restricting laws have been implicated as reasons for youth susceptibility to e-cigarettes (14). An analysis of e-cigarette retail websites, marketing, and promotional campaigns demonstrated frequent appeals to adolescents such as use by celebrities, feature cartoons, and enhanced social activity as well as sexual appeal (15).
It is a common assumption that adolescents have higher rates of impulsivity, and therefore proclivity of adopting dangerous behaviors, rather than other age groups (16). According to the theoretical model of planned behavior, individuals' perceptions influence their choice to participate in a specific behavior (17). Consistent with the aforementioned theory, many youths perceive e-cigarettes as safer, easier to conceal, and healthier alternatives compared with combustible cigarettes (18,19). Youths who have lower harm perceptions may be particularly susceptible to e-cigarette and polytobacco use (11,(20)(21)(22)(23)(24), conversely those who perceive e-cigarettes as more harmful would be less possible to use them (11).
Marketing, especially through social media, has a salient role in vaping promotion among adolescents; whereas retail stores are a prominent source of e-cigarette display (25). Four Scottish communities participated in a recent observational study in which a potential concern has emerged due to the placement of e-cigarettes, in 36% of stores, near to products popular to children (26). E-cigarettes are often marketed and displayed on countertops near till points and next to products of particular interest to children and adolescents; this may lead to the embracing of e-cigarettes as a broadly used and accepted product (26). However, several US jurisdictions have passed laws that increased the minimum age of sale for all tobacco products, including e-cigarettes, to 21 years (27).
Future research is imperative to illustrate the motivations behind teenagers' experimentation with e-cigarettes, while continued monitoring is warranted to clarify the temporal relationship between e-cigarette and tobacco products (28), with firmer tobacco control and social networking policies to prevent smoking initiation and lifetime continuation.

Search Strategy
We performed an electronic search in the following databases: PubMed, EMBASE, and Google Scholar, using combinations of the following keywords: adolescents, teenagers, e-cigarettes, vaping. The search included all types of articles written in English until August 2017. A total of 100 articles were found, and 25 were finally included in the present review. Exclusion criteria were the following: included participants older than 18 years (n = 30 articles), not original research (n = 2 articles), not relevant data, for example, e-cigarette marketing issues, consumers' preference in certain products, etc. (n = 31 articles), use of conventional cigarettes (n = 8 articles), use of alternative tobacco products (n = 4). The followed strategy and search results are displayed in Figure 1.

ReSULTS
The large body of evidence points to an increased interest in exploring the characteristics among adolescent e-cigarette users. Table 1 summarizes the current literature about the characteristics of adolescent users regarding e-cigarettes. E-cigarette has a large dispersion and penetration among teenagers, and is becoming the most commonly used tobacco product (9). Many researchers declared that the most frequent reason behind adolescents' e-cigarette experimentation was curiosity and the irresistible urge to try something new (29)(30)(31). On the contrary, major causes of e-cigarette smoking cessation were the following: losing interest, perceiving them as uncool, and enunciating anxiety about health (14).

DiSCUSSiON
As most studies report, male gender, older age, higher amount of pocket money, and tobacco smoking-related characteristics, such as regular and heavier smoking, and having peers who smoke, are the most common trends in characteristics of adolescent e-cigarette users.
The increased prevalence of vaping among males can be due to sociocultural characteristics or marketing messages and current trends. In many cases, males are most likely to be early adopters of technology, having easier access to e-cigarettes and they can also get exposed to e-cigarettes because they represent a newcomer product (29,33). Additionally, it was demonstrated that boys' higher risk of e-cigarette use may exist, partially, due to their lower harm perception (48). Generally, males tend to appraise lower risk comparative with females and stay away of risky behaviors only when they perceive severe risk (49).
One might say that the observed frequent use of e-cigarettes in older age is expected since older students are more informed about e-cigarettes, and e-cigarettes are easier to obtain from retail shops and through the internet (19,44), given the lack of regulation of age restrictions laws, a situation which is recently being reviewed in numerous countries (27,50).
Another characteristic, the observed relationship between higher amount of pocket money and e-cigarette usage may be due to the fact that adolescents can afford to buy e-cigarettes (51). This characteristic may suggest that having an adequate allowance at the adolescent's disposal may influence smoking practice, suggesting that guardians, who provide youths with pocket money, should pay attention in how that cash is spend.
Several studies also support the association between vaping and tobacco use related characteristics. Indeed, it has been proposed that e-cigarettes can be used as a method for smoking cessation. However, previous research has shown that vaping among adolescents was faced more for experimentation rather than smoking cessation (33), and that heavier e-cigarettes smokers are least likely to consider smoking cessation (29). A great percentage of young vapers had never tried conventional cigarettes (29,30,33,35,44), while in other studies e-cigarette ever-use was non-significantly related neither with quit intention nor attempts (24,39). On the contrary, among adults, e-cigarettes are seen as a potential cessation aid (11,52), while among adolescents who have never before smoked, e-cigarette use is associated with willingness to smoke, and vaping may act as a "one-way bridge" to smoking (47,53). Moreover, an association was recently reported -E-cigarette use was associated with intention to smoke. The associations were also significant in experimental and former smokers but not in current smokers -In current smokers, e-cigarette use was significantly associated with heavier smoking and morning smoking, and non-significantly associated with lower quit intention and attempts  -E-cigarette ever-use was increasing, and among never-smokers -The most common reason of e-cigarette everuse was the urge to try something new -Advertisements and parental smoking were associated with e-cigarette experimentation -Daily use was most common among 18 years old boys -Tobacco-related factors were stronger determinants for e-cigarette use than socioeconomic factors -Among socioeconomic characteristics, adolescents' academic performance was more strongly related to e-cigarette use than family structure, parental education, or working status -Higher prevalence of current e-cigarette use in highschool students, non-Hispanic white, male gender, and in those who had someone in their home who currently smoked -Both current and lifetime e-cigarette users were also more likely than their peers to use other tobacco products  -Primary-school children were more likely to have used e-cigarettes than tobacco -The proportion of children who had ever used an e-cigarette and reported currently smoking increased in higher grades -Current weekly smokers were 100 times more likely than non-smokers to report regular e-cigarette use -Regular e-cigarette use was more likely among those who had used cannabis Lee et al. (45) -6,655 middle and high-school students in South Korean -Assessment through nationally representative cross-sectional survey -The percentage of frequent e-cigarette users (≥10 days/month) was 3.5% in adolescents who did not smoke within a month, but 28.7% among daily smokers -The most ordinary reason for e-cigarette initiation was curiosity (22.9%), followed by the belief that they are less harmful than traditional cigarettes (18.9%), the motive to cease smoking (13.1%), and the capacity for indoor use (10.7%) -Frequent and intensive cigarette smoking was associated with frequent e-cigarette users. Frequent users were older -The most common reason for e-cigarette use was curiosity, followed by the belief that they are least harmful than conventional cigarettes, the desire to quit smoking, and the capacity for indoor use -Non-smokers who had used e-cigarettes presented more willingness to smoke cigarettes compared with non-smokers -Willingness predicted smoking onset between e-cigarette use and initiation or escalation of cigarette smoking (28). Only in two Korean studies, e-cigarette usage was associated with the desire to quit smoking (32,45). A number of limitations of studies included should be considered in order to allow interpretation of the described findings. Firstly, the cross-sectional design in many studies preclude us from exporting causal inferences about the results; since they can only indicate associations among the studied characteristics but not causality (6, 20, 24, 29-33, 35, 36, 38, 39, 41-47). Secondly, the self-reported assessment could introduce reporting bias (6, 20-24, 29-46, 52). Thirdly, findings may not be generalizable to populations outside of the samples geographical areas or other countries or regions (20,21,23,29,30,(32)(33)(34)(35)(36)(37)(38)(41)(42)(43)(44)(45)(46)(47).
It is urgent, moreover, to include e-cigarettes in tobacco prevention programs; targeting in vulnerable groups through early intervention efforts. Given their overwhelming acceptance, prevention campaigns via social media, appear to be an effective mechanism for influencing trends when targeting youth populations. Prospective surveys should be directed toward addressing the potential long-term effects on health and the probable nicotine addiction of consumers. The findings underscore the need of constructing persuasive e-cigarette prevention messages promoting public health welfare.

AUTHOR CONTRiBUTiONS
EPP contributed in designing and drafting the manuscript, PS contributed in the initial conception and critical revision, EP contributed in the design and interpretation, TC contributed in the interpretation and critical revision, EN contributed in the conception, interpretation, and critical revision. All authors provide their approval for the final version to be published.