Trend analyses suggest gender-focused, peer-aware, and parent-targeted interventions could reduce teenage e-cigarette use


 E-cigarette ever-use and current-use among teenagers has been increasing worldwide, including in Ireland. Boys are widely observed to have increased prevalence compared with girls.We use data from two waves of the European School Survey Project on Alcohol and other Drugs (ESPAD) in Ireland and focus on gender and teenage e-cigarette use. We examine changes in ever and current-use among 16-year-olds (n = 3,421) between 2015 and 2019. We use multivariable logistic regression analyses to explore relative and gendered effects of covariates on teenage e-cigarette use.E-cigarette ever-use increased from 23–39% and current-use from 10–18%, (in 2015 vs 2019, respectively). The rate of increase in ever-use is significantly greater for girls AOR 2.67 (C.I. 2.02, 3.54) vs 2.04 (C.I. 1.55, 2.68) for boys. Smoking and e-cigarette use are linked but never-smokers who try e-cigarettes have risen from 33–67%. Peer smoking is also significantly associated with e-cigarette ever- and current-use, particularly for boys if “Most/All friends smoke” AOR 5.90 (C.I. 3.31, 10.52) vs 3.50 (C.I. 1.79, 6.84) for girls. Less parental monitoring is associated with greater e-cigarette use, for boys AOR 5.50 (C.I. 2.85, 10.61) vs AOR 5.31 (C.I. 3.01, 9.37) for girls.Boys currently have a higher prevalence of e-cigarette use but girls are increasingly at risk. Targeting by industry through advertising, gaming and social media may contribute to use. Peer influences and parental monitoring are significant risk factors for use and provide a mechanism for intervention to prevent an increasing risk of nicotine addiction.

Introduction E-cigarette use is increasing worldwide, and e-cigarettes are the most commonly used tobacco product among adolescents (1), including in Ireland (2). A secondary analysis of ve Irish health datasets (all strati ed random samples in school-based settings) which included data on teenage tobacco use found that e-cigarette use has risen rapidly among adolescents in Ireland in recent years (from 23% ever-use in 2014 to 39% in 2019, and from 3% current-use in 2014 to 18% in 2019) (3).
Concerns about e-cigarettes at the level of public health and tobacco control have been aired for some time (4), including the effects of the mainstream tobacco industry's entry into the e-cigarette marketplace and the potential for "renormalization" of combusted cigarette use (4)(5)(6). Currently, concerns about e-cigarette harms include negative impact on cardiovascular health (7) and potential hazards such as obstructive lung disease from avorants in e-cigarettes (8). Passive exposure to second hand aerosol (SHA) from e-cigarettes is also a cause for concern, especially among those with respiratory diseases such as COPD (9,10), and symptoms of sensory irritation, and general complaints have been reported by non-smokers (11).
Among adolescents, additional and speci c potential negative consequences of e-cigarette use have been identi ed (12,13). There is "compelling" evidence that nicotine exposure during adolescence causes both long-term structural and functional changes in the brain with multiple adverse health consequences (14). The series of risks identi ed from nicotine exposure, include altered development of cerebral cortex and hippocampus in the developing adolescent brain (12). The link between teenage e-cigarette use and increased smoking is widely accepted and supported by several reviews (15)(16)(17). The gateway theory (18), the common liability theory (19), the catalyst model (20), and the catalyst/ diversion model (whether e-cigarettes are a "catalyst" to conventional smoking or whether they divert adolescents from more harmful products (21)) have all been considered.
The upward trend in teenage e-cigarette use in Ireland has occurred against the historical backdrop of a two-decade downward trend in teenage cigarette smoking, down from 41% in 1995 to 14% in 2015 (22). Between 2015 and 2019, at the same time that e-cigarette use was increasing rapidly, the downward trend in teenage current cigarette use came to a standstill, remaining at 14% overall in 2019 and, in fact, reversed for boys, among whom smoking prevalence rose to 16% in 2019 (2). The decline of ever cigarette smoking in Ireland seems to continue, albeit not statistically signi cantly. Elsewhere, it has been suggested that ecigarette users do not t the traditional risk pro le of cigarette smokers (23), pointing to a separate need to understand ecigarette use.
Teenage e-cigarette users are more likely to be male gender, older, have higher amounts of pocket money, and tobacco smokingrelated characteristics (including regular and heavier smoking, and having peers who smoke) (24). Gender differences in teenage smoking have been widely researched (e.g. (25)), but gender differences in teenage e-cigarette use are "relatively unknown" (1). A recent review of literature on US adolescent e-cigarette use concluded that boys appear to have greater ecigarette use, but "girls may be at increased risk if e-cigarettes are targeted to them, as it has been for cigarettes" (1). The Irish data with its clear upward trend point to much higher levels of e-cigarette use among boys than among girls, but little is known about why this is so.

Study
Data are drawn from two Irish waves (2015 and 2019) of the European School Survey Project on Alcohol and other Drugs (ESPAD) (2,22), which contained comparable questions on e-cigarettes.

Sample
In both 2015 and 2019, students were surveyed in a strati ed random sample of schools (n = 50) in Ireland, based on geographic region and school type (secondary, vocational, community/comprehensive), religious a liation (Roman Catholic, Church of Ireland, inter-denominational), gender (all-boys, all-girls, mixed), and school-level disadvantage status (Delivering Equality of Opportunity in Schools (DEIS vs. non-DEIS)). Completed survey data were entered manually into SPSS v22 as they appeared in the survey. Data entry was cross-checked via double entry for 20% of surveys. Full accounts of the data cleaning procedure have been reported elsewhere (2,22). The total valid sample for this study was 3,421 16-year-olds, comprising 1,472 students (born in 1999) in the 2015 sample, and 1,949 students (born in 2003) in the 2019 sample.

Measures for multivariable logistic regression analysis
A full list of the questions, and answer categories, from which variables were drawn is in Additional Files1&2 (ESPAD questionnaires 2015 and 2019).
Outcome variables: Ever-use and current-use of e-cigarettes Two variables measured prevalence, e-cigarette ever-use ('Have you ever used e-cigarettes'? yes/no, more than 12 months ago, in the last 12 months, in the last 30 days; recoded 'ever-use' -no vs yes) and e-cigarette current-use ('How often have you smoked e-cigarettes during the last 30 days?' not at all; less than once per week; at least once a week; and almost every day; recoded 'current-use' -no vs yes). Variables in model Socio-demographic variables Variables included gender (male, female), perceived wealth (recoded much better off, better off, about the same, less well off) and household composition (one-parent, two-parent, blended).

Individual, familial, and peer variables
Students were asked about: Use of cigarettes Their use of cigarettes (ever-use, current-use, recoded -no vs yes).

Reasons for trying e-cigarettes
Reasons for rst trying e-cigarettes, and to select all that applied: out of curiosity, to quit smoking, because friends were using it.
Relationship with tobacco when rst tried e-cigarettes Their relationship with tobacco when they rst tried e-cigarettes: had never smoked tobacco, smoked occasionally, smoked regularly.

Absenteeism
To report the number of days they had skipped or "cut" school in the last 30 days (recoded none, 1-4 days, 5 + days). Parental monitoring If their parents (mother or father) know where they spend their Saturday nights ('know always', 'know quite often', 'know sometimes', 'usually don't know').

Relationship with mother
Were they satis ed with their relationship with their mother (recoded as satis ed, neither nor, not satis ed). Peer smoking How many of their friends smoke cigarettes, ('none', 'a few', 'some', 'most' and 'all'; recoded none, a few/some, most/all).

Statistical analysis
Pearson's chi-square test was used to compare the demographic variables between 2015 and 2019 and to determine whether the differences in the variables between the two waves are statistically signi cant (Table 1). E-cigarette ever-and current-use was examined using a multivariable logistic regression model with e-cigarette ever-use (Table 2) and current-use (Table 3) as the dependent variable, analysed for all and by gender. All analyses and results are presented as adjusted odds ratios (AOR) and 95% con dence intervals. A p-value of .05 was used as a cut-off for signi cance. All statistical analyses were conducted using STATA version 16.

Sample differences 2015-2019
Regarding socio-demographic, individual, familial and peer characteristics, there are very few statistically signi cant differences between the 2015 and 2019 ESPAD waves (Table 1).

* Parents know where child is on Saturday nights
Two features of the student samples stand out. Firstly, in 2015, 33% (n = 101) of the e-cigarette sample had never used tobacco; in 2019, this rose to 67%, (n = 461). The number who said that they were regular tobacco users when they rst used an ecigarette dropped from 14.9% (n = 45) to 9% (n = 62) between waves.
Trends in e-cigarette ever-use Table 2 shows the results of multivariable logistic regression analyses of e-cigarette ever-use in the 2015 and 2019 ESPAD surveys, for the total sample, and separately for boys and girls. Girls who reported that their families were less well-off than other families had increased odds (AOR 1.76, 95% CI:1.11-2.78) of being ever-users of e-cigarettes. Boys living in blended families also had increased odds (AOR 1.85, 95% CI:1.02-3.35).
As with ever-use of e-cigarettes increased parental monitoring was also important for adolescents' current-use of e-cigarettes. The odds were increased (AOR 4.48, 95% CI:2.83-7.11) in young people who say that their parents "usually don't know" where they are on a Saturday night (reference group "parents always know"). Parental monitoring was more important for boys (AOR 5.50, 95% CI:2.85-10.61) than for girls (3.50, 95% CI:1.79-6.84).

Discussion
Results from our two ESPAD samples of 3,421 16-year-olds show that e-cigarette ever-and current-use increased signi cantly between 2014 and 2019 in Ireland. There was a signi cant rise in never-smokers trying e-cigarettes, with an increase from onethird (33%) to two-thirds (67%) of the sample who had never used tobacco when they rst tried an e-cigarette.
Gender differences in e-cigarette use From the outset, boys in our trend analyses were more likely to be both everand current-users of e-cigarettes. This is in line with many other studies (1,24). Various theories have been offered to explain gender and substance use including tobacco and e-cigarettes, such as Connell's (2005) in uential construct of hegemonic masculinity and how it puts men at risk of harmful health behaviours and consequences that can be destructive for them (26), including for teenage boys (27), and Butler's (28) consequential theory of gender performativity -that gender is not an essential, biologically determined quality or an inherent identity, but is repeatedly performed, based on, and reinforced by, societal norms, this repeated performance of gender being also performative -applied to smoking by women in Australia by Gilbert (29). They argued that smoking is "a gender act that can be internalised and which, when repeatedly performed by women in gender-appropriate ways, constructs a 'feminine' gender identity" (28,29). Such theories and how they relate to our ndings on gendered e-cigarette use are outside the scope of our data. We raise them here to acknowledge that our ndings may have a broader, and deeper, context within discourses on gender and substance use.
Boys have higher prevalence of e-cigarette use but the rate of increase in this study is signi cantly greater for girls, and this was particularly pronounced for current-use, with the trend analysis showing girls having more than 50% higher odds (AOR 3.11, 95% CI 2.10-4.61) than boys (AOR 1.96, 95% CI 1.37-2.82) of being e-cigarette current-users in 2019 compared with 2015. This gendered pattern of substance use showing initial high male use, with females subsequently potentially over-taking males may re ect a particular historical pattern of gendered tobacco use, driven by the tobacco industry's gendered marketing and exploitation of social change and social disruption (30)(31)(32), such as the post-war targeting of women by the tobacco industry "as an equality and freedom issue" (32). The latter comprised advertising and marketing by the industry, speci cally and successfully targeted to women and girls, a market identi ed as a large untapped lucrative reservoir (30)(31)(32). Among the tactics were brand names chosen to appeal to women, development of "slim" cigarettes, and emphasis on qualities identi ed as desirable such as thinness and female independence (30)(31)(32). E-cigarette advertising and direct and covert marketing uses strikingly similar techniques to those used previously by the cigarette industry (33) -featuring young, attractive models, sponsorship of sports events and parties, product placement, and direct payments to social media in uencers (33). We add support to Kong et al. (2017) who observed that, while boys in the U.S. appear to have greater use of e-cigarettes, girls may be at increased risk if e-cigarettes are targeted to them "as it has been for cigarettes" and we join in calling for further research on gender differences in e-cigarette use, particularly in gendered rates of increase, and on the role of industry advertising and marketing including the gendered nature of such activities on the internet (1).
Secondly, we suggest that online messaging about e-cigarettes, particularly as a "healthier" alternative to traditional combustible cigarettes, may be especially effective for increasing e-cigarette use among girls and young women. including targeted and sophisticated messaging on social media (33). Teenagers are at the forefront of the transition from traditional to electronic media, especially social media (34) and the vast majority of adolescents search for health-related information online as opposed to more traditional media (34,35). Teenage girls appear to be at greater risk of consuming unreliable health-related information on social media than are teenage boys (35). One mixed-methods pilot study (34) found that, while girls recognised the potential to encounter false information in health-related content that they saw on social media (often in paid advertisements), their methods for fact-checking were not stringent; rather, for example, they tended to trust posts from people they knew personally (34). As Wartella and colleagues point out, teenagers "virtually always have a digital device at their ngertips" and so "public health interventions and informational campaigns must be tailored" (35). We agree with O'Leary et al. (2019) that, while the state and use of social media are ever changing, the potential to use social media as a form of promotion for healthy behaviours, especially among adolescents, will continue to offer promise (34). Thus, we extend to the domain of tobacco and e-cigarette use their call for education interventions for teenagers (34).
Thirdly, we point to the gendered nature of teenagers' online activities and how these may be exploited in particular ways through marketing, advertising and particularly social in uencing (36). Speci cally, we draw attention to ndings regarding the potentially different online worlds inhabited by teenage girls (social media platforms) and teenage boys (gaming platforms) that have been identi ed (2,37). This leads us to speculate, for example, that boys may be targeted through gaming platforms and that girls' rapidly increasing e-cigarette use may be related to their greater social media use. We know that heavier exposure to advertisements and e-cigarette content in social media posts is associated with a greater risk for e-cigarette use among adolescents, and even brief exposure to e-cigarette content on social media has been found to be associated with greater intention to use and more positive attitudes toward e-cigarettes (38). The scope within these parallel gendered domains for targeted marketing of e-cigarettes by industry merits further research and we also support calls for regulatory action to prohibit sponsored e-cigarette content on social media platforms used by youth (38,39).
Gender differences, e-cigarette use and smoking The link between cigarette smoking and e-cigarette use has been well-established (15)(16)(17) and our ndings support this as, but with gender differences. Girls who had ever-smoked had higher odds (AOR 1.56, 95% CI: 1.12-2.18) of ever-using e-cigarettes. Boys who were current smokers had more than twice the odds of being e-cigarette ever-users (AOR 2.60, 95% CI: 1.71-3.93).
Differences in experimentation and continuation of both smoking and e-cigarette use appear to be gendered, pointing to different characteristics between the smoking and e-cigarette populations or to gender differences that require further exploration, lending support to the ndings of Creamer et al. (2021) that, regarding psychosocial risk factors for cigarette smoking, e-cigarette users do not t the traditional risk pro le of cigarette smokers, and require further research (23).

Peer pressure and gender
Adolescent peer social networks have been found to be important for health behaviour choices, with health behaviour similarity found to be driven by homophilic social selection and/or social in uence (40). Studies of adolescent social networks, including online networks, suggest that friends' online behaviours are a viable source of peer in uence (41). Those with more peers who smoke have much higher odds of being ever-users of e-cigarettes, and this pattern was particularly strong for boys. Peer smoking was similarly implicated in e-cigarette current-use and, again, gender differences showed a somewhat stronger in uence of peers on boys than on girls in relation to current-use of e-cigarettes. A review of 26 studies examining adolescents' susceptibility to peer pressure to engage in risky behaviours identi ed two primary trends: one, that adolescent males appear to more susceptible to peer in uences that encourage risk-taking behaviours; and the other, that there are no consistent gender differences (42). McCoy and colleagues conjecture that, as attitudes about gender-appropriate behaviour shift across historical time, it may be that male and female teenage experiences are becoming increasingly similar, for example in experiencing comparable levels of deviant peer pressure around substance use in particular and also that differences across types of risky behaviours may "even out", causing gender differences to disappear. Adding to this, given that many marginalised groups smoke at higher rates and that socioeconomic inequities in smoking prevalence are widening (43), we draw attention to a broader lack of focus on intersectional understandings (44) of adolescent behaviour, noting that there is little or no data on young people, peers, and substance use -and speci cally, tobacco/ e-cigarette use -synthesised, for example, across social categories of gender, race, class/SES, disability, sexuality, and religion. Moreover, intersectionality has been identi ed as a promising framework for addressing health inequities in tobacco cessation, aiming to capture the complex set of factors that may account for tobacco cessation inequities, such as "multiple aspects of identity that may in uence access to tobacco cessation treatment and exposure to certain stressors that impede cessation efforts" (43). In turn, this may lead to better design and interventions for individuals at high risk (43), and may be particularly useful in dealing with newly-emergent patterns of ecigarette use across gender and other identities.

Parental monitoring and gender
Parental monitoring was a separately important factor in explaining e-cigarette use and we add further support regarding ecigarette use to previous ndings from ESPAD -dating as far back as 2003 (45) and as recently as 2018 (46) -showing that lack of parental monitoring remained a signi cant predictor for all illicit substance use in the best-tting models. This nding does not lend support to the suggestion (e.g. (23)) that e-cigarette users have a different risk pro le to traditional cigarette users. This nding about parental monitoring was also gendered, being more important for teenage boys than for teenage girls.In our study, lack of satisfaction with the maternal relationship increased the odds of current-use.

Conclusion
Teenage boys are currently more likely to be both ever-and current-users of e-cigarettes but trend analyses show that the risk to girls has increased signi cantly more than to boys in the years between 2015 and 2019. This differential in rate of increase may re ect differences in how girls and boys are targeted through advertising, gaming and social media platforms. Peer in uences operate differently for girls and for boys, as does parental monitoring. While further research is needed, to understand how ecigarette use, like other tobacco use, is a gendered activity, and into ways, in which industry advertising and marketing is gendered in order to support regulation of this rapidly evolving market of new tobacco products, for now we highlight the role of parental monitoring and peer smoking in uence as targets for prevention of this increasing addiction to e-cigarettes. Declarations