Foreword

Chapter 2 of the Surgeon General’s report describes patterns of e-cigarette use among youth (12–17 years) and young adults (18–24 years). The report makes repeated statements about ‘the number of young people who use e-cigarettes’; for example, “E-cigarette use has increased considerably in recent years, growing an astounding 900% among high school students from 2011 to 2015” (p. 7). This statement is true when ‘use’ is measured as the proportion of young people who have ever tried using an e-cigarette, even once, in their lifetime. However, one-time or experimental use of an e-cigarette is extremely unlikely to increase any individual’s risk for developing any disease known to be caused by smoking. Rather, pre- and post-market population models of the public health impact of e-cigarette use are more meaningfully informed by data on the prevalence of three characteristics of e-cigarette use that were afforded only limited discussion by the Surgeon General: frequency of e-cigarette use, the nicotine concentration of e-cigarettes used, and the smoking status of the e-cigarette user.

Ever-use vs. frequent use

The Surgeon General’s report draws heavily from the National Youth Tobacco Survey (NYTS) and the Monitoring the Future Study (MTF) [5,6,7,8], two nationally representative surveys of US youth. Despite being reported by these surveys, the Surgeon General’s report provides little discussion of the rate of frequent e-cigarette use, defined as use on at least 20 of the past 30 days, among US youth. Measures of e-cigarette use, which do not capture frequency, intensity, or reasons for use, are largely uninformative and provide misleading conclusions about the individual as well as public health impact of e-vapor products [9].

While these surveys do indeed show youth rates of ever-use of an e-cigarette have increased by several hundred per-cent in recent years, they also show youth rates of frequent use of an e-cigarette, which is more strongly indicative of a behavior likely to be sustained, and so, more strongly associated with health outcomes, have remained very low between 2011 and 2015. Data from the 2015 NYTS, for example, reveal that, among middle school students, 13.5% have ever used an e-cigarette (i.e., ever-users) and 5.3% have used an e-cigarette at least once in the past 30 days, but only 0.6% have used an e-cigarette on at least 20 of the past 30 days (i.e., frequent users) [4] (p. 29). Among high school students, the respective rates were 37.7, 16.0, and 2.5% [4] (p. 30). Similar patterns have been observed among young adults too, with the 2013–2014 National Adult Tobacco Survey finding rates of ever-use, current use, and frequent use among those aged 18–24 years to be 35.8, 13.6, and 2.0%, respectively [4] (p. 38). The 2014 MTF survey, too, revealed that although past 30-day e-cigarette use was reported by 17.2% of 12th graders, only 6.6% had used e-cigarettes for > 5 days in the past month [10].

Frequent e-cigarette use according to smoking status

The 2014 MTF survey showed that frequent e-cigarette use was extremely rare among never-smoking youth: only 1.7 and 0.7% of never smokers were using e-cigarettes for > 5 days and 20–30 days of the past month, respectively [7]. In contrast, 14.7% of current regular smokers and 15.0% of youth who smoked regularly in the past were currently using an e-cigarette frequently. A secondary analysis of the 2014 NYTS data showed that 87% of past 30-day e-cigarette users had ever used a tobacco product while 63% reported using a tobacco product in the past 30 days [11]. However, less than 0.1% of never-users of tobacco had used e-cigarettes for > 10 days of the past month [11]. Similarly very low rates of frequent e-cigarette use among never-smoking US youth have also been observed among youth in several other countries [12, 13]. For example, although approximately 12% of adolescents in the UK reported ever-use of e-cigarettes, only 0.7–1% reported use of an e-cigarette more than once weekly, with most being smokers [14, 15].

Nicotine-containing e-cigarette use

Next, the Surgeon General states that “…most e-cigarettes contain nicotine, which can cause addiction and can harm the developing adolescent brain.” [1] (p. 7). Consumption of nicotine via e-cigarette aerosol is an important determinant of an individual’s risk for dependence on e-cigarettes and progression to frequent use or transition to smoking tobacco cigarettes. However, while most e-cigarettes may indeed contain nicotine, data suggest the majority of ever-user US youth use e-cigarettes that do not contain nicotine. Miech et al. recently reported data from the 2015 MTF survey showing that 65–66% of e-cigarette ever-user students in the 8th, 10th, and 12th grade had, at last use, used an e-cigarette that did not contain nicotine (i.e., only contained flavorings); only 13–22% of ever-user students were using nicotine-containing e-cigarettes [16]. For current e-cigarette users, 59–63% reported using e-cigarettes containing ‘just flavorings’ at last use. Similar findings have been observed in Canada, where 70% of high school e-cigarette ever-users had never used an e-cigarette that contained nicotine [17]. Thus, the rate of nicotine-containing e-cigarette use appears to be a small proportion of overall e-cigarette use in this population.

Rate of youth e-cigarette use over time

The alarm raised by the Surgeon General comes at a time when prevalence of ever-use of an e-cigarette among US youth appears to be stabilizing, even declining. The MTF survey found no change in prevalence of past 30-day use between 2014 and 2015, despite the fact that, in 2015, the survey questions were broadened to say “electronic vaporizers such as e-cigarettes” instead of just “e-cigarettes”. While the 2015 NYTS reported an increase in past 30-day use from 2014 (9.3%) to 2015 (11.3%) the rate of increase has significantly declined compared to the change from 2013 (3.1%) to 2014. Recently, data from the 2016 MTF survey [18] revealed a substantial decline in e-cigarette use compared to 2015. Specifically, the 2016 prevalence of past 30-day use was 6.2% in 8th graders (from 9.5% in 2015), 11.0% in 10th graders (from 14% in 2015), and 12.5% in 12th graders (from 16.0% in 2015). Similar to 2015, the majority of adolescents were using e-cigarettes that did not nicotine.

Youth e-cigarette use and smoking rates

The greatest public health concern about e-cigarettes, however, is not the rate at which youth are currently using e-cigarettes, but the rate at which youth use of e-cigarettes may increase rates of youth use of more harmful tobacco products (e.g., starting to smoke cigarettes). Setting aside momentarily the evidence that the vast majority of youth e-cigarette use in the USA is found among youth who are already smoking, the rate at which non-smoking youth who use e-cigarettes may become smokers as a consequence of having used e-cigarettes is a legitimate concern and a pressing public health policy question.

Broadly, there are two trajectories in which a youth’s likelihood of initiating smoking varies as a function of his/her e-cigarette use, and two trajectories in which the likelihood of initiating smoking occurs independently of e-cigarette use. In the worst-case scenario for public health, e-cigarette use may (for example, by habituating a young non-smoker to the effects of nicotine and the sights, smell, and feel of inhaling and exhaling a visible vapor) increase interest in smoking cigarettes among youths who were unlikely to have started smoking had they not started using e-cigarettes first (i.e., a putative causal effect), resulting in net harm to the youth population. In the best-case scenario for public health, use of an e-cigarette (particularly those that do not physically resemble a conventional cigarette and those containing flavors that are not available through conventional cigarettes) may provide a sufficiently pleasurable experience that discourages initiation of cigarette smoking among youth who were more likely to have started smoking cigarettes had they not started using e-cigarettes first (i.e., a putative protective effect), resulting in net avoided harm to the youth population. Then there are two trajectories in which cigarette smoking and e-cigarette use are unrelated: youth who are likely to start smoking even if they do not start using e-cigarettes, and youth who are unlikely to start smoking even if they do start using e-cigarettes. Studies that report data on the prevalence of these four mutually exclusive relationships between e-cigarette use and smoking initiation will provide a good basis for estimating the net health impact of e-cigarettes on US youth. Thus, while the low rates of current use and frequent use of an e-cigarette use among US youth revealed by multi-year cross-sectional surveys do not depict e-cigarette use among US youth as a major public health concern at present, monitoring for evidence of an increased use of e-cigarettes by non-smoking youth, and an increased rate of smoking initiation among the small proportion of non-smoking youth who use e-cigarettes is essential.

Determining the rates at which e-cigarette use prevents, causes, and coincides with smoking initiation among youth requires longitudinal studies to follow young people over time, ideally over several years, to adequately characterize the rates of youth smoking initiation associated with prior regular, experimental, one-time, and no use of an e-cigarette. These studies should ideally also seek to identify the combinations of e-cigarette device formats, flavors, nicotine strengths, and use settings most strongly associated with youths’ increased and reduced odds for future smoking initiation.

Five longitudinal studies cited in the Surgeon General report claimed that e-cigarette use at baseline predicted smoking at follow-up [19,20,21,22,23]. However, all studies suffered from the issues discussed above. They only assessed ever-use [19,20,21,22] or past 30-day e-cigarette use [23] and did not assess the nicotine content of e-cigarettes used. There was no evidence that adolescents were regular e-cigarette users at baseline, and no evidence that they were smoking cigarettes regularly at follow-up. These aspects are crucial in supporting a gateway hypothesis, i.e., that adolescents became addicted to e-cigarettes and then transition to addiction to cigarette smoking. Moreover, it is not clear how e-cigarettes could be causally linked to cigarette smoking, unless adolescents became addicted to nicotine and/or the act and rituals of inhalation through e-cigarette use and then were curious to try smoking cigarettes. It is possible that these adolescents may have become smokers even in the absence of e-cigarettes; in that case, initiation of e-cigarette use may be related to easier access or cheaper price of first-generation disposable products and a predisposition of these subjects to engage in an inhalational habit. Thus, the ‘gateway hypothesis’ can be neither supported nor rejected by the findings of these studies. In contrast, two studies found that the implementation of restrictions on e-cigarette sales to adolescents was associated with an increased smoking rate among adolescents [24, 25] and another study reported an association between restrictions on e-cigarette sales an increased smoking rate among pregnant youth and young adults [26]. Although the evidence is not conclusive, the potential for e-cigarettes to have a primary prevention role, and the potential for restrictions on e-cigarettes sales to increase smoking initiation among US youth, should be carefully considered. Continuous monitoring of youth transitions between cigarette smoking and e-cigarette use through prospective cohort studies are a public health imperative, but such studies should assess as their primary dependent variable the prevalence of regular or frequent e-cigarette and cigarette smoking at baseline and follow-up, respectively.

It should also be emphasized that the increasing rate of ever-use of e-cigarettes among US youth has coincided with the sharpest declines in youth smoking rates in many decades. Data from the NYTSs show past 30-day smoking prevalence in high school students decreased from 15.8% (2011) to 12.7% (2013) to 9.2% (2014), while in 2015 no further decrease in prevalence was observed. The 2015 MTF survey showed a continuous decline in past 30-day smoking prevalence to 7.0% in 2015 compared to 11.7% in 2007. In 2016, further declines were observed in all school grades (12th grade = 10.5%; 10th grade = 4.9%; 8th grade = 2.6%) [27].

Among young adults too, between 2010 and 2015, the period in which the Surgeon General points to rapidly increasing use of e-cigarettes among US young adults, the prevalence of smoking reduced by 54% among 18–19 year-old males and by 64% among 18–19 year-old females. These reductions are three times and five times larger, respectively, than the reductions observed between 2005 and 2010, when e-cigarette use was essentially zero. Given the cross-sectional design of the MTF, NYTS, and NATS, no conclusive determination can be made of the role played by e-cigarettes in the observed incremental declines in smoking prevalence among US youth and young adults between 2010 and 2015. This possibility certainly exists, though is not addressed in the Surgeon General’s report. At the very least, available data appear reassuring that e-cigarettes are not decelerating let alone reversing declining rates of youth smoking.

Use of flavored e-cigarettes

The Surgeon General report presents data from population surveys that indicate most adolescents who have ever used an e-cigarette have used flavored e-cigarettes. Again, however, the important question is not what flavors are being used, but with what effect are different flavors being used by youth, young adults, and adults? It should be expected that good flavors will attract consumers of all ages. Flavors appear to play an important role in perceived satisfaction and self-reported effectiveness of e-cigarettes among adults who have used e-cigarettes to stop smoking. A survey of adult e-cigarette users, most of whom were former smokers, indicated that flavors played an important role in their efforts to reduce or quit smoking with the use of e-cigarettes [28]. Most participants were using multiple flavors on a regular basis. Additionally, adult smokers appear to prefer tobacco-flavor when they start using e-cigarette, but as e-cigarette use develops, preferences appear to dimish for tobacco flavor and grow for sweet and fruit flavors [28, 29]. The use of these non-tobacco flavors may help suppress craving for cigarettes and so help the e-cigarette user to sustain abstinence from smoking, since such flavors should be less likely than tobacco flavor to cue smoking as a conditioned response. Other surveys have also shown that a small minority of adult e-cigarette users are using flavorless liquids [30]. Thus, the decision to ban or restrict flavors should depend on the balance between the health benefits to adults who manage to reduce or quit smoking by switching to use of flavored e-cigarettes, and the need to protect youth who likely would have never smoked, if it is shown that flavors are indeed a significant determinant of regular e-cigarette use and subsequent smoking initiation.