An International Systematic Review of Prevalence, Risk, and Protective Factors Associated with Young People’s E-Cigarette Use

While the prevalence of young people’s conventional cigarette use has decreased in many countries, the use of e-cigarettes has risen. To effectively counteract the growing popularity of e-cigarettes among young people internationally, researchers should know the exact prevalence as well as the protective and risk factors associated with vaping. Based on five eligibility criteria, 53 articles were chosen and analyzed by general characteristics, prevalence, sample characteristics, gender difference, protective factors, and risk factors. In this study, the international pooled prevalence of young people’s lifetime e-cigarette use was 15.3%, the current use was 7.7%, and dual use was 4.0%. While the highest lifetime, current, and dual prevalence were found in Sweden, Canada, and the United Kingdom, respectively, the lowest prevalence was found in Germany, followed by South Korea and Sweden. Some protective and risk factors include perceived cost and danger of vaping, parental monitoring, internal developmental assets, cigarette use, family and peer smoking, exposure to online advertisements, and the presence of nearby retail stores. Based on this review, researchers and practitioners can develop different intervention programs and strategies for young smokers.


Introduction
The use of electronic cigarettes (e-cigarettes) among young people, including adolescents and emerging adults, is a rising concern worldwide. E-cigarettes are handheld, small, rechargeable, and battery-powered electronic devices that deliver nicotine to users through aerosols or vapors [1,2]. Since their invention in the early 2000s, e-cigarettes have become trendy among young people [1,3]. Fadus et al. [3] indicate a swiftly increasing trend in e-cigarette use, while traditional combustible cigarette use has decreased slightly. In addition, many studies suggest that the use of e-cigarettes is an international phenomenon and is increasing globally [4,5]. For instance, the past 30-day use of e-cigarettes among young Americans rose from 1.5% in 2008 to 20.8% in 2018 [6]. To be more specific, the highest ever e-cigarette use was reported among young adults (aged 20-28), followed by adults (above 18) and adolescents (aged [11][12][13][14][15][16][17][18][19], in the United States [7]. In Asia, the rate of young Korean who had ever used e-cigarettes rose from 0.5% in 2008 to 42% in 2017 [8,9]. Similarly, the current e-cigarette use of young Taiwanese men grew from 2.5% in 2014 to 6.4% in 2017 [10]. In European countries, the prevalence of having ever used e-cigarettes varied by 17-62% [1, 5,11]. Vapers in many European countries were previously cigarette users or started smoking at a young age, and some of these nations even reported a higher prevalence of e-cigarette use than traditional cigarette use (e.g., Iceland, Monaco, Lithuania, Poland, Ireland, Germany, Czechia, Hungary, France, and Norway) [12]. After the implementation of the EU Tobacco Products Directive regulations, e-cigarette use among young people in European nations, including the UK, is slowing down [13,14]. However, unlike other countries, England still shows an increase in past and current e-cigarette use from 2014 to 2016 [13,14]. Despite the significance of these findings, there has been a lack

Selection Procedure
Following the PRISMA guideline [36], we first identified 206,867 records through database searching and then removed duplicates. Of the 159,014 records, we removed 130,697 because the title and keywords did not match the study's aim. By screening the abstracts, we excluded 27,072 records. Out of 1245 full-text articles assessed for eligibility, we excluded 1198 because they did not meet the eligibility criteria. There were four eligibility criteria in this review: (1) examining either protective or risk factors of ecigarette use, (2) measuring lifetime, current, or dual e-cigarette use as the outcome variable, (3) published in English, and (4) targeting young people (9-25 years old) [37]. All records had to be peer-reviewed journal articles and doctoral dissertations.
Researchers conducted a quality assessment for the selected studies to ensure their "relevance, methodological rigor, and credibility" [38,39]. Each study was evaluated based on the following questions: (1) "Is the research question clear and adequately substantiated?" (2) "Is the design appropriate for the research question?" (3) "Was the sampling method appropriate for the research question and design?" (4) "Were data collected and managed systematically?" and (5) "Were the data analyzed appropriately?" [38] Each study was scored from one to five points, and only studies with more than four points were included in the analysis.
Moreover, to establish the same criteria and time frame for a lifetime, current, and dual e-cigarette use, we have defined lifetime use as "having ever used e-cigarettes, even just once," current use as "the use of a tobacco product on at least one day in the past 30 days," and dual use as "the use of both products (i.e., cigarette and e-cigarette) within the same reporting period (e.g., the past 30 days)" [40]. After these stages, a total of 53 articles were chosen for analysis ( Figure 1) [1, 4,5,8,9,11,23,[28][29][30]. electronic nicotine delivery systems (ENDS). In addition, we used all the e-cigarett lated terms suggested by the Centers for Disease Control and Prevention [35].

Selection Procedure
Following the PRISMA guideline [36], we first identified 206,867 records throug tabase searching and then removed duplicates. Of the 159,014 records, we rem 130,697 because the title and keywords did not match the study's aim. By screenin abstracts, we excluded 27,072 records. Out of 1245 full-text articles assessed for eligib we excluded 1198 because they did not meet the eligibility criteria. There were four bility criteria in this review: (1) examining either protective or risk factors of e-ciga use, (2) measuring lifetime, current, or dual e-cigarette use as the outcome variabl published in English, and (4) targeting young people (9-25 years old) [37]. All records to be peer-reviewed journal articles and doctoral dissertations.
Researchers conducted a quality assessment for the selected studies to ensure "relevance, methodological rigor, and credibility" [38,39]. Each study was evalu based on the following questions: (1) "Is the research question clear and adequately stantiated?" (2) "Is the design appropriate for the research question?" (3) "Was the pling method appropriate for the research question and design?" (4) "Were data colle and managed systematically?" and (5) "Were the data analyzed appropriately?" [38] study was scored from one to five points, and only studies with more than four p were included in the analysis.
Moreover, to establish the same criteria and time frame for a lifetime, current dual e-cigarette use, we have defined lifetime use as "having ever used e-cigarettes, just once," current use as "the use of a tobacco product on at least one day in the pa days," and dual use as "the use of both products (i.e., cigarette and e-cigarette) withi same reporting period (e.g., the past 30 days)" [40]. After these stages, a total of 53 ar were chosen for analysis ( Figure 1) [1, 4,5,8,9,11,23,[28][29][30].

Coding and Analysis
Two raters individually reviewed each article by general characteristics (i.e., year published, study site (country), sampling method), prevalence, sample characteristics, gender differences, protective factors, and risk factors. The coding scheme referred to several other systematic reviews [81][82][83][84]. We organized the list of identified protective and risk factors into individual, family, peer, school, community, and societal levels, following the socio-ecological model (SEM) [85]. We further divided the individual level into three subsections, including status (e.g., academic competence), behavior (e.g., cigarette use), and perception (e.g., perception of the harms of e-cigarettes). If any disagreement arose during coding, the authors reviewed the matrix and discussed the issue to reach an agreement. The Kappa coefficient for the two raters was calculated as the interrater reliability; it was 0.846, indicating a high consistency. Regarding the pooled prevalence, all the prevalence and sample sizes reported in each study were entered into the Comprehensive Meta-Analysis (CMA) software. Because prevalence usually varies by sample size, we assumed that observed estimates would be different across studies; thus, the random-effects model was considered when estimating the pooled prevalence [86].

General Characteristics
The published studies included in our review were from 2014 to 2021. Out of the 53 studies, 13 (24.5%) were published in 2020, eight (15.1%) in 2016, seven (13.2%) in 2017 and 2019, six (11.3%) in 2015, and three (5.7%) in 2021. Approximately 73% of the selected studies used random sampling (n = 39), and 26.4% employed non-random sampling (n = 14), but a majority used random sampling methods to select participants. Researchers conducted the studies in 20 different countries. More than 50% of the studies (n = 30) were from the United States, and 7.5% were from South Korea (n = 4); Canada, Poland, and Sweden each represented 3.8% of the studies (each with n = 2). Only six studies (11.3%) were conducted in Asian countries, including South Korea (n = 4), Malaysia (n = 1), and Thailand (n = 1) (Table 1).

Prevalence
Except for six studies, all reported the prevalence of either lifetime or current e-cigarette use and current dual use. By country, the average lifetime e-cigarette use was 23.5% in the United States (out of 16 studies), 6.7% in South Korea (out of three studies), 23.7% in Sweden (out of two studies), and 21.3% in Poland (out of two studies). We estimated all other prevalence based on a single study: 34% in European countries, 20% in New Zealand, 17.3% in Scotland, 12.6% in Finland, 12.3% in Greece, 9.5% in Mexico, 7.6% in Argentina, 7% in Thailand, and 4.7% in Germany. We found the highest lifetime e-cigarette use in Sweden and the lowest in Germany. The international average of lifetime e-cigarette use under the random effect model was 15.3% (95% CI: 11.1-20.6%; Figure 2).
For current e-cigarette use, the average rate was 8.04% in the United States (out of 20 studies), 31.4% in Canada (out of two studies), 17.1% in Poland (out of two studies), and 1.7% in South Korea (out of two studies). We based all other estimates on a single study: 13.3% in the United Kingdom, 10.9% in Mexico, 9% in Malaysia, 6.7% in Thailand, 5.1% in Ireland, 4.2% in Sweden, 2.7% in European countries, and 2.8% in Greece. We found the highest current e-cigarette use in Canada and the lowest in South Korea. Compared with the WHO average (i.e., the WHO's estimated prevalence of e-cigarette use in each country), the rates of current e-cigarette use in this study were higher in the following countries: the United States (8.03% vs. 4.9%), Thailand (6.7% vs. 3.3%), the United Kingdom (13.3% vs. 6%), and Canada (31.4% vs. 10%). The rates of e-cigarette use were lower than the WHO average in Malaysia (9% vs. 9.8%), Poland (17.1% vs. 23.4%), South Korea (1.7% vs. 2.7%), and Sweden (4.2% vs. 8%). Based on these average rates, the international pooled prevalence for current e-cigarette use in this study was 7.7% (95% CI: 6.1-9.7%; Figure 3). For current e-cigarette use, the average rate was 8.04% in the United States (out of 20 studies), 31.4% in Canada (out of two studies), 17.1% in Poland (out of two studies), and 1.7% in South Korea (out of two studies). We based all other estimates on a single study 13.3% in the United Kingdom, 10.9% in Mexico, 9% in Malaysia, 6.7% in Thailand, 5.1% in Ireland, 4.2% in Sweden, 2.7% in European countries, and 2.8% in Greece. We found the highest current e-cigarette use in Canada and the lowest in South Korea. Compared with the WHO average (i.e., the WHO's estimated prevalence of e-cigarette use in each coun try), the rates of current e-cigarette use in this study were higher in the following coun tries: the United States (8.03% vs. 4.9%), Thailand (6.7% vs. 3.3%), the United Kingdom (13.3% vs. 6%), and Canada (31.4% vs. 10%). The rates of e-cigarette use were lower than   [4,5,11,19,[40][41][42][43][46][47][48][50][51][52]54,[56][57][58][59][60][61][62]64,65,67,[69][70][71]73,76,79,80]. Note 1. + Includes six European countries (Belgium, Finland, Germany, Ireland, Italy, Netherlands, Portugal). Note 2. National average is presented with the shape of a triangle and the total pooled prevalence as rhombus. pooled prevalence for current e-cigarette use in this study was 7.7% (95% CI: 6.1-9.7%; Figure 3).  Among seven studies that reported young people's current dual use, the average was 5.4% for the United States (out of six studies) and 3.3% for South Korea (out of three studies). We calculated all other dual-use prevalence based on a single study: 11% for the United Kingdom, 9.3% for Ireland, and 1.7% for Sweden. We found the highest dual use in the United Kingdom and the lowest in Sweden. Based on these numbers, the international average of dual use was 4.0% (95% CI: 2.5-6.5%; Figure 4). of a triangle, WHO average as a red box marked with 'X', and the total pooled prevalence as bus.
Among seven studies that reported young people's current dual use, the averag 5.4% for the United States (out of six studies) and 3.3% for South Korea (out of three ies). We calculated all other dual-use prevalence based on a single study: 11% f United Kingdom, 9.3% for Ireland, and 1.7% for Sweden. We found the highest du in the United Kingdom and the lowest in Sweden. Based on these numbers, the in tional average of dual use was 4.0% (95% CI: 2.5-6.5%; Figure 4). . Prevalence of current dual use by country [8,23,27,30,48,49,54,56,66,68,74,77]. Note tional average is presented with the shape of a triangle and the total pooled prevalence as rh bus.

Gender Differences
Of the 53 studies, 67.9% (n = 36) reported gender differences, and 32.1% (n = 1 not examine the lifetime or current e-cigarette use between males and females. Out 36 studies that examined gender differences, 22 (61.1%) reported statistically sign differences between males and females in e-cigarette use, four (11.1%) estimated the alence of e-cigarette use by gender but without statistical analysis, and 10 (27.7%) rep no significant difference. The 22 studies, which reported statistically significant ences, all indicated that males were more likely to use e-cigarettes than females o  . Prevalence of current dual use by country [8,23,27,30,48,49,54,56,66,68,74,77]. Note. National average is presented with the shape of a triangle and the total pooled prevalence as rhombus.

Gender Differences
Of the 53 studies, 67.9% (n = 36) reported gender differences, and 32.1% (n = 17) did not examine the lifetime or current e-cigarette use between males and females. Out of the 36 studies that examined gender differences, 22 (61.1%) reported statistically significant differences between males and females in e-cigarette use, four (11.1%) estimated the prevalence of e-cigarette use by gender but without statistical analysis, and 10 (27.7%) reported no significant difference. The 22 studies, which reported statistically significant differences, all indicated that males were more likely to use e-cigarettes than females or that being male increases the odds of smoking e-cigarettes. Males had a higher prevalence of lifetime e-cigarette use, current e-cigarette use, and dual use (Table 1).

Protective Factors
Twenty-four (45.2%) studies identified protective factors against young people's ecigarette use. This review found some of the common protective factors at the individual and family levels. For instance, at the individual level, the perceived cost and danger of vaping (n = 7), engaging in physical activities (n = 2), internal developmental assets (i.e., positive identity, empowerment, social competency; n = 2), and academic achievement (n = 1) played a buffering role against e-cigarette use. At the family level, parental monitoring (n = 3), a mother's higher education level (n = 2), and parental support (n = 2) reduced the likelihood of vaping (Table 1). Although school and community-level protective factors were examined only in a single study, it is worth noting that young people with high college aspirations and positive teacher engagement were less likely to use e-cigarettes [66,77]. Furthermore, those who did not see or know anyone using e-cigarettes or who lived in a place with high neighborhood cohesion were less susceptible to using e-cigarettes [40,69]. Table S1 (Supplementary Material) provides more information on an additional set of protective factors. In the United States, we found the perceived cost and danger of vaping as the common protective factors [1, 19,45,52,61].

Risk Factors
All the studies (N = 53, 100%) reported the risk factors for young people's e-cigarette use. Many studies indicated risk factors at the individual level, including cigarette use (n = 22), alcohol use (n = 16), marijuana use (n = 12), other substance/drug use (n = 9), sensation seeking (n = 6), externalizing problems (n = 5), internalizing problems (n = 4), poor academic performance (n = 4), sexual intercourse (n = 3), and perceiving e-cigarettes as less harmful (n = 3). Next, we noted family-level factors, such as having family members (e.g., parents, siblings, etc.) as smokers (n = 17) and having incarcerated family members (n = 2). At the peer level, we found friends smoking (n = 16) to be a risk factor, and at the school level, risk factors included attending school with a high portion of students participating in free and reduced lunch programs (n = 2). Furthermore, the presence of retailers near schools (n = 2) was associated with an increase in young people's vaping. Exposures to online commercials/advertisements on e-cigarettes and/or cigarettes (n = 6) also heightened the risk of e-cigarette use (Table 1). Table S1 (Supplementary Material) lists other significant factors. By country, we found marijuana and alcohol use to be significantly correlated with e-cigarette use for many participants in the US setting [1, 28,68,79]. In addition, two studies from South Korea confirmed that sexual intercourse was a significant factor in increasing the risk of vaping among Korean adolescents [8,48], and waterpipe and snus use were significant risk factors reported in northern European countries such as Sweden and Finland [5,58].  2015) did not report beta or odds ratio, but instead presented the M (SD) by groups and compare these numbers in four different user groups (e.g., Group 1 vs. Group 2). Only those variables that showed statistically significant difference in the e-cigarettes only group were included in this Table with their mean presented inside the parentheses.

Discussion
Considering these 53 studies, we estimated 15.3% as the overall pooled prevalence of international lifetime e-cigarette use, 7.7% for current use, and 4.0% for dual use among young people ranging from 9 to 25 years old. More than half of the studies reported gender differences, and males generally showed higher chances of lifetime, current, and dual ecigarette use than females. We found the perceived cost and danger of e-cigarettes, parental monitoring, the mother's education level, engaging in physical activity, parental support, internal developmental assets, and academic achievement as common protective factors. Moreover, we identified the following as common risk factors: friends smoking, cigarette use, alcohol use, marijuana use, family members smoking, sensation seeking, sexual intercourse, other substance/drug use, exposure to online commercials/advertisements, poor academic performance, perceiving e-cigarette as less harmful (compared to conventional cigarette use), family incarceration, and presence of retail stores nearby.
Most studies on young people's e-cigarette use were conducted in the United States. However, there is also a growing interest in Asian countries on this issue. Aligning with this trend, Chun [88] purported that tobacco use in Western countries is decreasing, but it is rising in many Asian countries; specifically, e-cigarettes are becoming more prevalent among young people in Asia. This alarms us that the prevalence of e-cigarettes may be increasing more broadly in Asian countries. However, one must acknowledge that there are distinctive smoking cultures in Asian nations [89], and the extent of e-cigarette use, sales, and importation policies vary across Asian countries [90,91]. Therefore, researchers should design more detailed studies in the future to gain culturally sensitive insights and interpretations of e-cigarette use among youngAsian, separate from Western cultures.
The international pooled prevalence of this review (lifetime: 15.3%, current: 7.7%) is similar to the prevalence of adolescent ENDS used by Yoong et al. [17], who reported the lifetime and current ENDS use at 16.4% and 5.6%, respectively. It is important to track how this trend changes over time and be aware that 15.3% of the world's young people have ever used an e-cigarette and that 7.7% of them are currently using e-cigarettes. This result suggests a need for active implementation of e-cigarette control and/or prevention programs for adolescents and emerging adults, separate from those preventing conventional cigarette use. For example, authorities should consider implementing a free program such as "This is Quitting," which is proven to be an effective e-cigarette cessation text message program, especially for adolescents and young adults [92].
We found that current e-cigarette uses in Canada, Thailand, the United Kingdom, and the United States was higher in this review than the WHO average. However, Malaysia, Poland, South Korea, and Sweden's prevalence were lower than the WHO average [16]. The prevalence of current e-cigarette use among young Americans in this review was consistent with Yoong et al.'s [17] systematic review (8.03% vs. 8.7%). However, we found some contrasting results concerning Canada (31.4% vs. 2.6%), the United Kingdom (13.3% vs. 1.0%), and Poland (17.1% vs. 29.9%) [17]. The methods used to estimate prevalence may account for the variances in current e-cigarette use across studies and reviews. According to the US National Institute of Mental Health [93] and Boyle [94], data collection timing, sample design, instruments, definitions, and analysis methods can change prevalence estimates. Therefore, we encourage researchers to obtain accurate estimates of e-cigarette use by choosing a representative sample from the target population, using probability sampling and validated instruments, and reporting confidence intervals [95]. If researchers do not meet any of these criteria in their studies, there is a danger of overgeneralizing results to the entire adolescent population in their countries. For instance, our study found that 96.2% of the studies did not include all age groups, but included only certain age groups (e.g., early, middle, and late adolescents). Setting a different age range for young people would make the results more difficult to interpret and may disable the comparison of the aggregated results in an international context. Thus, it is necessary to clarify and standardize the definition and criteria of young people. Further, we suggest including all age groups in the adolescent population to better represent the young population and to be able to generalize the results. In turn, we recommend that researchers present both their findings (e.g., prevalence) and the global or national prevalence to make an objective comparison of the result.
Regarding gender, approximately 70% of the studies reported gender differences and examined lifetime or current e-cigarette use between males and females. Specifically, the results found that males were more likely than females to have lifetime, current, and dual e-cigarette use. This finding is consistent with prior studies that reported significant gender differences in the prevalence and characteristics, patterns, awareness, reasons, and expectancies for e-cigarette use [96][97][98]. It further implies that e-cigarette use is a genderspecific behavior. Nonetheless, only a few studies have explored gender differences in e-cigarette use [96]. Thus, future studies should examine the role of gender in e-cigarette use to develop and implement effective prevention and intervention programs [97,98].
While the perceived cost and dangers of vaping were among the most recurrent protective factors in this review, the perception that cigarettes are harmful was also a risk factor. Other literature had also reported these contrasting stances [57]. For example, Amrock et al. [21] reported that 73% of adolescents believed that e-cigarettes were less harmful than cigarettes, and 47.1% thought they were less addictive than cigarettes. Perception of risks, benefits, and harm from e-cigarettes are frequently mentioned, possibly due to the underlying premise that e-cigarettes are a substitute or alternative to cigarettes. There remain conflicting assertions about whether an e-cigarette is a safer and better alternative to cigarette smoking. Some assert that e-cigarettes are effective for smoking cessation or reduction. Others purport that e-cigarettes can increase nicotine dependence and become a gateway product to other tobacco products [99]. Regardless of their effectiveness as an abstinence substitute, we cannot overlook the negative health impacts of e-cigarettes on young people [100]. Thus, we must alert the young population that e-cigarettes are not the safest alternative to smoking.
We found risk factors associated with young people's vaping, such as peers smoking and attending a school with a high proportion of students receiving free/reduced lunch. In this regard, school-based smoking cessation programs combining social competence and social influence components [101] could be effective in preventing or reducing ecigarette use of young people in school. Moreover, we found an association between socioeconomically disadvantaged neighborhoods and increased smoking [102], and an association among increased e-cigarette use, nearby retail stores, and communities with problems with alcohol and drugs. Hence, there should be a strict regulation of retail stores near schools and in the neighborhood area to ensure the safety of young people and to avoid them from using it. In other words, they need to be culturally responsive when designing the e-cigarette cessation programs at the neighborhood-or community-level [103].
In this systematic review, we found several risk factors at the societal level. These include exposure to e-cigarette commercials and marketing and an awareness of e-cigarettes through social media. This observation is consistent with previous findings that exposure to e-cigarette commercials and favorable perceptions of these commercials increased the use of e-cigarettes in adolescents [104]. E-cigarette companies actively engaging online, many young people can easily access information regarding e-cigarettes through various social media channels, which motivate them to search for vaping products without a careful consideration or validation [105][106][107]. Further, e-cigarette products are sold online with small health warning signs and a relaxed age verification process [107], which in turn increased the number of successful e-cigarette purchases by minors [108]. Currently, the restriction on sales of e-cigarettes-especially to those over a certain age-varies by country. For instance, the Tobacco & Vaping Products Act in Canada regulates the production, selling, labeling, and publicity of e-cigarettes sold in the country [109][110][111], and forbids the sale of e-cigarettes to young people under 18 years of age [112]. In addition, Canada also prohibits the advertisement of e-cigarette products that could be appealing to the youth [110]. In the US, the Prevent All Cigarette Trafficking Act and its recent amendment on Preventing Online Sales of E-Cigarettes to Children Act specifically banned the sales of cigarette products, including e-cigarettes, to minors [112][113][114]. Further, North Carolina's age verification law prohibits the use of e-cigarettes in minor groups [115]. The Food & Drug Administration in the US has also implemented regulations on tobacco products, including e-cigarettes, and restricts the minimum age of purchase [116,117]. However, e-cigarette products are on sale without a strict age verification process in South Korea because the Tobacco Business Act does not categorize e-cigarettes as tobacco products [114,118]. It is insufficient to simply restrict the minimum purchasing age to 18 years of age; at present, agerestricted online sales protocols place the responsibility for age cross-checking on vendors and delivery companies. According to Williams et al. [112], the loosened regulation of sales of e-cigarettes online allow minors to obtain e-cigarettes more easily. The government should thus implement or make necessary changes to the current tobacco laws in a way that strictly limits the purchase and use of e-cigarettes by minors [107,108]. Kim [114] also suggested a stronger age verification process and the development of a protocol for age-restricted online sales.
While systematic reviews on e-cigarettes focus on specific outcomes, such as weight gain or its subsequent impact on cigarette smoking [119,120], reviews on prevalence and precedent factors were limited. In this regard, the significance of the current study adds to the point whereby it systematically reviewed the protective and risk factors associated with young people's e-cigarette use in 20 different countries worldwide. Based on these results, future researchers and practitioners should have a clearer idea of what motivates and prevents these young people from using e-cigarettes. In addition, other reviews on the global prevalence of e-cigarettes were either limited to current prevalence or did not estimate dual prevalence. To overcome these limitations, this study updated the international pooled prevalence of lifetime, current, and dual e-cigarette use among young people and presented these results with a graphic representation.
However, the first limitation of the current study is concerned with the exclusion of eligible journal articles and doctoral dissertations not written in English, as well as master's theses. If we had also analyzed these sources, the overall results of the current study (i.e., prevalence, risk factor, protective factor) could have been slightly different. Second, e-cigarette keywords used to screen the relevant studies may not have been exhaustive. For example, researchers may have used the name of specific e-cigarette products and/or brands (e.g., JUUL) in their research articles, instead of the general terms. If we had considered all these keywords during screening, the scope of this study could have been more extensive. Third, prevalence may vary across years and age groups, but this study did not consider these potential variations when pooling. However, if published year and age groups are strictly limited to certain times and/or ages, the estimated prevalence may again lead to a biased result. Fourth, we tried to estimate the international prevalence and investigate major protective and risk factors by country. However, the results are still exploratory, given that some studies have their sample population hailing from certain regions of the country. Hence, future research must validate whether this prevalence and these distinct characteristics have fully reflected the current status of each nation. Finally, future research is suggested to separately estimate the prevalence for random and nonrandom sampling studies and perform meta-analysis in order to strengthen the rigor of the selected studies.

Conclusions
This study estimated the international pooled prevalence of young people's lifetime, current, and dual e-cigarette use and visualized these numbers in graphs. Based on what was found in this study, researchers would be able to address those who are more likely to use e-cigarettes and clarifies what is needed to protect adolescents from using e-cigarettes. Cigarette and e-cigarette use may share some common properties; however, there are also clear distinctions between these two products. The findings of this systematic review can be utilized as a foundational resource to enhance our understanding of e-cigarette use among young population.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/ijerph191811570/s1, Table S1: List of protective and risk factors associated with adolescent e-cigarette use, based on SEM.