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biochemically. Measurement of nicotine in blood or cotinine in urine or saliva could provide more detailed information on nicotine dependence and exposure to cigarette smoke. However, self-declared smoking statuses were confirmed by measured levels of exhaled carbon monoxide (CO), which has been described in another article [35]. The daily number of cigarettes smoked by the smokers group was lower compared to the general population. However, we are aware that our study is limited to a group of young, educated adults and the smoking habits in such a group may differ compared to the general population, meaning our results should not be generalized to the whole population of e-cigarette users. Moreover, follow-up studies are needed to examine whether the high Fagerström scores obtained in our study translate into a high risk of long-term e-cigarette use. This will also allow us to better understand the natural history of smoking and e-cigarette use as well as dual use. E-cigarettes are promoted as a safer alternative to traditional cigarettes. This view is also shared by e-cigarette users [8,36]. Exclusive e-cigarette users perceive e-cigarettes to be less harmful than people Int. J. Environ. Res. Public Health 2019, 16, 2279 8 of 10 using traditional cigarettes or dual users [36]. Studies performed in New Zealand and Germany have shown that there is a public belief that e-cigarettes can help in quitting smoking [37,38]. Our findings suggest that the use of e-cigarettes carries a great risk of addiction and the risk of addiction may be even higher than that from smoked tobacco cigarettes, at least among young adults. The higher Fagerström scores among dual users could suggest that these individuals are at particularly high risk of addiction. In jurisdictions adopting a harm-reduction approach to tobacco control, balanced regulatory approaches may be needed that seek to minimise the availability and use of e-cigarettes among adolescents and young adults, whilst ensuring access to established smokers who wish to use e-cigarettes to help quit or as substitutes for cigarettes. 5. Conclusions In conclusion, in this work the use of e-cigarettes among young adults was shown to result in higher nicotine dependence levels than nicotine dependence related to tobacco cigarette use. In addition, a stronger dependence on the e-cigarette compared to the traditional cigarette identified in dual users indicates that e-cigarettes may be highly addictive. Further research is necessary to provide evidence on the role of e-cigarettes in smoking cessation, especially in young adults. Social normalization of smoking behaviors by allowing e-cigarette use should be monitored. Supplementary Materials: The following are available online at http://www.mdpi.com/1660-4601/16/13/2279/s1. File S1: Study questionnaire. Author Contributions: M.J.: M.K., J.E.Z., and G.B. conceived of the survey study and its design, as well as questionnaire development. P.M. and J.L. were responsible for crude data collection and interpretation. M.J., M.K., and G.B. were the authors of the conception of current research questions addressed within the article. J.E.Z., M.J., J.A.L., and G.B. were responsible for the methodology of advanced data management and statistical analysis. M.J., M.K., J.E.Z., J.A.L., and G.B. worked on the manuscript. J.A.L. was responsible for linguistic correction and adjustment of the manuscript. All authors read and approved the final manuscript. Funding: The study was funded by a scientific grant from the Medical University Silesia (KNW-640-2-1-158/19). Conflicts of Interest: The authors declare no conflict of interest. References 1. Benowitz, N.L. Nicotine addiction. N. Engl. J. Med. 2010, 362, 2295–2303. [CrossRef] [PubMed] 2. Kandel, D.B.; Hu, M.C.; Griesler, P.C.; Schaffran, C. On the development of nicotine dependence in adolescence. Drug Alcohol Depend. 2007, 91, 26–39. [CrossRef] [PubMed] 3. McLaughlin, I.; Dani, J.A.; De Biasi, M. Nicotine withdrawal. Curr. Top. Behav. Neurosci. 2015, 24, 99–123. [PubMed] 4. Goriounova, N.A.; Mansvelder, H.D. Short- and long-term consequences of nicotine exposure during adolescence for prefrontal cortex neuronal network function. Cold Spring