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additional group of 30 people was drawn randomly from a group of cigarette smokers (n = 433). 2.2. Measures During the preparation of the study protocol, we analyzed the currently available data on nicotine addiction among e-cigarette users [23–25]. The questionnaire from the cross-sectional study was used to assess the behavioral aspects of cigarette and e-cigarette smoking and included 25 items (items and content are located in Supplementary Materials File S1) related to traditional and electronic cigarettes. The Fagerström test for nicotine dependence (FTND) [26] and its adapted versions for e-cigarettes [23,27] were used to measure nicotine dependence. The scores from each of the six questions of the FTND were summed and an overall total score for nicotine dependence was calculated. All the questionnaires were administered in person to each of the participants. Validity and reliability of the questionnaire was tested in a pilot study of 28 students. First, content validity was considered acceptable based on a literature review [23–25] and scientific discussion during a research team meeting and conference presentation. Second, reliability was assessed by having the students complete the survey, twice, five days apart. Cronbach’s alpha for the survey items ranged between 0.76 to 0.98. Participation in the study was voluntary and anonymous. The study protocol Int. J. Environ. Res. Public Health 2019, 16, 2279 3 of 10 was reviewed and approved by the Ethical Review Board at the Medical University of Silesia, Poland (consent number: KNW/0022/KB1/37/I/17). 2.3. Statistical Analysis Data were analyzed using Statistica 12 Software (TIBCO Software Inc., Palo Alto, CA, USA). Statistical significance of differences between continuous variables was analyzed using an independent samples t-test or a Mann-Whitney U test where appropriate. Statistical testing to compare categorical variables was completed using an independent samples chi-square test. We assessed strength of association by calculating odds ratios (OR). We used 95% confidence intervals (CI) and defined statistical significance as p < 0.05. 3. Results Of the adults included (39.8% female; mean age 22.4 ± 2.2 years old), the mean smoking duration was 50.0 ± 32.0 months among smokers and 67.3 ± 30.5 months among dual users (p = 0.03). Duration of e-cigarette use was comparable between exclusive e-cigarette users and dual users being 29.0 ± 24.1 and 27.7 ± 17.4 months, respectively (p = 0.6). There were no statistically significant differences in sex, age, or the daily number of cigarettes or e-cigarette smoking sessions between groups (p > 0.05). 3.1. Behavioral Aspects and Patterns of E-Cigarette Use Habits and behaviors associated with the use of e-cigarettes, including nicotine content in the e-liquid, e-liquid consumption, type of e-liquid used, and the number of e-cigarettes used, did not differ significantly (p > 0.05) between e-cigarette users and dual users. Users of e-cigarettes (n = 60) consumed an average of 4.2 mL of e-liquid per day, with the most frequently chosen e-liquid being that containing 6 mg of nicotine in 1 mL of e-liquid. None of the subjects used nicotine free e-liquids. The study population was dominated by individuals who prepared e-liquids themselves: 66.7% of e-cigarette users and 74.1% of dual users (Table 1). Over half (52.7%) of the e-cigarette users (53.6% of exclusive e-cigarette users and 51.9% dual users; p = 0.9) had chosen devices that allow technical modifications of the e-cigarette such as voltage, power, and resistance of the heater. Almost all exclusive e-cigarette users (96%) had ever tried cigarette smoking. Out of the 60 e-cigarette users, 18.5% initiated nicotine use by e-cigarette. E-cigarette, as a first nicotine product, was declared by 16% of exclusive e-cigarette users and 20.7% of dual users (p = 0.6). The most common reasons stated for starting to use e-cigarettes were it being less harmful to health, price, and desire to quit traditional smoking. The reasons stated for using e-cigarettes are presented in Figure 1. Figure 1. Motivations for e-cigarette use. Int. J. Environ. Res. Public Health 2019, 16, 2279 4 of 10 Table 1. Patterns and habits of e-cigarette use among e-cigarette users and dual users. Overall n = 60 Exclusive E-Cigarette Users n = 30 Dual Users n = 30 p * Average concentration of nicotine in the e-liquid [mg/mL] Mean ± SD 7.5 ± 3.8 6.8 ± 3.4 8.3 ± 4.0 0.1 Median (min–max) 6.0 (2–18) 6.0 (2–12) 6.0 (3–18) Milliliters of e-liquid used per day [mL] Mean ± SD 4.2 ± 3.4 4.1 ± 3.9 4.3 ± 3.1 0.8 Median (min–max) 3.0 (0.15–15) 3.0 (0.3–15) 3.5 (0.15–10) Milliliters of e-liquid used per month [mL] Mean ± SD 107 ± 102.2 113.9 ± 121.2 100.5 ± 81.9 0.6 Median (min–max) 90.0 (10–550) 90.0 (10–550) 70.0 (10–300) Type of e-liquid used Purchased in the store % (95%CI) 29.6 (19.1–42.8) 33.3 (18.6–52.2) 25.9