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245 different fields of study within Universities in Katowice. The questionnaire was addressed to all the students within each selected field of study. Data were collected from 3002 participants. Out of all the participants, we identified 39 exclusive e-cigarette users and 54 dual users. All subjects who used e-cigarettes (n = 93) were invited to participate in the second part of the study. Exclusion criteria included occurrence of any chronic diseases, history of lung diseases (e.g., asthma or bronchial hyperactivity in childhood), presence of any allergic diseases, any medication intake (current or in the last 2 weeks), acute illnesses or infections in the last 2 weeks, influenza vaccination in the last 2 weeks, and current pregnancy or lactation. Among exclusive e-cigarette users, three met the exclusion criteria and six refused. Finally, 30 exclusive e-cigarette users were included. Among dual users, four met the exclusion criteria. To keep an equal number of participants per group, out of 50 eligible dual users, we randomly selected a group of 30 dual users, using randomization procedures available in the Statistica package. An 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