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Flow chart of selection of participants in the present study. A total of 28,059 participants of the Korean National Health and Nutrition Examination Survey (KNHANES) from 2013 to 2017 were classified into the following three groups; non-smokers, smokers (current smokers and ex-smokers of combustible cigarettes only), and e-smokers (current smokers and ex-smokers of e-cigarettes regardless of combustible cigarette use).
