Reduced Expiratory Flow in Young Adult Vapers and Smokers: A Cross-Sectional Study

Background/Objectives: Differences in respiratory function associated with e-cigarette use in young adults remain incompletely characterized, particularly in comparison with conventional smoking and dual use. This study aimed to compare resting spirometric function among young adults according to smoking and vaping status, with particular attention to expiratory-flow indices. Methods: A cross-sectional study was conducted in 99 adults aged 18–29 years, classified as Controls (n = 29), exclusive Vapers (n = 21), Tobacco smokers (n = 22), or Dual users (n = 27). Resting forced spirometry was performed using a MIR SpiroLab device. Seven core respiratory outcomes were evaluated: FEV1, FEV1/FVC, PEF, FEF25, FEF50, FEF75, and FEF25–75. Between-group differences were assessed using one-way ANOVA followed by Tukey’s HSD test, with Benjamini–Hochberg false discovery rate correction. Sensitivity analyses were performed using linear models adjusted for age, sex, muscle mass and visceral fat. Results: All seven core spirometric outcomes showed significant global group effects after false discovery rate correction. Pairwise reductions relative to Controls were most consistent among Vapers and Dual users, whereas FEF75 and FEF25–75 were also significantly reduced in conventional cigarette smokers. FVC did not differ significantly between groups. In sensitivity analyses, the overall group effect remained significant for all seven core outcomes after adjustment for age, sex, muscle mass, and visceral fat. Conclusions: Significant between-group differences in expiratory-flow indices were observed according to smoking and vaping status, whereas FVC remained similar across groups. Given the cross-sectional design and potential residual confounding, these findings should be interpreted as group-level associations and do not establish that smoking or vaping caused the observed spirometric differences.