FIGURE 1 Changes in expression levels of tumour necrosis factor (TNF)-α, transforming growth factor (TGF)-β1 and matrix metalloproteinase (MMP)-9 mRNAs and activity levels of MMP-9 in the upper airways of study participants. a) Relative expression levels of TNF-α, TGF-β1, and MMP-9 mRNAs in nasal epithelial cells (NECs), and relative activity levels of MMP-9 in nasal epithelial lining fluid (ELF), collected from smokers who abstained from smoking with the aid of nicotine-replacement therapy or varenicline (NRT/VAR group) measured at baseline (before) and after a 12-week period of successful abstinence from combustible cigarettes (after). b) Relative expression levels of TNF-α, TGF-β1 and MMP-9 mRNAs in NECs, and relative activity levels of MMP-9 in nasal ELF, collected from smokers who continued to smoke combustible cigarettes measured at week 5 (before) and week 16 (after). c) Relative expression levels of TNF-α, TGF-β1, and MMP-9 mRNAs in NECs, and relative activity levels of MMP-9 in nasal ELF, collected from smokers who switched exclusively to electronic cigarettes (e-cigarettes) measured at week 5 (before) and week 16 (after). Measurements from some participants were not included because mRNA levels were below the limit of detection. Bars show mean values. All data were analysed using Prism software (GraphPad, San Diego, CA, USA). ns: not significant. *: p<0.05 by Wilcoxon test.