We found that the differences in DNA methylation between current and former smokers was lower compared to the difference between current and never smokers in 10 of the 15 CpG sites. This suggests that the effect of tobacco smoking on DNA methylation of these CpG sites is relatively sustained after smoking cessation. To explore the time needed for these CpG sites to recover, we did an analysis on methylation and cessation time and found that cessation time was associated with DNA methylation in 3 of the 15 CpG sites. These results are in line with findings from a recent paper on the relation between tobacco smoking cessation and DNA methylation [6]. We further found that 2 of the 15 CpG sites were associated with cumulative tobacco smoking exposure in current smokers, which is concordant with results from previous studies [6, 7].
