Changes in median exhaled CO levels from baseline to 12 weeks in quitters (n=183) and continuous smokers (n=94). Changes in median exhaled CO levels from baseline to 12 weeks in quitters (n=183) and continuous smokers (n=94). Decreases in exhaled CO levels from baseline to 12 weeks in quitters (−10.1±7.8) as well as continuous smokers (−9.0±11.2) were statistically significant (p<0.0001, respectively). Numerical data are presented as mean±SD. Hiromi Tomioka et al. BMJ Open Resp Res 2014;1:e000047 ©2014 by British Thoracic Society