(a) k13< 2.6×10−2 ml/(cell day) corresponds to resistant smokers (TD<30%), while k13≥2.6×10−2 ml/(cell day) is associated with susceptible smokers. (b) Effects of smoking cessation after 2500 days of CS exposure. 2.6×10-2ml/(cell day) ≤ k13 < 0.31ml/(cell day) corresponds to reversible susceptible smokers and COPD is reversible. k13≥0.31 ml/(cell day) is associated with severely susceptible smokers. In this case, COPD is not reversible.