I agree with the above answer. I would like to add a few thoughts. Smoking is a constant irritant to the mucous membrane of the respiratory tissue. This is one reason smokers suffer from illnesses like chronic bronchitis more so than nonsmokers. Additionally, in the disease of COPD, chronic obstructive pulmonary disease, the cilia in the trachea become damaged over time from the smoke. The purpose of the cilia that line the trachea is to help the person expectorate mucus particles that line the mucous membrane. When the cilia are damaged, they can't effectively help push the mucus up towards the mouth. This series of events leads to a trapping of mucoid material in the lower airways and causes obstruction of the airways. Hence the word obstructive in COPD. Note the two different spellings......mucous is the membrane and mucus is the material expectorated.