If you suffer from COPD, you must stop smoking to prevent further lung damage. You cannot undo the damage done by smoking, but you can prevent further damage.
Smoking increases the likelihood of infection, cough and sputum production.
The nicotine and tar contained in cigarettes hinder the lungs' cleaning mechanism. As a result, the dirt breathed in from the air settles down in the lungs, and may lead to infection. The nicotine and tar are also irritants to the air tubes. This causes excess mucus production and thickening of the walls of the airways. As a result, the cilia that clean the respiratory airways cannot function properly, often resulting in infection.