A lung condition that develops over time in which the bronchi (large air passages that lead to the lungs) become inflamed and scarred. This causes the bronchi to make large amounts of mucus and can lead to a chronic cough and breathing problems. The most common cause of chronic bronchitis is cigarette smoking. It may also be caused by infection or by breathing in secondhand tobacco smoke, chemical fumes, or other forms of air pollution. Chronic bronchitis usually does not go away completely. It is a type of chronic obstructive pulmonary disease (COPD).