Cigarette smoking is the most common cause of chronic bronchitis.
Air pollution and allergies also may cause chronic bronchitis. Chronic bronchitis often can lead to emphysema if left untreated and exposure to lung irritants continues. In the early stages of chronic bronchitis a cough usually occurs in the morning. As the disease progresses, coughing persists throughout the day, eventually interfering with a person's quality of life. Mucous production in the lungs provides a good environment for infection, causing inflammation and swelling of the bronchial tubes, which result in a reduction in the amount of air flow in and out of the lungs and shortness of breath.