Chronic bronchitis occurs when the lungs are constantly exposed to an irritant in the air. This long-standing exposure causes the airways to swell and produce mucus, leading to the symptoms of bronchitis. Smoking is the most common irritant that causes chronic bronchitis.

Smoking destroys the cells that line the bronchial tubes. It interferes with the immune system's ability to fight infection. Smokers also have difficulty getting rid of mucus from their lungs. This makes bacterial lung infections more likely. Eventually, smoking destroys lung tissue.

Chronic obstructive pulmonary disease, or COPD, occurs when smoke or irritants cause permanent damage to the airways. This causes an obstruction to air entering (during inhalation) and leaving (during exhalation) the lungs. Chronic bronchitis is usually present in those who have COPD.

Other irritants that cause chronic bronchitis include:

• Mine dust: black lung
• Air pollution
• Chemical fumes
• Silicosis
• Asbestosis

Additional illnesses that cause chronic bronchitis include:

• Asthma
• Cystic fibrosis