The main cause of chronic bronchitis is smoking. Smoke and sometimes other air pollutants can irritate the airways, causing them to swell and produce mucus. The swelling makes the inside of the airways become smaller. The airways become blocked by the mucus, making it hard for air to pass in and out of your lungs. This causes wheezing and trouble breathing. Chronic bronchitis slowly gets worse, gradually reducing your ability to breathe.
Frequent bacterial infections of the upper respiratory system can also cause chronic bronchitis. The upper respiratory system includes the nose, sinuses, voice box (larynx), and the trachea (windpipe).