Acute bronchitis is generally caused by lung infections, 90% of which are viral in origin. Repeated attacks of acute bronchitis, which weaken and irritate bronchial airways over time, can result in chronic bronchitis.
Industrial pollution is another culprit. Chronic bronchitis is found in higher-than-normal rates in industries such as coal mining where workers are exposed to dust and fumes. But the main cause is heavy, long-term cigarette smoking, which irritates the bronchial tubes and causes them to produce excess mucus. The symptoms of chronic bronchitis are also worsened by high concentrations of sulphur dioxide and other pollutants in the atmosphere.