This leads to lots of mucus and thus narrowing of the tubes. Smokers may consider this to be a normal smokers' cough, but there is nothing normal about it. As time goes by, this productive cough increases. Later on, wheezing and breathlessness become a problem. If you are breathless when you exert yourself, you probably have significant lung damage.
Smoking is the main cause.
People who work in dusty atmospheres are also at risk. Air pollution is a minor factor. At first the bronchitis gets worse with bad colds or influenza, but eventually even a mild cold can bring on a nasty flare-up. Colds or other infections can cause deterioration, especially in winter. However, chronic bronchitis is not caused by chronic infection.