Smoke is toxic. Both the heat of the smoke and the chemicals within it irritate the linings of the respiratory tract. It takes 10 years or more to produce chronic bronchitis in most smokers.
There . There are hundreds of chemicals in smoke that irritate and inflame cells in the airway. This leads to spasm of airway muscles and an influx of inflammatory cells that lead to chronic cough. In addition there is an increase in the number of mucus producing cells that causes increased phlegm production. If you have a cough on most days of most months then that defines chronic bronchitis. This develops at various rates and times of cumulative smoking, not all people have the same susceptibility to the effects of cig smoking. 10 pack years (1 pack a day x 10 years) is an average amount however.