Answer
Ciliated cells of the respiratory tract are damaged by smoking. In the non-smoker, cilia of these cells and mucus normally trap particles and bacteria and move them towards the pharynx where they can be expelled by expectoration.
Work Step by Step
Because this mechanism is impaired in smokers, coughing serves the purpose of forcibly bringing up bacteria and dust particles from the trachea and preventing them from doing damage by reaching the lungs.
