Frequent coughing is a common symptom of hyperventilation and in particular, habitual mouth-breathing. The cough may be the result of post-nasal drip or cough variant asthma. If it is the result of a respiratory infection the sputum will be a yellow-greenish in colour.
A chronic cough may be symptomatic of a more serious health condition such as COPD and it is advisable that you consult with your doctor. Smokers cough because the tar from inhaled smoke damages the hair-like cilia lining the mucous membranes of the airways so that they are unable to clear the lungs naturally.
A persistent cough is also a side-effect from a class of drugs known as ACE inhibitors used in the treatment of high blood pressure. Consult your prescribing doctor if you suspect this to be the case.