What Causes Chronic Prolonged Coughing?

Hi, it is about 12 years that I am coughing. After examining by lung specialists doctors, CT scanning differe prescribing different kinds of medicines such as Airokast, symbicort, fluimucil, beclex, nasocrom, felazon, cimetidine and etc. Finally my cough did not cured. Of course they say that my lungs are healthy. My coughs are usually very severe in the mornings. I have used many kinds of anti acid medicines for preventing my reflex, but all of them could not treat my reflex ( if I had nay reflex). Of course any smell, such as the smell of hot and fresh bread, the smell of new newspaper and the smell of frying onion will cause coughing. Awaiting to receive your useful guidance. Thanks and best regards. A.N. Fard.