Cough without phlegm is known as dry cough. It is actually a defense mechanism employed by the body to clean the airway from substances which are causing a hindrance in breathing effectively. When dry cough has been persistent for a long period of time, it may cause the lungs to produce more phlegm. The causes of this condition are varied and range from viral and bacterial infection to foreign body obstruction in the airway, smoking, congestive heart failure, etc.