Chronic and excessive phlegm is a possible indication of chronic obstructive pulmonary disease, or COPD, according to the American Thoracic Society. It is also a sign of cystic fibrosis, says Mayo Clinic.
Normally, the lungs produce phlegm to clear the lungs of irritants, claims the American Thoracic Society. Smoking and constant exposure to pollutants destroy the cilia that line the airway. These cilia also help clean out the bronchial passages. When too many of them are destroyed, the phlegm that has built up cannot be cleared out properly.
In cystic fibrosis, the phlegm is unusually thick and sticky, claims Mayo Clinic. This clogs the air passages in the lungs and causes complications such as breathlessness and susceptibility to infection.