Although excess mucus in the lungs can have various causes, it is usually a defense response to infections or irritants, states the American Lung Association. If excess mucus persists for more than a month, the cause could be lung disease.
Various lung diseases that cause excess mucus are asthma, COPD (emphysema and chronic bronchitis) and lung cancer, explains the American Lung Association. Cystic Fibrosis can also cause a mucus build up in the lungs, although cystic fibrosis is a disease affecting all mucus membranes in the body, not only the mucus membranes the lungs, states Mayo Clinic.
Although asthma is a disease that inflames the airways, it also causes mucus, according the American Lung Association. When a person with asthma encounters irritants in the air or other triggers, the airways swell and create excess mucus.
COPD is also responsible for excess mucus, as inflamed airways secrete mucus, states the Respiratory Health Association. However, unlike asthma, COPD is often a result of too much smoking, as it reduces the lungs' defenses, narrows air passages, causes swelling in air tubes and destroys air sacs. Chronic bronchitis and pneumonia infections (which cause mucus in the lungs) can also be a sign of lung cancer, explains the American Lung Association.
The appearance of the mucus differs depending on the type of disease, states Everyday Health. For example, mucus with a either a yellow or greenish color usually indicates infection, such as one caused by pneumonia or bronchitis. However, the color can also be indicative of the irritants in the air.