COPD is a disease in which the lung airways are partly damaged and obstructed, making it difficult to breathe. The most common cause is cigarette smoking, but breathing in other types of lung irritants, including pollution, dust, and chemicals, over a long period of time may also contribute to COPD. It is the fourth leading cause of death in the United States. Symptoms include coughing, excess mucus production, shortness of breath, wheezing, and chest tightness.