COPD, or chronic obstructive pulmonary PULL-mun-ary disease, is a progressive disease that makes it hard to breathe. “Progressive” means the disease gets worse over time. COPD can cause coughing that produces large amounts of mucus a slimy substance, wheezing, shortness of breath, chest tightness, and other symptoms. Cigarette smoking is the leading cause of COPD. Most people who have COPD smoke or used to smoke. Long-term exposure to other lung irritants—such as air pollution, chemical fumes, or dust—also may contribute to COPD.