Emphysema is a long-term (chronic), irreversible lung disease that occurs when the tiny air sacs in the lungs are damaged and lose their stretch, usually as a result of long-term smoking. It causes difficulty breathing and shortness of breath that often gets worse over time. Emphysema is usually a type of chronic obstructive pulmonary disease (COPD).

A less common type of emphysema is caused by the lack of a substance in the lungs called alpha-1 antitrypsin. This type of emphysema is usually inherited.