Emphysema  is an abnormal condition of the lungs in which the walls of the alveoli are over-inflated and distended and changes in their structure occur. This produces large air-filled spaces that do not contribute to the respiratory process.

Acute cases can be caused by whooping cough, tuberculosis or bronchopneumonia, and chronic cases often accompany chronic bronchitis, which itself is caused in great part by tobacco smoking. In elderly people, the alveolar membrane may collapse, producing large air sacs, with decreased surface area.