Emphysema is a chronic obstructive disease of the lungs. The lungs contain millions of tiny air sacs called alveoli. In emphysema, the sacs lose their elasticity and air becomes trapped in the sacs. It becomes difficult to expel oxygen-depleted air from the lungs so the normal exchange of new and used air is diminished. Emphysema is classified as a chronic obstructive pulmonary disease (COPD).