Results in the alveolar walls disintegrating, leading to large air spaces that do not close during exhalation (see figure below). The resultant reduction in surface area for gaseous exchange and accompanying lung elasticity make exhalation a very difficult task. Clear-cut causes of emphysema are long-term irritation of the air passages from, for example, smoking and other air pollution. The effect of cigarette smoke is to deactivate the enzyme I-antitrypsin, which under normal circumstances inhibits the protease (degenerate elastic tissues) that attack the alveoli.
