Long-term smoke causes chronic airway inflammation in patients with COPD, stimulating airway mucosa to produce TGF-β and MMP-9, which leads to protease/anti-protease imbalance, alveoli destruction, hydrolyzed alveolar elastic fibers, alveolar fusion, and finally emphysema (Vlahos and Bozinovski, 2014). Here, MMP9 is linked to chronic obstructive pulmonary disease.