Chronic obstructive pulmonary disease (COPD), one of the most common public health problems worldwide, is the fourth leading cause of death in the world. In China, respiratory diseases (of which COPD is a major component) are the third leading cause of death in rural areas and the fourth leading cause of death in urban area, accounting for 1 million deaths and over 5 million disabilities each year. COPD ranks first among the burdens in China.~1