Chronic obstructive pulmonary disease. Illustration of the bronchi (airways) in chronic obstructive pulmonary disease (COPD), a common lung disease where breathing is difficult. COPD is a combination of chronic bronchitis, the symptoms of which include a long-term cough with quantities of mucus (yellow) produced by the cells lining the airways (inset left), and emphysema (the gradual breakdown of the tissues of lungs). COPD is caused by noxious particles or gas, most commonly from tobacco smoking, which trigger an abnormal inflammatory response in the airways.