Caption:
Illustration of chronic obstructive pulmonary disease (COPD), a common lung disease where breathing is difficult. There are two main forms of COPD: Chronic bronchitis, whose symptoms include a long-term cough with mucus) (inset left) and emphysema (the gradual breakdown of the lungs) (inset right). Most people with COPD have a combination of both forms. COPD is caused by noxious particles or gas, most commonly from tobacco smoking, which triggers an abnormal inflammatory response in the lung.