Chronic Obstructive Pulmonary Disease (COPD) is a isease of the lungs where the airways become narrowed. This leads to a limitation of the flow of air to and from the lungs causing shortness of breath. In contrast to asthma, the limitation of airflow is not fully reversible and usually gradually gets worse over time. COPD is associated with an abnormal inflammatory response of the lung to noxious particles or gases, most commonly tobacco smoke. The inflammatory response in the larger airways is known as chronic bronchitis which often results in people with COPD regularly coughing up sputum or mucus. In the smaller airways, the inflammatory response causes small airways inflammation. In the alveoli, the inflammatory response causes destruction of the tissue of the lung, a process known as emphysema. Individuals with COPD vary in the way the disease affects their lungs. Some have mostly chronic bronchitis, others have mostly emphysema and many have a mixture of inflammatory responses.