Chronic Obstructive Pulmonary Disease (COPD) involves two diseases that are related; bronchitis and emphysema. Both bronchitis and emphysema involve a chronic obstruction of airflow out of a person's lungs which is commonly both permanent and progressive over time.

Asthma; another pulmonary disease involving obstruction of airflow out of the lungs, is unlike chronic bronchitis or emphysema in that asthma is commonly reversible. Airflow between asthma attacks is usually good.