How are chronic bronchitis and emphysema related to smoking?

Tobacco smoking is the primary cause of both chronic bronchitis and emphysema, two conditions that frequently coexist and are collectively referred to as chronic obstructive pulmonary disease (COPD).123

Chronic Bronchitis
Smoking accounts for over 90 percent of chronic bronchitis cases. The habit causes the mucous glands in the large airways to increase in size and number, leading to the production of excessive, viscous mucus and a persistent, deep cough. Over time, this chronic inflammation causes irreversible damage to the bronchial walls, narrowing the passages and obstructing airflow.14

Emphysema
Smoking is also the most common cause of emphysema, which involves the destruction of the alveoli (gas-exchanging tissues) and the loss of elastic lung tissue. This damage reduces the surface area available for oxygen exchange and causes the lungs to expand, which further compresses the small bronchi and increases resistance to breathing.2

While some individuals may smoke for decades without severe symptoms, others experience rapid respiratory decline. Genetic factors, such as a deficiency in the enzyme alpha-1 antitrypsin, can significantly increase a smoker's susceptibility to severe emphysema.42