Respiratory diseases—diseases of the lungs and airways such as asthma, chronic bronchitis, emphysema, and lung can cer—have diverse causes. While cigarette smoking is the leading cause of most major respiratory diseases, air pollutants and workplace toxins can also contribute to respiratory illness.

The lungs are equipped with an elaborate defense system to repel toxins and invading organisms. Before air reaches the lungs, it passes through the nose, throat, and bronchi that are lined with mucus to trap irritants. Within the bronchi, smaller bronchial tubes and bronchioles are covered with cilia that sweep particles out. Lungs are far from vulnerable, however. The defenses can fail, leading to any number of respiratory diseases.

Cigarette smoking is the most important cause of Chronic Obstructive Pulmonary Disease (COPD), a name covering pulmonary emphysema and chronic bronchitis. These lung diseases damage the air passageways and interfere with the lung's...