What Causes Mucus To Form In Bronchial Tubes?

What Causes Mucus To Form In Bronchial Tubes? Mucus buildup in the lungs can be cased by infection, gastroesophageal reflux disease (GERD), smoking, cystic fibrosis, allergies, bronchiectasis, and chronic obstructive pulmonary disease (COPD). What is bronchial mucus? The walls of the bronchi produce mucus to trap dust and other particles that could otherwise cause irritation.