Chronic bronchitis is characterized by a recurrent or chronic productive cough for a minimum of 3 months/year for at least 2 years. It is caused by physical or chemical irritants and recurrent lung infections. Cigarette smoking is by far the most common cause of chronic bronchitis. Workers exposed to dust, such as coal miners and grain handlers, also are at higher risk. The underlying process is an impairment of cilia, so they can no longer move secretions. Mucous gland hypertrophy causes hypersecretion, altering cilia function. Excessive mucus is trapped in edematous airways, obstructing airflow. The lining of the bronchial tubes becomes inflamed and eventually scarred. The patient cannot clear tenacious mucus and it becomes a medium for bacteria and infection. This increased airway resistance leads to bronchospasm. The condition results in altered oxygen–carbon dioxide exchange, hypoxia, and hypercapnia.