Tobacco smoking results in inflammatory, sometimes fibrotic, lesions of the membranous (terminal) bronchioles and respiratory bronchioles, referred to as membranous bronchiolitis and respiratory bronchiolitis, respectively. The extent and severity of these lesions vary widely, and they may be accompanied by emphysema, chronic bronchitis, or interstitial lung disease in the form of respiratory bronchiolitis-associated interstitial lung disease or desquamative interstitial pneumonia. More often, the lesions are unaccompanied by clinically significant findings, although physiologically, measurements of FEV₁, FEF_25%-75%, or FEF_75% in smokers may indicate small-airway obstruction.

Clinically significant small-airway obstruction is often referred to as small-airway disease, but FEV₁, FEF_25%-75%, and FEF_75% measurement cannot distinguish between proximal (membranous or terminal bronchiole) and distal (respiratory bronchiole) obstruction. In patients with clinical chronic obstructive pulmonary disease, the severity of disease and obstruction appears to correlate with the severity of bronchiolitis.