Less commonly it has been reported in person with a variety of exposures to industrial or environmental pollutants. In most cases, patients are asymptomatic.
Symptomatic patients are usually young and there is no reported sex predominance.
The most common symptom is dyspnea of 1-2 years' duration, typically associated with cough. Pulmonary function tests shows mixed mild obstructive and restrictive physiology. The prognosis is good. Progression to fibrosis, respiratory failure, or death has not been reported. Respiratory bronchiolitis is characterized by PAS-positive macrophages containing brown pigment that fill the respiratory bronchioles and adjacent alveolar ducts and sacs.