Bronchiolitis obliterans is an inflammatory obstruction of the lung's tiniest airways, called bronchioles. The bronchioles become damaged and inflamed by chemical particles or respiratory infections, particularly after organ transplants, leading to extensive scarring that blocks the airways. The disease is sometimes referred to as constrictive bronchiolitis, a similar condition in which the small airways become constricted in diameter because of inflammation and scarring.
The disease can be caused by breathing in irritant fumes, such as chlorine, ammonia, oxides of nitrogen or sulfur dioxide. Diacetyl, a chemical used to provide butter flavor in many foods, has also been suspected of causing bronchiolitis obliterans in workers who manufacture it or mix it into foods, such as butter-flavored popcorn. Bronchiolitis obliterans also can result from respiratory infections, a connective tissue disorder such as rheumatoid arthritis, a medication reaction, and after a bone marrow, lung or heart-lung transplant. Also, the disease may be idiopathic (without a known cause).
Another similarly named disease, bronchiolitis obliterans organizing pneumonia, is a completely different disease.