Bronchiolitis obliterans has many possible causes, including: collagen vascular disease, transplant rejection in organ transplant patients, viral infection (RSV, adenovirus, PCP, HIV, CMV), drug reaction, aspiration and complications of prematurity (bronchopulmonary dysplasia), and exposure to toxic fumes, including: diacetyl, sulfur dioxide, nitrogen dioxide, ammonia, chlorine, thionyl chloride, methyl isocyanate, hydrogen fluoride, hydrogen bromide, hydrogen chloride, hydrogen sulfide, phosgene, polyamide-amine dyes, and ozone. Additionally, the disorder may be idiopathic (without known cause). There are many industrial inhalants that are known to cause bronchiolitis.