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Bronchiolitis obliterans (BO), also called obliterative bronchiolitis (OB) and constrictive bronchiolitis (CB), is a rare and life-threatening form of non-reversible obstructive lung disease in which the bronchioles (small airway branches) are compressed and narrowed by fibrosis (scar tissue) and/or inflammation. Bronchiolitis obliterans is also sometimes used to refer to a particularly severe form of pediatric bronchiolitis caused by adenovirus.
Bronchiolitis obliterans is a lung disease characterized by fixed airway obstruction. Inflammation and scarring occur in the airways of the lung, resulting in severe shortness of breath and dry cough.
FEV1 (forced expiratory volume in 1 second) should be above 80% of predicted values to be considered normal. Bronchiolitis obliterans reduces this to 16% to 21%.
Bronchiolitis obliterans has many possible causes, including: collagen vascular disease, transplant rejection in organ transplant patients, viral infection (respiratory syncytial virus, adenovirus, HIV, cytomegalovirus), Stevens-Johnson Syndrome, Pneumocystis pneumonia, 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, mustard gas and ozone. It can also be present in patients with rheumatoid arthritis. Certain orally administrated emergency medications, such as activated charcoal, have been known to cause it when aspirated. Additionally, the disorder may be idiopathic (without known cause).
There are many industrial inhalants that are known to cause various types of bronchiolitis, including bronchiolitis obliterans. 
Industrial workers who have presented with bronchiolitis:
In rare instances, bronchiolitis obliterans may be caused by inhalation of airborne diacetyl, a chemical used to produce the artificial butter flavoring in many foods such as candy and microwave popcorn and occurring naturally in wines. This first came to public attention when eight former employees of the Gilster-Mary Lee popcorn plant in Jasper, Missouri, developed bronchiolitis obliterans. In 2000, the Missouri Department of Health called in NIOSH to make a determination of the cause, and to recommend safety measures. After surveying the plant and each patient's medical history, NIOSH recommended respiratory protection for all workers in microwave popcorn production. Due to this event, bronchiolitis obliterans began to be referred to in the popular media as "Popcorn Lung" or "Popcorn Workers Lung".
In 2007 a heavy consumer of microwaved popcorn was diagnosed by a doctor in Denver with "popcorn lung," the first known case involving a consumer. On 16 January 2008, it was announced that Wayne Watson, the Denver consumer who developed "popcorn lung" after inhaling fumes from microwaved popcorn, was suing the Kroger grocery store chain and its affiliates. In the lawsuit, filed in U.S. District Court, Watson's attorney claimed that the companies "failed to warn that preparing microwave popcorn in a microwave oven as intended and smelling the buttery aroma could expose the consumer to an inhalation hazard and a risk of lung injury." On September 19, 2012 a jury in U.S. District Court in Denver awarded $2.3 million in actual damages and $5 million in punitive damages to Wayne Watson of Centennial, Colorado who habitually consumed microwave popcorn for a decade. Defendants included Glister-Mary Lee, the manufacturer; Kroger, and Kroger's subsidiary Dillons, owners of King Soopers & City Market, a Colorado regional supermarket chain.
In September 2007, Dr. Cecile Rose, pulmonary specialist at Denver's National Jewish Medical and Research Center, warned federal agencies that consumers, not just flavoring or food factory workers, may be in danger of contracting bronchiolitis obliterans. David Michaels, of the George Washington University School of Public Health, first published Rose's warning letter on his blog.
On 4 September 2007, the Flavor and Extract Manufacturers recommended reduction of diacetyl in butter-like flavorings. The next day ConAgra Foods announced that it would soon remove diacetyl from its popcorn products.
Diacetyl is approved by the Food and Drug Administration as a safe flavor ingredient, but there is evidence to suggest that inhalation in large amounts is dangerous. There are currently no warnings from federal regulators about diacetyl.
Diagnosis may include the following tests:
This disease is irreversible and severe cases often require a lung transplant. Evaluation of interventions to prevent bronchiolitis obliterans relies on early detection of abnormal spirometry results or unusual decreases in repeated measurements.