Bronchiolitis obliterans is a serious lung condition in which the smallest airways, called bronchioles, become scarred and narrowed, permanently restricting airflow. Sometimes called “popcorn lung,” it is irreversible and can range from mild breathlessness to severe respiratory disability. It develops after the lungs are exposed to a damaging trigger, whether that’s a chemical inhalation, an infection, or an immune reaction following an organ transplant.
What Happens Inside the Lungs
Your lungs branch into progressively smaller tubes, ending in tiny passages called bronchioles that deliver air to the gas-exchanging tissue. In bronchiolitis obliterans, inflammation damages the walls of these bronchioles, and the body’s repair process goes wrong. Instead of healing normally, scar tissue (collagen) builds up beneath the lining and around the outer walls of the airways. This fibrotic scarring thickens the walls, narrows the opening, and in some cases completely blocks the airway.
The scarring doesn’t hit every bronchiole equally. Studies of affected lung tissue show that collagen buildup occurs in roughly 29% of bronchioles beneath the lining and about 16% in the tissue surrounding the airways. But even partial involvement is enough to cause significant airflow obstruction because the bronchioles are so small that even modest narrowing dramatically increases resistance to airflow. Other changes include overgrowth of the smooth muscle around the airways, distortion of the airway shape, and mucus plugging. Together, these changes lock the lungs into a state of chronic obstruction that doesn’t respond to the inhalers typically used for asthma or COPD.
Common Causes and Risk Factors
The condition has several well-established triggers, grouped loosely into chemical exposures, immune-related injury, and infections.
Chemical and Occupational Exposure
The “popcorn lung” nickname comes from a cluster of cases among workers at a microwave popcorn factory in the early 2000s. The culprit was diacetyl, a butter-flavoring chemical that, when heated and inhaled as a vapor, damages bronchiolar tissue. Its chemical substitute, 2,3-pentanedione, carries a similar risk. These chemicals show up across the food-production industry, not just popcorn. Workers in bakery mix manufacturing, flavored coffee production, and facilities that produce strawberry, caramel, vanilla, and butterscotch flavorings have also been affected. NIOSH has flagged over a thousand flavoring chemicals as potential respiratory hazards, though diacetyl remains the best-studied offender. Other industrial chemicals linked to the condition include nitrogen oxides, sulfur dioxide, ammonia, and chlorine gas.
Lung and Bone Marrow Transplants
Bronchiolitis obliterans is the most common form of chronic rejection after a lung transplant. The recipient’s immune system attacks the donor airways, triggering the same inflammation-to-scarring cascade. About 48% of lung transplant recipients develop the condition within five years, and that number climbs to 76% within ten years. Bone marrow transplant recipients face a lower but still significant risk: roughly 1 in 10 develop it within five years, as part of a broader immune reaction called graft-versus-host disease.
Infections
Severe respiratory infections, particularly certain viral infections in childhood, can trigger bronchiolitis obliterans. This is more common in children and is sometimes seen after particularly aggressive lower respiratory tract infections.
Symptoms and How They Progress
You may not have symptoms at first. The disease often begins quietly, with a dry cough or mild shortness of breath during exercise that’s easy to dismiss. Over weeks to months, breathing becomes progressively harder. The timeline from initial trigger to noticeable symptoms varies. After a heavy chemical exposure, symptoms can appear within weeks. After a lung transplant, the decline typically emerges months to years later.
As the disease progresses, common symptoms include a persistent dry cough that doesn’t produce much mucus, increasing breathlessness with everyday activities like climbing stairs or walking uphill, wheezing, and fatigue. Because the airflow obstruction is fixed (the scarring doesn’t relax the way asthma-related narrowing does), symptoms tend not to fluctuate much from day to day. They gradually worsen over time rather than coming in flares.
How It’s Diagnosed
Diagnosis relies on a combination of breathing tests and imaging, because the condition can mimic other obstructive lung diseases.
Pulmonary function tests (spirometry) reveal a pattern of airflow obstruction. The key measurement, the amount of air you can blow out in one second, drops significantly and doesn’t improve after using a bronchodilator inhaler. This “fixed obstruction” pattern helps distinguish it from asthma, where the airways open up with medication.
CT scans of the chest show a characteristic patchwork pattern called mosaic attenuation: some areas of the lung appear lighter (less air getting in) while neighboring areas look normal. When the scan is repeated during a full exhale, the damaged areas trap air and fail to deflate, a finding called air trapping. Widened bronchial tubes and thickened airway walls are often visible too. Air trapping combined with bronchial wall thickening is considered the most reliable imaging indicator of the condition. Direct signs of bronchiolar scarring are rarely visible on a scan because the affected airways are too small; instead, doctors rely on these indirect clues.
In some cases, a lung biopsy is needed to confirm the diagnosis by showing the characteristic scarring pattern under a microscope.
Treatment Options
There is no cure for bronchiolitis obliterans. The scarring is permanent, so treatment focuses on slowing further damage, managing symptoms, and preserving whatever lung function remains.
For lung transplant recipients, the first-line approach is typically a long-term course of a macrolide antibiotic (a class of drugs that also has anti-inflammatory properties). The treatment is given for at least three months to assess whether it stabilizes lung function. It doesn’t reverse existing scarring, but in some patients it slows or halts the decline. If lung function continues to drop, doctors may adjust the immunosuppressive medications the patient is already taking to better control the immune attack on the donor airways.
For non-transplant patients, treatment depends on the underlying cause. Removing the source of exposure is critical for occupational cases. Corticosteroids and other anti-inflammatory medications are sometimes used, though their effectiveness varies and long-term high-dose steroids are generally avoided because the risks outweigh the benefits. Supplemental oxygen helps when blood oxygen levels fall. Pulmonary rehabilitation, a structured program of exercise and breathing techniques, can improve daily function even though it doesn’t change the underlying disease. In the most severe cases, lung transplantation itself becomes the treatment option, though this carries its own risk of the condition recurring in the new lungs.
Vaping and Popcorn Lung
The link between e-cigarettes and bronchiolitis obliterans has gotten significant attention, largely because some e-liquid flavorings once contained diacetyl. However, there have been no confirmed cases of popcorn lung linked to vaping. In the UK, diacetyl was banned from e-cigarettes and e-liquids in 2016. While vaping carries other documented respiratory risks, the specific claim that it causes bronchiolitis obliterans remains unsubstantiated by clinical evidence at this point.
Workplace Prevention
For people who work around flavoring chemicals, prevention is the most effective strategy because the damage, once done, cannot be undone. NIOSH has expressed concern not only about diacetyl and 2,3-pentanedione but also about other volatile flavoring compounds that could have similar toxic effects. Protective measures include engineering controls like enclosed mixing systems and local exhaust ventilation, proper respiratory protection, and air monitoring to keep chemical concentrations as low as possible. If you work in food production and develop an unexplained cough or shortness of breath, early evaluation with pulmonary function testing can catch the disease before significant lung function is lost.