Bronchiolitis obliterans is a serious lung condition in which scar tissue builds up inside the smallest airways (bronchioles), narrowing or even completely blocking them. Unlike asthma or COPD, the obstruction is structural, not caused by temporary muscle spasms or reversible inflammation. Once the scarring sets in, standard inhalers and bronchodilators do little to open the airways back up. The condition is sometimes called “popcorn lung” because of its well-known link to a chemical used in microwave popcorn flavoring, but the causes extend well beyond that single exposure.
How It Damages the Airways
Your lungs branch into progressively smaller tubes, ending in tiny bronchioles less than 2 millimeters wide. In bronchiolitis obliterans, something injures the delicate lining of these small airways. The body attempts to repair the damage, but the healing process goes haywire. Instead of restoring healthy tissue, it produces excessive granulation tissue and collagen that fills in the airway walls from the inside out.
The result is a concentric narrowing of the bronchiole’s opening, like scar tissue slowly cinching a tube shut. The smooth muscle around the bronchioles thickens, mucus accumulates in the narrowed passages, and in severe cases the airway lumen can be completely sealed off. Critically, the deeper lung tissue where oxygen exchange happens (the alveoli) remains largely unaffected. The problem is getting air through the plumbing, not a failure of the gas-exchange surfaces themselves.
Known Causes and Risk Factors
Bronchiolitis obliterans has several distinct triggers, but they all share the same endpoint: damage to the bronchiolar lining followed by an abnormal scarring response.
- Lung or bone marrow transplant. This is the most common clinical context. The immune system can attack the transplanted tissue (or vice versa), triggering chronic inflammation in the small airways. In lung transplant recipients, a majority develop either bronchiolitis obliterans syndrome (BOS) or die within four years of transplantation, and nearly 90% reach one of those outcomes within 10 years.
- Chemical inhalation. Diacetyl, a butter-flavoring compound used in food manufacturing, is the most notorious occupational cause. A structurally similar substitute called 2,3-pentanedione carries comparable risks. NIOSH recommends exposure limits as low as 5 parts per billion for diacetyl over an 8-hour shift, reflecting how little it takes to cause harm. Despite this, OSHA has no enforceable standard for these chemicals.
- Vaping. Case reports have confirmed vaping-associated constrictive bronchiolitis. In one documented case, a woman in her 40s who vaped developed rapid-onset shortness of breath and cough, with severe obstruction on lung function testing and extensive air trapping on CT imaging. A lung biopsy confirmed the diagnosis.
- Severe respiratory infections. Certain viral infections, particularly in childhood, can leave behind enough airway damage to trigger the fibrotic process.
- Autoimmune conditions. Rheumatoid arthritis and other connective tissue diseases can provoke an immune attack on the bronchiolar lining.
What It Feels Like
The hallmark symptoms are a dry cough and progressive shortness of breath, particularly with exertion. Because the scarring develops gradually, many people initially attribute their symptoms to being out of shape or getting older. The onset can be insidious over weeks to months, or it can appear more rapidly after a clear exposure event like a toxic inhalation.
One key distinction: bronchiolitis obliterans does not respond meaningfully to bronchodilators. If you use an inhaler and feel no relief, that’s a significant clinical clue. Wheezing may or may not be present. As the condition progresses, shortness of breath worsens and can eventually limit basic daily activities like walking across a room.
How It’s Diagnosed
Standard chest X-rays often look normal or near-normal, which can delay diagnosis. The imaging tool that reveals the condition is a high-resolution CT scan, particularly one that includes images taken during exhalation.
The characteristic finding is called mosaic attenuation: a patchwork of lighter and darker areas across the lungs. The dark patches represent trapped air in regions where narrowed bronchioles won’t let air escape during exhalation, while the lighter patches show relatively normal lung tissue. In one study of 115 patients with confirmed bronchiolitis obliterans, 100% showed this mosaic pattern on CT. Ninety percent also had secondary changes like bronchial wall thickening, signs of fibrosis, or bronchiectasis (widened, damaged airways).
Lung function testing shows an obstructive pattern, meaning the ratio of air you can force out in one second compared to your total lung capacity is reduced. The critical detail is that this obstruction does not improve after inhaling a bronchodilator, which helps distinguish it from asthma. In some cases, a lung biopsy is needed to confirm the diagnosis, revealing the telltale submucosal and adventitial scarring, smooth muscle thickening, and narrowed or obliterated bronchioles under the microscope.
Treatment Options
There is no cure that reverses the scarring already in place. Treatment focuses on slowing progression and managing symptoms.
For post-transplant bronchiolitis obliterans syndrome, azithromycin (a common antibiotic) has become a first-line therapy, not for its antibacterial properties but for its anti-inflammatory and immune-modulating effects. It was first reported to halt or even partially reverse lung function decline in BOS patients in 2003 and has since shown effectiveness in both short- and medium-term use. A meta-analysis found that azithromycin therapy is both effective and safe for improving lung function and may improve survival after lung transplantation. Some transplant centers use a combination of azithromycin with an inhaled steroid and a leukotriene blocker.
Other approaches that have been tried in transplant-related cases include adjustments to immunosuppressive medications, total lymphatic irradiation, and extracorporeal photopheresis (a procedure that treats immune cells outside the body). For non-transplant cases, removing the offending exposure is the single most important step. Corticosteroids are sometimes used to reduce active inflammation, though they cannot undo established fibrosis.
Long-Term Outlook
Prognosis varies considerably depending on the cause and how early the condition is caught. Occupational cases that are identified early, before extensive scarring, tend to stabilize once exposure stops, though lost lung function rarely returns. Cases tied to transplant rejection or autoimmune disease often follow a more progressive course.
When bronchiolitis obliterans becomes severe enough to cause respiratory failure, lung transplantation may be considered. However, the condition can recur in the transplanted lungs. Registry data from the International Society for Heart and Lung Transplantation shows that bilateral lung transplant recipients have a median survival of about 8.5 years, while single lung transplant recipients have a median survival of roughly 5.75 years. Five-year survival rates are approximately 66% for bilateral and 56% for single lung transplants.
The condition that develops in the new lungs, BOS, remains the leading cause of long-term transplant failure. Median BOS-free survival after bilateral lung transplant is about 3.6 years, meaning half of recipients develop the syndrome or die within that window. For single lung transplants, that figure drops to about 3.2 years.