Constrictive bronchiolitis is a serious lung disease in which scar tissue builds up around the smallest airways (bronchioles), permanently narrowing them and making it harder to move air out of the lungs. Unlike asthma, which causes reversible tightening, the airflow obstruction in constrictive bronchiolitis does not improve with inhalers or bronchodilators. The condition is also called bronchiolitis obliterans or, colloquially, “popcorn lung” when linked to chemical flavoring exposure. It develops over weeks to months and can lead to significant, lasting disability.
How the Airways Become Damaged
The disease targets the terminal and respiratory bronchioles, the tiniest branches of your airway tree, while leaving the larger airways and the air sacs (alveoli) relatively untouched. After some kind of injury, whether from a toxic chemical, an infection, or an immune reaction, the normal repair process goes wrong. Instead of healing cleanly, the tissue around the bronchioles triggers an overproduction of fibrous scar tissue. This scar tissue wraps concentrically around the airway walls, shrinking their openings.
Several changes happen at once. The smooth muscle surrounding the bronchioles thickens. Inflammatory cells accumulate in the tissue around them. Mucus collects inside the narrowed passages. The combined effect is a progressive, irreversible obstruction that traps air in the lungs. Because the damage is deep in the small airways, standard chest X-rays often look normal, which can delay diagnosis.
Common Causes and Triggers
Constrictive bronchiolitis has a wide range of triggers, but they generally fall into a few categories: toxic inhalation, post-transplant immune reactions, autoimmune conditions, and infections.
Chemical and Occupational Exposures
Workplace inhalation injuries are among the best-documented causes. The most well-known example is diacetyl, a butter-flavoring chemical used in microwave popcorn factories, flavoring manufacturing, and diacetyl production. Workers exposed to diacetyl often developed the disease insidiously over the course of their employment rather than after a single dramatic event. Other documented chemical triggers include sulfur dioxide, nitrogen oxides (from silo gas), ammonia, hydrogen sulfide, styrene in fiberglass boat manufacturing, and iron oxide dust from copy machines. Even household cleaners have been implicated in case reports.
Military and Environmental Exposures
A landmark study published in the New England Journal of Medicine examined 80 soldiers from Fort Campbell, Kentucky, who developed unexplained shortness of breath after serving in Iraq and Afghanistan. Of the 49 who underwent lung biopsies, 38 had constrictive bronchiolitis. Most were young, with a median age of 33, and 25 of the 38 were lifelong nonsmokers. Their exposure histories tell a striking story: 87% reported exposure to dust storms, 74% to smoke from a massive sulfur mine fire near Mosul in 2003, 63% to burn pits incinerating solid waste with jet fuel, and 47% to incinerated human waste. Biopsy samples from most soldiers showed inhaled particulate matter lodged in the tissue, even in those who had never smoked. A separate 2009 study of 46,000 military personnel confirmed higher rates of respiratory problems among those who served in Iraq compared with those deployed elsewhere.
Post-Transplant and Autoimmune Causes
Constrictive bronchiolitis is the most feared long-term complication of lung transplantation, where it is called bronchiolitis obliterans syndrome (BOS). It develops when the recipient’s immune system attacks the transplanted airways as foreign tissue. It also occurs after bone marrow transplants as a form of graft-versus-host disease. Autoimmune conditions like rheumatoid arthritis have also been linked to the disease.
Symptoms and How They Develop
The hallmark symptom is progressive shortness of breath, particularly with physical activity. Many people first notice they can no longer exercise at their previous level. A dry cough is common. Symptoms typically build over weeks to months, not suddenly. This gradual onset distinguishes constrictive bronchiolitis from asthma, where symptoms come and go in episodes. Some people exposed to a single toxic event may not even seek medical attention at the time of exposure, only to develop fixed airway obstruction in the following weeks.
Because the symptoms overlap with more common conditions like asthma or COPD, and because standard imaging can appear unremarkable, people are sometimes treated for the wrong diagnosis for months or longer before the true cause is identified.
How It Is Diagnosed
Diagnosis relies on a combination of breathing tests, specialized imaging, and sometimes a lung biopsy.
Pulmonary function testing is essential. Spirometry typically shows an obstructive pattern, meaning air gets trapped and can’t be fully exhaled. The key finding is that this obstruction does not improve after using a bronchodilator, which separates it from asthma. A characteristic pattern includes a reduced ratio of the air you can blow out in one second relative to your total exhaled breath (FEV1/FVC ratio), alongside a significantly elevated residual volume, the air that remains trapped in the lungs after a full exhale. In one documented case, residual volume reached 213% of the predicted normal value. The lungs’ ability to transfer oxygen into the blood (measured by a test called DLCO) often remains relatively preserved, since the air sacs themselves are not the primary target.
High-resolution CT scanning is the most useful imaging tool. It reveals a pattern called mosaic attenuation: a patchwork of lighter and darker areas in the lung tissue. The darker patches represent areas where air is trapped behind narrowed airways. This pattern becomes more obvious on scans taken during exhalation. Standard chest X-rays frequently miss the disease entirely.
In some cases, particularly when the diagnosis remains uncertain, a surgical lung biopsy is needed. The biopsy shows the characteristic scarring and narrowing around the small airways. However, the findings can be subtle and patchy, so even biopsies can occasionally miss affected areas.
Treatment Options
There is no cure for constrictive bronchiolitis, and the scarring that has already occurred is generally irreversible. Treatment focuses on slowing progression, managing symptoms, and addressing the underlying cause when possible.
For post-transplant cases, adjusting immunosuppressive therapy is a primary strategy. Certain antibiotics with anti-inflammatory properties have shown some benefit in slowing the decline of lung function in transplant-related disease. Inhaled corticosteroids and bronchodilators may provide modest symptom relief for some patients, even though the obstruction itself doesn’t fully reverse. Pulmonary rehabilitation, which involves structured exercise and breathing techniques, can help people maintain the best possible function with their remaining airway capacity.
For occupational or environmental cases, the single most important step is removing the ongoing exposure. If a worker is still inhaling the offending chemical, continued exposure will drive further damage. Once exposure stops, some people stabilize, though the existing scarring persists.
In the most severe cases, lung transplantation becomes the only remaining option. This creates a difficult reality: the disease can recur in the transplanted lungs as BOS.
Long-Term Outlook
Prognosis depends heavily on the cause, how early the disease is caught, and whether the trigger can be eliminated. People whose disease is identified early and whose exposure is removed may stabilize at a reduced but functional level of lung capacity. Others experience a steady, progressive decline.
The post-transplant form carries particularly sobering numbers. In a study of bilateral lung transplant recipients who developed BOS, median survival after onset was 2.5 years. Five-year survival was just 26%. Patients whose BOS appeared within two years of transplant fared worse, with a median survival of 1.47 years, compared with 2.51 years for those who developed it later. About 39% of patients whose disease was caught at an early grade stabilized and did not progress further, while 61% worsened within a median of roughly seven months.
For non-transplant constrictive bronchiolitis caused by occupational or environmental exposures, long-term data is more limited. The 50,000 Iranians exposed to sulfur mustard gas during the Iran-Iraq war have been followed for over 20 years, providing one of the longest observational windows. Their cases confirm that the disease can persist and evolve over decades, underscoring why early recognition and exposure prevention matter so much.