Bronchiolitis in adults is inflammation and scarring of the bronchioles, the smallest airways in the lungs, those with an internal diameter of about two millimeters or less. Unlike the common infant illness triggered by respiratory syncytial virus, adult bronchiolitis is a diverse family of disorders with causes ranging from toxic fume exposure and autoimmune disease to organ transplantation and certain medications.1PubMed Central. Recent advances in the understanding of bronchiolitis in adults Because the bronchioles are too small to see on a standard chest X-ray, adult bronchiolitis often goes unrecognized for months or years, and its symptoms are frequently mistaken for asthma or chronic obstructive pulmonary disease.
Why “Bronchiolitis” Means Something Different in Adults
Most people hear the word bronchiolitis and picture a wheezing baby in the emergency room. That form of the disease is overwhelmingly caused by viral infection, especially RSV, and it resolves on its own in almost all cases. Adult bronchiolitis shares the same anatomical target, the bronchioles, but tends to be chronic rather than acute. Instead of a single virus doing temporary damage, the adult versions usually involve ongoing injury that leads to either new tissue growing inside the airways or scar tissue tightening around them. The result is progressive breathing difficulty that does not go away when the initial trigger is removed.
Two Broad Patterns of Damage
Pathologists divide adult bronchiolitis into two major types based on what the disease does to the airway wall. In proliferative bronchiolitis, inflammatory tissue and loose connective tissue grow into the airway lumen, partially plugging the bronchioles from the inside. On imaging, this tends to show up as hazy patches where the lung looks whiter than it should. In constrictive bronchiolitis, the damage is the opposite: scar tissue forms in and around the airway wall, squeezing it shut from the outside. This pattern shows up on imaging as patchy areas where the lung looks too dark, because trapped air cannot escape during exhalation.2PubMed. Proliferative and constrictive bronchiolitis: classification and radiologic features
The distinction matters because the two patterns have different causes and different outlooks. Proliferative bronchiolitis, including organizing pneumonia (sometimes called BOOP), is often treatable with corticosteroids and can partially or fully reverse. Constrictive bronchiolitis is generally irreversible. Once the scar tissue constricts the airway, it stays constricted. Treatments can slow progression and manage symptoms, but the lost airway caliber rarely comes back.
Causes in Adults
The list of things that can trigger bronchiolitis in an adult is surprisingly long. Most causes fall into a handful of categories.
Inhaled Toxins and Occupational Exposures
The most dramatic example is so-called “popcorn lung.” Workers in microwave popcorn factories and other food-flavoring plants developed constrictive bronchiolitis after prolonged inhalation of diacetyl, a butter-flavoring chemical. A study of workers at a diacetyl-producing plant found cases of bronchiolitis obliterans syndrome with air trapping on CT scans, including in lifelong nonsmokers.3PubMed. Bronchiolitis obliterans syndrome in chemical workers producing diacetyl for food flavorings The disease has since been linked to other occupational chemicals, including nitrogen dioxide, sulfur dioxide, fly ash, and inorganic dusts.4PubMed Central. Constrictive Bronchiolitis in Soldiers Returning from Iraq and Afghanistan U.S. soldiers returning from deployment in Southwest Asia have also been diagnosed with constrictive bronchiolitis, potentially linked to burn pit smoke and other environmental exposures in conflict zones.5PubMed. Inhalational Constrictive Bronchiolitis: The Evolution of our Understanding of this Disease
Diacetyl exposure has also raised concerns around e-cigarettes, since some vaping liquids contain the chemical. Although the concentrations are generally lower than in an industrial setting, bronchiolitis obliterans is now recognized as a potential risk associated with vaping.6PubMed Central. Repetitive diacetyl vapor exposure promotes ubiquitin proteasome stress and precedes bronchiolitis obliterans pathology
Smoking
Cigarette smoke causes its own form of the disease: respiratory bronchiolitis. In this variant, pigmented macrophages (a type of immune cell stained brown by smoke exposure) accumulate in the bronchioles, accompanied by low-grade inflammation and early fibrosis.7PubMed. Two Sides of the Same Smoke: Decoding Respiratory Bronchiolitis-Associated Interstitial Lung Disease and Alveolar Macrophage Pneumonia Respiratory bronchiolitis is almost universal in current smokers, but most people with it never develop significant symptoms. A subset, however, progresses to respiratory bronchiolitis-associated interstitial lung disease, which causes clinically meaningful breathlessness and abnormal lung function. The mechanisms driving this progression involve oxidative stress, immune activation, and fibrotic remodeling of the airway wall.
Transplant Rejection
Bronchiolitis obliterans syndrome is the leading long-term threat to survival after lung transplantation. The immune system attacks the transplanted lung’s bronchioles, producing progressive constrictive scarring. The process is classified as a form of chronic rejection. Despite improvements in transplant outcomes overall, this complication remains the single biggest obstacle to long-term survival in lung transplant recipients.8PubMed Central. A review of bronchiolitis obliterans syndrome and therapeutic strategies Bronchiolitis obliterans syndrome also occurs after bone marrow or stem cell transplantation, where it is a manifestation of graft-versus-host disease.
Autoimmune and Connective Tissue Diseases
Rheumatoid arthritis is one of the most common autoimmune conditions linked to small-airway disease. Research suggests that the bronchioles may be a direct target of the autoimmune process in rheumatoid arthritis, with inflammation causing mucosal swelling that narrows the airways and creates an environment favorable for recurring infections.9PubMed Central. Small airway obstruction in patients with rheumatoid arthritis Higher disease activity scores, joint erosions, and elevated autoantibody levels have all been associated with the presence of small-airway disease in rheumatoid arthritis patients.10PubMed. Small airway disease and its predictors in patients with rheumatoid arthritis Other connective tissue diseases associated with bronchiolitis include Sjögren’s syndrome, lupus, and inflammatory bowel disease.
Medications
Certain drugs can trigger organizing pneumonia, a proliferative form of bronchiolitis. Symptoms often include a dry cough and shortness of breath, sometimes accompanied by fever or rash. Imaging usually shows patchy opacities in both lungs. The list of implicated medications is long and includes some chemotherapy agents, certain antibiotics, and a number of cardiovascular drugs. Stopping the offending medication is usually the first step in treatment, and the condition often improves with corticosteroids.
Infections
While viral bronchiolitis is primarily a childhood disease, severe respiratory infections in adults can occasionally trigger bronchiolitis, particularly in immunocompromised patients. Post-infectious bronchiolitis may follow adenovirus, influenza, or other serious respiratory infections. The initial infection resolves, but the damage it leaves behind leads to chronic airway narrowing.
Symptoms and How They Show Up
The hallmark symptoms of adult bronchiolitis are a persistent cough and progressive difficulty breathing. On physical examination, a doctor may hear crackles or wheezing through a stethoscope. These findings are nonspecific, which is a big part of why the diagnosis is so often delayed. A persistent cough with gradually worsening breathlessness over months could be asthma, COPD, heart failure, or a dozen other things. Bronchiolitis tends to be considered only after the more common diagnoses have been tested and ruled out.
The pace of symptom development varies dramatically depending on the cause. Inhaling a high concentration of a toxic gas can produce symptoms within days or weeks. Post-transplant bronchiolitis obliterans syndrome typically develops gradually over months to years. Smoking-related respiratory bronchiolitis may smolder for decades with minimal symptoms. Some forms of constrictive bronchiolitis are remarkably indolent. Biopsy-confirmed cases have been documented in patients with normal spirometry results, meaning the standard breathing test came back clean even though the disease was present on tissue examination.11PubMed Central. Occupational causes of constrictive bronchiolitis
Lung function testing, when it does catch abnormalities, typically shows a pattern of airflow obstruction that does not improve with bronchodilator inhalers, along with hyperinflation (too much air trapped in the lungs) and reduced gas transfer. These results resemble COPD, which adds to the diagnostic confusion.
How Doctors Diagnose It
High-resolution CT scanning is the most useful tool for evaluating small-airway disease. Although the bronchioles themselves are too small to see directly on any imaging, their damage leaves characteristic indirect signs.12European Respiratory Journal. Small airways diseases: detection and insights with computed tomography The most important of these are mosaic attenuation and the “tree-in-bud” pattern.
Mosaic attenuation shows up as a patchwork of lighter and darker areas in the lung. The darker patches represent areas where air is trapped behind narrowed or scarred bronchioles. This finding becomes more obvious on expiratory scans, where the patient breathes out during the scan and the trapped air becomes more conspicuous. The tree-in-bud pattern looks exactly as the name suggests: tiny branching structures at the edges of the lung, resembling a budding tree, representing mucus or inflammatory material filling small airways.13European Society of Radiology. Bronchiolitis: A Practical Approach for the General Radiologist
Neither finding is unique to bronchiolitis, and some patients with biopsy-proven disease have normal-appearing CT scans. When imaging and lung function tests are inconclusive but suspicion remains high, a surgical lung biopsy may be necessary. However, this is an invasive procedure, so doctors weigh the risks against the potential benefit of knowing the exact diagnosis, particularly when the result would change treatment.
Specific Types Worth Knowing About
Diffuse Panbronchiolitis
Diffuse panbronchiolitis is a rare inflammatory disease primarily diagnosed in people of East Asian descent, though cases have been documented in other populations as well.14PubMed Central. Diffuse panbronchiolitis: not just an Asian disease: Australian case series and review of the literature It produces widespread inflammation centered on the respiratory bronchioles, and chronic bacterial airway infection, particularly with Pseudomonas, is a common complication.15PubMed Central. Characterization of Pseudomonas aeruginosa from subjects with diffuse panbronchiolitis Before effective treatment was discovered, the prognosis was grim. The breakthrough came with low-dose macrolide antibiotics, discussed below.
Follicular Bronchiolitis
Follicular bronchiolitis is characterized by an overgrowth of lymphoid tissue around the small airways. These lymphoid follicles, complete with germinal centers, develop in the walls of the bronchioles.16PubMed Central. Follicular Bronchiolitis: A Literature Review It can occur on its own but is more commonly associated with connective tissue diseases or immunodeficiency states. In a study of Chinese adult patients, the main pathological finding was lymphocytic proliferation with reactive germinal centers surrounding the bronchioles.17Scientific Reports. The Clinical Characteristics and Outcomes of Follicular Bronchiolitis in Chinese Adult Patients Treatment typically targets the underlying condition when one is identified.
DIPNECH
Diffuse idiopathic pulmonary neuroendocrine cell hyperplasia, usually shortened to DIPNECH, is an increasingly recognized condition in which specialized hormone-producing cells in the airway lining multiply abnormally. This overgrowth can lead to constrictive bronchiolitis and progressive airflow obstruction.1PubMed Central. Recent advances in the understanding of bronchiolitis in adults DIPNECH predominantly affects middle-aged women and is sometimes discovered incidentally on CT scans done for other reasons. Its recognition has grown considerably in recent years as radiologists and pulmonologists have become more aware of its imaging patterns.
Treatment Approaches
Because adult bronchiolitis is not one disease but many, treatment depends entirely on the underlying cause. The heterogeneity of the condition means there is no single medication or protocol that works across the board.18Respiration. Bronchiolitis in Adults: Etiology, Diagnostic, and Therapeutic Approach
For proliferative forms like organizing pneumonia, corticosteroids are the standard first-line treatment and often produce a good response. For constrictive forms, the picture is less encouraging. Removing the ongoing exposure (quitting smoking, changing workplaces, stopping a medication) can halt further damage but usually cannot undo what has already occurred. Immunosuppressive therapies are used in post-transplant bronchiolitis obliterans syndrome to try to slow the immune-mediated airway destruction, with variable success.
The standout success story in treatment involves macrolide antibiotics, specifically erythromycin and azithromycin used at low doses over long periods. These drugs transformed outcomes in diffuse panbronchiolitis. Their benefit in that disease appears to come not from killing bacteria but from their separate anti-inflammatory and immune-modulating properties. At sub-antimicrobial doses, macrolides tamp down prolonged inflammation, reduce airway mucus production, inhibit bacterial biofilms, and modulate neutrophil activity.19PubMed. Macrolide antibiotics as immunomodulatory medications: proposed mechanisms of action Their success in diffuse panbronchiolitis led researchers to explore macrolide therapy in other chronic airway diseases, and long-term macrolide treatment is now considered for a range of inflammatory airway conditions.20PubMed. Long-term macrolide treatment for chronic respiratory disease
Supportive care is relevant across all types. Pulmonary rehabilitation can help patients maintain exercise tolerance and quality of life. Supplemental oxygen may be needed as the disease progresses. In the most severe cases of constrictive bronchiolitis, lung transplantation is the only remaining option, though the irony is not lost on anyone that bronchiolitis obliterans syndrome can recur in the transplanted lungs as well.
Why It Gets Missed
Several features of adult bronchiolitis conspire to delay diagnosis. The symptoms are generic: cough and shortness of breath describe a hundred conditions. Standard chest X-rays usually look normal because the affected airways are below the resolution threshold of plain radiography. Spirometry may show an obstructive pattern that is attributed to asthma or COPD, and many patients get treated with inhalers for months or years before the lack of response prompts further investigation. Even high-resolution CT findings, while helpful, are not always present or may be subtle enough to overlook.
The rarity of the condition in adults adds to the problem. Clinicians who see dozens of patients with COPD every week may see one or two cases of adult bronchiolitis in their entire career. This means the diagnosis often depends on a pulmonologist or radiologist who has the pattern on their mental radar. If you have a persistent cough and worsening shortness of breath that is not responding to standard asthma or COPD treatment, asking your doctor about small-airway disease is reasonable, especially if you have a relevant risk factor like an occupational chemical exposure, a connective tissue disease, or a history of organ transplantation.
The Occupational and Environmental Angle
The popcorn lung saga brought adult bronchiolitis into public awareness in the early 2000s, but occupational constrictive bronchiolitis extends well beyond the food-flavoring industry. Case series have been documented in chemical plant workers, textile workers exposed to nylon flock, and military personnel exposed to sulfur mustard gas.11PubMed Central. Occupational causes of constrictive bronchiolitis What makes occupational bronchiolitis particularly concerning is that affected workers may have normal results on routine spirometry. The disease can be present on biopsy while the standard screening test shows nothing wrong, which means workplace health surveillance programs relying solely on spirometry may miss cases entirely.
The military exposure story illustrates another challenge. Soldiers returning from Iraq and Afghanistan were found to have constrictive bronchiolitis on lung biopsy, but their clinical presentation differed from the flavoring-worker pattern. Their symptoms and test results did not follow the template that doctors had come to expect from the popcorn lung cases, yet the tissue under the microscope told the same story.5PubMed. Inhalational Constrictive Bronchiolitis: The Evolution of our Understanding of this Disease The takeaway is that constrictive bronchiolitis can look quite different clinically depending on the exposure, even though the underlying pathology is the same. A high index of suspicion, anchored to the patient’s exposure history, is often the most important diagnostic tool a clinician has.