Popcorn lung doesn’t produce dramatic visible changes you can see from the outside. The damage happens deep inside the lungs, in the smallest airways called bronchioles, where scar tissue gradually narrows or blocks the tubes that carry air. What it “looks like” depends on the tool used to find it: a CT scan, a microscope, or a breathing test that maps airflow patterns. Each reveals a different layer of the same disease.
What Happens Inside the Lungs
Popcorn lung, formally called bronchiolitis obliterans, targets the tiniest airways in your lungs while leaving the larger structures mostly untouched. The walls of these small tubes become inflamed, and instead of healing normally, the body lays down scar tissue in the wrong places. This scar tissue builds up in rings around the inside of the airway, progressively shrinking the opening. In some cases, the scarring completely seals off the airway so no air can pass through at all.
The surrounding air sacs, where oxygen actually enters your blood, typically look normal. That’s what makes popcorn lung tricky: the damage is concentrated in a narrow zone of tissue that’s hard to see on standard imaging and easy to miss in early stages. You can also see thickened smooth muscle around the bronchioles, mucus plugging the narrowed openings, and clusters of inflammatory cells packed into the airway walls.
What It Looks Like on a CT Scan
A standard chest X-ray is not great at catching popcorn lung. It may show hazy patches in the lungs without the usual shrinkage you’d see in other chronic lung diseases, but the findings are subtle and nonspecific. High-resolution CT scanning is far more revealing.
The hallmark CT finding is something called mosaic attenuation, a patchwork pattern where some areas of the lung appear darker than others. The darker patches represent trapped air: those sections of lung filled during a breath in but can’t empty properly because the small airways feeding them are scarred shut. When radiologists ask you to breathe out during a scan, this contrast between healthy and trapped areas becomes even more pronounced. In one comparison study, mosaic attenuation showed up in 50% of patients with bronchiolitis obliterans but only 3% of patients with asthma, making it one of the most reliable features for distinguishing the two conditions.
CT scans may also reveal bronchial dilation, where larger airways widen to compensate for the blocked smaller ones. But because the actual amount of abnormal tissue in and around the tiny bronchioles is so small, direct signs of scarring often don’t appear on CT at all. Radiologists instead rely on these indirect clues, the trapped air and the mosaic pattern, to piece the diagnosis together.
What It Looks Like Under a Microscope
Lung biopsy remains the gold standard for confirming popcorn lung. Under a microscope, pathologists look for two main patterns. The more classic form shows concentric or lopsided deposits of fibrous scar tissue building up just beneath the lining of the bronchioles, sometimes with very little visible inflammation. The airway opening is visibly narrowed or obliterated by this dense, pale scar tissue.
A second pattern, called lymphocytic bronchiolitis, looks quite different. Here the bronchiole walls are packed with immune cells, and the airway lining itself shows signs of active damage and inflammation. This pattern can exist alongside or precede the scarring pattern, and the distinction matters because the two may respond differently to treatment. In both cases, the tissue surrounding the airways, the delicate air sacs where gas exchange happens, looks relatively normal. The disease is remarkably focused on those small tubes.
What It Feels Like From the Outside
You won’t see popcorn lung by looking at someone. There are no skin changes, no visible swelling, no outward signs. What you will notice are the symptoms, which tend to creep in gradually. Early on, the disease may cause no symptoms at all. As more airways scar over, the first signs are usually shortness of breath during physical activity and a persistent dry cough. Wheezing and deep fatigue follow as the disease progresses.
These symptoms overlap heavily with asthma and other obstructive lung diseases, which is one reason popcorn lung is often misdiagnosed or caught late. The onset timeline varies. In factory workers exposed to diacetyl (the butter-flavoring chemical that gave the disease its name), symptoms developed after months to years of repeated exposure. In case reports linked to vaping, some patients experienced rapid onset of breathlessness and cough.
How Breathing Tests Reveal the Damage
Pulmonary function tests offer another “view” of popcorn lung, this time through airflow measurements rather than images. The key finding is an obstructive pattern: air goes in but can’t get out efficiently because the small airways are blocked. Specifically, doctors look for a drop in the volume of air you can forcefully exhale in one second compared to your total lung capacity.
A clinically significant decline, 20% or more from your personal best, is one of the standard diagnostic thresholds. But research on post-transplant patients found that relying solely on this measurement misses early disease in about 61% of cases. Measurements of airflow in the middle and end of exhalation, which reflect what’s happening in smaller airways, tend to decline earlier and catch the disease sooner. Residual volume, the air left in your lungs after you exhale as hard as you can, also rises above normal because trapped air has nowhere to go.
How It Gets Worse Over Time
Popcorn lung is staged based on how much lung function you’ve lost compared to your personal best. The earliest recognizable stage, sometimes called “potential” or “probable” disease, reflects a 10 to 20% drop in exhaled air volume. Beyond that, the disease is graded in progressively severe stages as lung function continues to decline. The scarring is generally irreversible, so the pattern is one of stepwise worsening rather than flare-and-recovery cycles.
On imaging, later-stage disease shows more widespread mosaic patterns and more pronounced air trapping. The bronchial dilation visible on CT may become more obvious as the lungs struggle to route air through fewer open pathways. Under the microscope, the balance shifts from active inflammation toward dense, established fibrosis as the disease matures.
Known Causes and Exposures
The nickname “popcorn lung” comes from workers in microwave popcorn factories who developed the disease after inhaling diacetyl, a chemical used to create artificial butter flavor. NIOSH researchers found extremely high particle concentrations in the breathing zones of quality control workers who opened freshly popped bags. Safe exposure limits recommended by federal agencies are very low, between 1 and 10 parts per billion, reflecting how potent the chemical is at damaging small airways even in tiny amounts.
Diacetyl isn’t the only cause. The disease also occurs after lung or bone marrow transplants, as the immune system attacks the new organ’s airways. Chemical inhalation in industrial settings, certain autoimmune conditions, and severe respiratory infections can all trigger it. More recently, case reports have linked vaping to constrictive bronchiolitis, including a 17-year-old boy who developed progressive breathing difficulty and low oxygen levels after vaping flavored e-liquids, and a woman in her 40s who experienced rapid onset of cough and shortness of breath. Many flavored e-liquids have contained diacetyl or chemically similar compounds, though the full spectrum of harmful chemicals in vape aerosol is still being characterized.