How Rare Is Popcorn Lung? Causes and Who’s at Risk

Popcorn lung, the colloquial name for bronchiolitis obliterans, is genuinely rare in the general population. Even among factory workers with heavy diacetyl exposure, documented cases number in the dozens rather than thousands, and confirmed cases in consumers are rarer still. But “rare” does not mean “irrelevant.” The condition is irreversible, poorly treated, and connected to chemical exposures that show up in surprising places, from microwave-popcorn factories to coffee roasting facilities to flavored e-cigarettes.

What Popcorn Lung Actually Is

Bronchiolitis obliterans is a condition in which the smallest airways in the lungs, called bronchioles, become scarred and narrowed. The scarring is permanent. Unlike asthma, where airways tighten and then relax, the obstruction in bronchiolitis obliterans is fixed. Breathing tests show a pattern of airflow limitation that does not improve with bronchodilators, and lung function tends to decline over time rather than fluctuate. Symptoms include a dry cough, shortness of breath on exertion, and wheezing that gradually worsens over weeks to months.

The nickname “popcorn lung” comes from a cluster of cases identified in May 2000 at a microwave-popcorn production plant in Missouri, where eight former workers were found to have severe bronchiolitis obliterans.¹ That outbreak brought the disease into public awareness and tied it firmly to a butter-flavoring chemical called diacetyl.

Diacetyl and How It Damages the Airways

Diacetyl (2,3-butanedione) is a naturally occurring compound that gives butter its characteristic smell. It is safe to eat. The problem arises when you inhale it in vapor form at high concentrations over time. When diacetyl vapor reaches the lining of the small airways, it triggers inflammation, oxidative stress, and changes in the way airway cells repair themselves. Lab studies using human bronchial cells show that exposure to diacetyl vapor increases markers of inflammation and tissue injury, and boosts secretion of proteins involved in tissue remodeling.² Researchers have also proposed that diacetyl can disrupt normal electron-transfer processes in cells, generate reactive oxygen species, and react directly with proteins in the airways.³

Diacetyl is not the only culprit. A closely related chemical, 2,3-pentanedione, was introduced by some manufacturers as a substitute after diacetyl’s dangers became known. Animal studies show it causes comparable or even worse bronchial fibrosis. In rats exposed to either chemical at similar concentrations, both produced scarring in the bronchi, and pentanedione sometimes caused more extensive damage in the deeper lung tissue after the exposure period ended.⁴ In other words, the replacement may not have been safer than the original.

The Microwave Popcorn Factory Outbreak

The Missouri cases that gave the disease its name were striking. Workers who mixed butter-flavoring ingredients in the plant had dramatically higher rates of airway obstruction than the general population. Across the facility, workers had about 3.3 times the expected rate of airway obstruction, but the risk was most pronounced in those who had never smoked: they showed nearly 11 times the expected rate, eliminating cigarettes as a plausible explanation.¹ The more cumulative diacetyl exposure a worker had, the more likely they were to have measurable obstruction. CT scans of affected workers showed thickened bronchial walls and a characteristic mosaic pattern of air trapping.⁵

Industrial hygiene studies at these plants confirmed that workers were breathing complex mixtures of volatile organic compounds, with diacetyl being the most prominent flavoring chemical in the air. The exact contribution of other compounds in the mix has not been fully resolved.⁶ Later litigation revealed that industry groups were aware of cases at multiple butter-flavoring and popcorn plants but often failed to implement safety improvements and actively concealed warning information from workers.⁷

Beyond Popcorn Factories

The disease is not limited to microwave-popcorn plants. Any workplace where diacetyl or similar flavoring chemicals become airborne can put workers at risk. Coffee roasting is one such setting: roasting green coffee beans naturally releases diacetyl, and some flavored-coffee operations add extra flavoring compounds. A report from the CDC described two workers at a small coffee-processing facility in Texas who developed obliterative bronchiolitis. Both were initially misdiagnosed, a recurring theme in this disease, because doctors did not connect their workplace to a condition most associated with popcorn factories.⁸

A larger study of coffee roasting and packaging workers found that those who worked near flavoring stations reported significantly higher rates of chronic bronchitis and asthma diagnoses compared to workers in other parts of the same facilities.⁹ The disease has also been documented in a worker at a British potato-crisp flavoring factory. That worker, a 36-year-old nonsmoker, developed fixed airway obstruction after a period of unusually high diacetyl exposure. Even after he was removed from the exposure, his lung function did not recover.¹⁰

The common thread across these industries is not the product being made. It is the inhalation of flavoring chemicals in vapor form, often in enclosed spaces with poor ventilation. Anywhere workers heat, mix, or aerosolize flavorings containing alpha-diketones like diacetyl and pentanedione, the risk exists.

Vaping and E-Cigarettes

This is the exposure route that worries the most people today. When researchers tested 51 flavored e-cigarette products, diacetyl was detected in 39 of them. Pentanedione, the substitute chemical with comparable toxicity, was found in 23. At least one flavoring chemical showed up in 47 out of 51 products tested.¹¹ Fruit, candy, and cocktail flavors were the most common carriers. The concentrations varied widely across brands and flavors.

Some diacetyl is lost during the aerosolization process. Transfer efficiency studies show that diacetyl and pentanedione transfer from the liquid to the aerosol at roughly 65%, meaning some of what is in the e-liquid never reaches the lungs.¹² That said, “lower than what is in the liquid” is not the same as “safe.” E-cigarette users may take hundreds of puffs daily, and the cumulative exposure over months or years is the concern.

The honest state of the evidence is that no definitive cases of bronchiolitis obliterans have been clearly attributed to vaping in published, peer-reviewed literature the same way they have been attributed to factory exposure. The concentrations inhaled per puff are far lower than what factory workers breathed during their shifts. But vaping delivers diacetyl directly into the deep lung, the exposure is daily and self-administered, and the habit is sometimes sustained for years. The worry is not unreasonable; it just has not been confirmed by the kind of case-cluster evidence that tied the disease to popcorn plants. Given that it can take years for the damage to become severe enough to detect, and that vaping is a relatively recent phenomenon, it may simply be too early to tell.

Can Eating Microwave Popcorn Cause It?

A small case series documented three consumers with biopsy-confirmed bronchiolitis obliterans whose primary diacetyl exposure was from eating butter-flavored microwave popcorn.¹³ These were not factory workers but people who ate popcorn frequently at home. The reported exposure route involved repeatedly inhaling the burst of steam from freshly opened bags, where diacetyl concentrations can spike briefly.

Three confirmed cases is an extremely small number, and it is hard to draw broad conclusions from them. For the overwhelming majority of people who eat microwave popcorn occasionally, the risk is effectively negligible. The cases involved heavy, habitual consumption combined with deliberate inhalation of the bag’s steam. Most major popcorn manufacturers have also reformulated their products since the mid-2000s to reduce or eliminate diacetyl from their butter flavorings. Still, if you are someone who eats several bags a day and leans in to breathe the steam, the case series is worth knowing about.

Popcorn Lung Without Chemical Exposure

Bronchiolitis obliterans is not exclusively caused by inhaled chemicals. The same pattern of small-airway scarring occurs in several medical contexts. The most well-known is after organ transplantation. Among lung transplant recipients, it is actually a common long-term complication and one of the leading causes of graft failure. It also occurs after bone-marrow (hematopoietic stem cell) transplants as a manifestation of chronic graft-versus-host disease, in which the donor’s immune cells attack the recipient’s tissues. Estimates suggest roughly 2 to 3 percent of all bone-marrow transplant recipients develop bronchiolitis obliterans, and about 6 percent of those who develop chronic graft-versus-host disease will go on to get it.¹⁴

Detection after transplant is tricky because the disease often goes unnoticed until lung function has already dropped substantially. In a study of children after stem-cell transplant, those who developed bronchiolitis obliterans had reductions in a key breathing measure of 40 to 53 percent compared to their pre-transplant values by the time it was caught.¹⁵ The disease can also follow severe respiratory infections, exposure to certain other toxic gases like mustard gas or nitrogen dioxide, and some autoimmune conditions, particularly rheumatoid arthritis.

Why It Is So Often Misdiagnosed

Popcorn lung’s rarity works against patients. The symptoms overlap heavily with asthma and COPD: progressive shortness of breath, wheezing, dry cough. Unless a doctor specifically suspects occupational chemical exposure or post-transplant complications, the diagnosis may never come up. The Texas coffee workers described earlier were both misdiagnosed before eventually being identified as having occupational bronchiolitis obliterans.⁸ A British potato-crisp factory worker similarly went through a diagnostic process before his condition was recognized.¹⁰

The disease does not always show up on a standard chest X-ray. High-resolution CT scans are more revealing, typically showing the mosaic attenuation pattern and air trapping that indicate small-airway disease.⁵ A definitive diagnosis sometimes requires a surgical lung biopsy, which is invasive and not routinely performed for what appears to be ordinary obstructive lung disease. The most telling clinical clue is an obstruction pattern on breathing tests that refuses to improve with inhalers, especially in someone who has never smoked. If that person also has a history of working near flavorings, heated chemicals, or is post-transplant, the pieces start to fit.

Treatment and Prognosis

There is no cure for bronchiolitis obliterans. Once the small airways are scarred, the damage is permanent. Treatment is focused on slowing further decline and managing symptoms. Anti-inflammatory medications, including systemic corticosteroids and sometimes immunosuppressive drugs, may help stabilize lung function, particularly in transplant-related cases where an overactive immune response is driving the damage. In occupational cases, the first step is removing the person from the exposure, though this alone does not reverse the disease. The British crisp-factory worker, for instance, saw no improvement in symptoms or lung function even after he left the job entirely.¹⁰

For the most severe cases, lung transplantation is the only option that can restore meaningful function, but it carries its own serious risks, and bronchiolitis obliterans can actually recur in the transplanted lung. The prognosis varies widely. Some patients stabilize with modest impairment and live relatively normal lives. Others progress to respiratory failure. Early detection matters enormously because catching the disease while function is still reasonably preserved gives treatment the best chance of preventing further decline.

Regulatory Response and the Current Landscape

In the years following the Missouri outbreak, diacetyl was banned or restricted in inhalation products in several jurisdictions. Its use has been phased out by many food-flavoring manufacturers, at least in products where workers would be exposed to heated vapors. However, enforcement has been inconsistent, and the substitute chemicals that replaced diacetyl are not necessarily safer. One review noted that despite bans, nearly 70 percent of e-liquids in a sample of 159 products still contained diacetyl.¹⁶

In the United States, OSHA and NIOSH have issued guidance on workplace diacetyl exposure but have not established a formal permissible exposure limit specifically for diacetyl. NIOSH recommended an exposure limit of 5 parts per billion as a time-weighted average, but this is a recommendation, not an enforceable standard. The gap between recognized hazard and binding regulation remains a frustration for occupational health advocates.

Who Is Actually at Risk

Putting all of this together, the populations with meaningful risk fall into a few groups:

Flavoring-industry workers: Anyone who mixes, heats, or handles flavoring chemicals containing diacetyl or pentanedione in enclosed or poorly ventilated settings. This includes popcorn manufacturing, coffee roasting with added flavorings, and food-flavoring production for snack foods.
Transplant recipients: People who have had lung transplants or bone-marrow transplants, particularly those who develop chronic graft-versus-host disease.
E-cigarette users: People who vape flavored products daily over extended periods. The risk is not confirmed by documented case clusters, but the chemical exposure pathway is real and the long-term picture is incomplete.
People exposed to other toxic inhalants: Certain industrial chemicals, toxic gases, and severe respiratory infections can also trigger bronchiolitis obliterans outside of the flavoring-chemical context.

For the general public, the disease remains very rare. A person who does not work in a flavoring factory, does not vape, and has not had an organ or bone-marrow transplant is extremely unlikely to develop it. Eating microwave popcorn at ordinary levels does not pose a meaningful risk, especially given reformulation efforts by manufacturers. The concern is real, but it is concentrated in specific exposure scenarios rather than being a broad public health threat.

The Substitute-Chemical Problem

One of the more frustrating aspects of the popcorn lung story is the pattern of replacing one harmful chemical with another that turns out to be similarly dangerous. When diacetyl’s hazards became clear, the food and e-cigarette industries turned to pentanedione and other alpha-diketones as replacements. But animal studies have shown that pentanedione causes comparable bronchial scarring. At similar exposure concentrations, pentanedione produced fibrosis in the same proportion of animals as diacetyl, and in some measures it caused more widespread damage to the deeper lung tissue.⁴ A growing body of preclinical work has examined the short- and long-term toxicity of diacetyl and related compounds, consistently finding that the chemical family as a whole poses inhalation hazards.¹⁷

This pattern shows up throughout chemical regulation: a harmful substance gets restricted, a structurally similar compound takes its place, and years later researchers discover the replacement acts through the same biological pathways. For workers and consumers, the practical takeaway is that a product labeled “diacetyl-free” is not automatically safe to inhale. The question worth asking is whether the replacement chemicals have been tested for inhalation toxicity at all, and in many cases the answer is that they have not been tested nearly as thoroughly as the compound they replaced.