What is Bronchiolitis Obliterans Syndrome?

Bronchiolitis obliterans syndrome, sometimes called popcorn lung, is a disease that affects the bronchioles, which are the tiniest airways in your lungs. Think of your airways like branches on a tree. The air you breathe travels through larger branches first, then moves into smaller and smaller branches. The bronchioles are the smallest of these branches, and when they become damaged, they can no longer work properly.^[1]

When someone has this condition, inflammation develops in these tiny airways. This inflammation then leads to scarring and narrowing of the passages. The term “obliterans” comes from a Latin word meaning to completely block or close off. Over time, the scarred tissue makes it harder and harder for air to flow through the lungs. The condition gets its nickname “popcorn lung” because researchers first identified it in workers at a microwave popcorn factory who had breathed in a butter-flavoring chemical.^[3]

The disease has several names depending on the situation. Doctors may call it obliterative bronchiolitis or constrictive bronchiolitis. When it happens specifically after someone receives a lung transplant or a hematopoietic stem cell transplant (a procedure where stem cells are transferred from one person to another to help produce healthy blood cells), it is called bronchiolitis obliterans syndrome.^[1]

This is different from regular bronchiolitis, which is a common infection in young children that usually gets better on its own. Bronchiolitis obliterans syndrome is a chronic condition that causes permanent changes to the lungs.^[7]

How Common is Bronchiolitis Obliterans Syndrome?

In the general population, bronchiolitis obliterans syndrome is considered rare. However, certain groups of people face much higher risks. The condition is particularly common among people who have received organ transplants.^[4]

Among lung transplant recipients, the numbers are quite striking. More than half of all people who receive lung transplants will develop some degree of bronchiolitis obliterans syndrome within five years after their surgery. Some sources report that about 75% of lung transplant patients develop this condition within ten years. The syndrome typically appears between 16 to 20 months after transplantation, though cases have been reported as early as three months after surgery.^[1][3]

People who receive hematopoietic stem cell transplants also face significant risk, though slightly lower than lung transplant recipients. About 5% to 14% of these patients develop bronchiolitis obliterans syndrome, which can appear several months to years after their transplant procedure. Around 10% of bone marrow transplant recipients develop the syndrome within five years.^[1][3]

For transplant recipients, bronchiolitis obliterans syndrome represents a form of chronic rejection, where the body’s immune system gradually damages the transplanted organ. It is considered the most common type of long-term complication affecting lung transplant patients.^[10]

In children, the condition is also rare, though we don’t know exactly how many children are affected. Studies show that children are more likely to develop it after a severe chest infection.^[7]

What Causes Bronchiolitis Obliterans Syndrome?

This condition is not contagious, meaning you cannot catch it from another person or spread it to others. Instead, it develops when something causes injury to the delicate tissues of the small airways in the lungs.^[3]

For transplant recipients, bronchiolitis obliterans syndrome happens as part of the body’s immune response to the transplanted organ. After a lung or stem cell transplant, the immune system may recognize the new tissue as foreign and mount an attack against it. This is called chronic allograft rejection. The inflammation caused by this immune response leads to scarring and narrowing of the bronchioles.^[1]

Beyond transplantation, several other causes can trigger this disease. Breathing in toxic fumes and chemicals represents one of the most common causes in the general population. Workers in certain industries have developed the condition after exposure to harmful substances. These include diacetyl, a chemical once widely used to create butter flavoring in microwave popcorn and other food products. Other dangerous substances include sulfur mustard gas, nitrogen oxides, ammonia, chlorine, formaldehyde, hydrochloric acid, sulfur dioxide, and acetaldehyde. Coffee roasting plants have also reported cases, and the condition has been linked to vaping or using electronic cigarettes that contain flavoring chemicals.^[1][3]

Respiratory infections can also lead to bronchiolitis obliterans syndrome, especially in children. Viral infections are the most common culprits, particularly adenovirus and respiratory syncytial virus (RSV). Other viruses that can cause the condition include influenza, measles, and certain herpes viruses. Bacterial infections, such as those caused by mycoplasma (which causes “walking pneumonia”), fungi, and HIV have also been associated with the disease.^[1][18]

Autoimmune disorders represent another category of causes. In these conditions, the body’s immune system mistakenly attacks its own tissues. Rheumatoid arthritis is particularly associated with bronchiolitis obliterans syndrome, as is systemic lupus erythematosus (SLE). Less commonly, inflammatory bowel disease can also trigger the condition.^[1]

Some rare medical conditions have been linked to the syndrome as well, including Stevens-Johnson syndrome (a severe skin reaction to medication), Castleman disease, and paraneoplastic pemphigus. Certain medications can also cause reactions that lead to lung damage.^[1][3]

In some cases, doctors cannot identify a specific cause. When this happens, the condition is called idiopathic bronchiolitis obliterans, meaning the cause is unknown.^[5]

Who is at Higher Risk?

Certain groups of people face elevated risks of developing bronchiolitis obliterans syndrome. Understanding these risk factors helps doctors monitor vulnerable patients more closely and may help in early detection.^[3]

The highest risk group includes people who have received organ transplants. Lung transplant recipients face the greatest risk, with the condition affecting the majority of long-term survivors. Hematopoietic stem cell transplant recipients, bone marrow transplant recipients, and heart-lung transplant recipients also have significantly elevated risk.^[1]

Workers in certain occupations face increased risk due to exposure to harmful chemicals and fumes. This includes employees at microwave popcorn factories, coffee roasting plants, and other food flavoring facilities. Workers who handle industrial chemicals, those exposed to metal oxide fumes, and military personnel exposed to burn pits or deployment-related particulate matter also have higher risk. People exposed to World Trade Center dust after the September 11 attacks have also developed the condition.^[5]

People with autoimmune diseases, particularly rheumatoid arthritis, systemic lupus erythematosus, and inflammatory bowel disease, have increased susceptibility. Children who experience severe respiratory infections, especially those caused by adenovirus or RSV, are at higher risk. People with compromised immune systems from HIV or other conditions also face elevated risk.^[1]

Those who use electronic cigarettes or vaping products, particularly those with flavoring chemicals, may be at risk. Smokers who develop a form of the disease called respiratory bronchiolitis are another affected group.^[5]

Symptoms of Bronchiolitis Obliterans Syndrome

The symptoms of bronchiolitis obliterans syndrome develop gradually over time and generally worsen over weeks to months. One of the challenging aspects of this condition is that some people may not have any symptoms initially, making early detection difficult.^[3]

The most common symptom is a persistent dry cough. Unlike a cough from a cold or flu that improves within a week or two, this cough continues and often gets worse over time. The cough may be especially noticeable during and after physical activity. Sometimes it may bring up mucus.^[3][6]

Shortness of breath, called dyspnea in medical terms, is another hallmark symptom. People often notice this first when they exercise or engage in activities that were previously easy for them. As the condition progresses, they may become short of breath even with minimal activity or at rest. Children with the condition may have trouble keeping up when running or playing.^[3][7]

Wheezing is also common. This is a whistling or squeaky sound that occurs when breathing, caused by air trying to move through narrowed airways. The sound may be noticeable both when breathing in and out.^[3][7]

Many people with bronchiolitis obliterans syndrome experience persistent tiredness or fatigue. This exhaustion goes beyond normal tiredness and doesn’t improve much with rest. The body is working harder to breathe, which can be physically draining.^[3]

Some patients develop additional symptoms including fever, night sweats, and skin rash. These symptoms may appear suddenly in some cases, or they may develop more gradually in others.^[3]

How to Prevent Bronchiolitis Obliterans Syndrome

While not all cases can be prevented, there are important steps that can reduce the risk of developing bronchiolitis obliterans syndrome.^[3]

For workers in industries where exposure to harmful chemicals is possible, proper workplace safety measures are essential. This includes using appropriate protective equipment such as respirators or masks, ensuring adequate ventilation in work areas, and following all safety protocols. Employers should monitor air quality and limit workers’ exposure to known harmful substances like diacetyl and other flavoring chemicals. If you work with chemicals, metal fumes, or other potential lung irritants, always use the protective equipment provided and report any safety concerns.^[5]

Avoiding or quitting vaping and electronic cigarette use can help prevent exposure to flavoring chemicals that may damage the lungs. Many vaping liquids contain diacetyl and other chemicals linked to lung injury.^[3]

Preventing respiratory infections is particularly important for children and those at high risk. Regular handwashing is one of the most effective ways to prevent the spread of viruses and bacteria. Family members and visitors should avoid close contact with vulnerable individuals when they are sick. Keeping up to date with vaccinations, including yearly flu shots, helps protect against infections that could trigger lung damage.^[18]

For transplant recipients, following all medical advice carefully is crucial. Taking immunosuppressive medications exactly as prescribed helps manage the immune response and may reduce the risk of chronic rejection. Regular monitoring and follow-up appointments allow doctors to detect early signs of the condition. Some research suggests that certain preventive treatments after transplantation may help, though the best approaches are still being studied.^[10]

For people with autoimmune diseases, working closely with healthcare providers to manage the underlying condition may help reduce complications. Taking prescribed medications and attending regular check-ups are important parts of care.^[1]

How the Disease Affects the Lungs

Understanding what happens inside the lungs when someone has bronchiolitis obliterans syndrome helps explain why the symptoms occur and why treatment is challenging. The disease process involves inflammation that ultimately leads to permanent scarring.^[1]

The process begins when something triggers inflammation in the bronchioles. This trigger might be the immune system attacking transplanted tissue, chemicals damaging the airway lining, or an infection causing injury. Whatever the initial cause, the body responds by sending immune cells to the affected area. These cells release substances intended to fight off the threat and begin healing.^[1]

However, in bronchiolitis obliterans syndrome, this inflammatory response becomes excessive or prolonged. Instead of resolving normally, the inflammation continues and causes more damage. The walls of the tiny airways become thickened and infiltrated with inflammatory cells.^[1]

As the inflammation persists, the body tries to repair the damage by laying down scar tissue, a process called fibrosis. This scar tissue is different from normal, healthy airway tissue. It is stiff and inflexible, and it narrows the opening of the bronchioles. In severe cases, some airways may become completely blocked or “obliterated,” which is where the condition gets its name.^[1]

The narrowing and blockage of these tiny airways creates several problems. First, it becomes harder for air to flow in and out of the lungs, particularly during exhalation. This creates an obstructive pattern of lung disease, similar to what happens in asthma or chronic obstructive pulmonary disease (COPD), but the changes are permanent and don’t respond to medications that open airways. Second, air can become trapped in parts of the lung beyond the narrowed airways, leading to what doctors call air trapping. This trapped air takes up space but can’t participate in normal oxygen exchange.^[1]

Over time, the reduced airflow means less oxygen reaches the bloodstream. The body has to work harder to get enough oxygen, which explains the shortness of breath and fatigue that patients experience. The narrowed airways also create turbulent airflow, which produces the wheezing sound.^[1]

The disease can progress at different rates in different people. Some individuals may experience rapid worsening over months, while others may have a slower progression. In transplant recipients, the condition typically leads to progressive decline in lung function over time, though the rate varies from person to person.^[3]

Unfortunately, once the scarring occurs, it is permanent. The damaged airways cannot return to normal, which is why early detection and treatment are so important. Treatment aims to stop further damage from occurring, but it cannot reverse changes that have already happened.^[3]