Hypersensitivity Pneumonitis

Also known as Extrinsic allergic alveolitis, Bird fancier’s lung, Farmer’s lung, Hot tub lung, Humidifier lung
Hypersensitivity pneumonitis is a rare immune system disorder that affects the lungs. It occurs in some people after they breathe in certain substances they encounter in the environment. These substances trigger their immune systems, causing short- or long-term inflammation, especially in a part of the lungs called the interstitium. This inflammation makes it harder for the lungs to function properly and may even permanently damage the lungs. If diagnosed, some types of hypersensitivity pneumonitis are treatable by avoiding exposure to the environmental substances or with medicines such as corticosteroids that reduce inflammation. If the condition goes untreated or is not well controlled over time, the chronic inflammation can cause irreversible scarring of the lungs that may severely impair their ability to function.

Explore the hypersensitivity pneumonitis Health Topic sections to learn more about this condition, NHLBI’s role in research and clinical trials to improve health, and where to find more information.

Causes - Hypersensitivity Pneumonitis

Hypersensitivity pneumonitis is caused by repeated exposure to environmental substances that cause inflammation in the lungs when inhaled. These substances include certain:

  • Bacteria and mycobacteria
  • Fungi or molds
  • Proteins
  • Chemicals

Learn more about common environmental sources of substances that may cause your condition.

Where can these substances be found in the environment?

Common environmental sources of substances that can cause hypersensitivity pneumonitis are:

  • Animal furs
  • Air conditioner, humidifier, and ventilation systems
  • Bird droppings and feathers
  • Contaminated foods such as cheese, grapes, barley, sugarcane
  • Contaminated industry products or materials such as sausage casings and corks
  • Contaminated metal working fluid
  • Hardwood dusts
  • Hay or grain animal feed
  • Hot tubs

Because this condition is caused by different substances found in many environmental sources, doctors once thought they were treating different lung diseases. Research has helped us understand hypersensitivity pneumonitis is triggered by different causative substances.

Why does hypersensitivity pneumonitis only occur in some people?

If you have hypersensitivity pneumonitis, your body’s immune system reacts strongly to certain substances. Differences in our immune systems may explain why some people have strong reactions after breathing in certain substances, while others who breathe those same substances do not.

Learn more about the immune system of the lungs and how it contributes to hypersensitivity pneumonitis.

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Normally, the immune system in the lungs monitors inhaled substances. The immune system is activated when it recognizes a portion of the substance called the antigen as foreign. The activated immune system produces molecules that cause normal levels of inflammation, such as increased levels of immune cells and factors including antibodies that recognize and help clear the foreign substance. Normally after clearing the substance, the immune system shuts off and the inflammation stops. Usually, these processes are well controlled.

The immune systems of people with hypersensitivity pneumonitis are unable to shut down these normal inflammation processes, especially in the lung interstitium. The interstitium is a space where the lung’s air sacs, called alveoli, come in contact with blood vessels and a small amount of connective tissue. When there is high level of inflammation in the lungs, immune cells begin to collect in this space. These uncontrolled levels of inflammation in the lungs cause the signs, symptoms, and complications of this condition.

lymphatic vessel diagram
In the lung interstitium, alveoli air sacs come into contact with the blood vessels and connective tissues of the lung. If you have hypersensitivity pneumonitis, your body’s immune system reacts strongly to certain inhaled substances, causing inflammation especially in the interstitium or interstitial space.

Risk Factors - Hypersensitivity Pneumonitis

Certain factors affect your risk of developing hypersensitivity pneumonitis. These factors include age, environment or occupation, family history and genetics, lifestyle habits, other medical conditions, and sex or gender.


Although hypersensitivity pneumonitis can occur at any age, people tend to be diagnosed with this condition between 50 and 55 years of age. Hypersensitivity pneumonitis is a common type of chronic interstitial lung disease in children.

Environment or occupation

Repeated exposure to certain substances that cause the condition, possibly while working in occupations where environmental sources are common, can increase your risk of developing hypersensitivity pneumonitis. Certain occupations—such as farmers or people who breed animals or birds, cheese washers, woodworkers, and wine makers—have a greater chance of exposure to causative substances. However, you may be exposed to environmental sources in your home or elsewhere. Even having pets such as birds in the home can increase your risk of hypersensitivity pneumonitis.

Alone, environmental exposure to causative substances is not enough to cause hypersensitivity pneumonitis. An estimated 85 to 95 percent of people exposed to causative substances either never develop hypersensitivity pneumonitis or they experience a mild immune reaction with no obvious signs or symptoms or disease.

Family history and genetics

Genetics is thought to predispose some people to have strong immune responses and develop hypersensitivity pneumonitis after repeat exposures to a causative substance. In some populations, family history of pulmonary fibrosis or hypersensitivity pneumonitis may increase the risk of developing hypersensitivity pneumonitis. When hypersensitivity pneumonitis occurs in relatives it is called familial hypersensitivity pneumonitis.

Researchers are beginning to map genetic variations in immune system proteins that may increase the risk for developing hypersensitivity pneumonitis. These differences may explain why immune cells respond differently between people who do or do not develop hypersensitivity pneumonitis after the same exposure to a causative substance.

Lifestyle habits

Smoking is not thought to increase the risk of developing hypersensitivity pneumonitis. However, smoking can worsen chronic hypersensitivity pneumonitis and cause complications. If you have hypersensitivity pneumonitis, learn why doctors recommend quitting smoking.

Other medical conditions

Some viral infections later in life may increase the risk of developing hypersensitivity pneumonitis.

Sex or gender

Men and women can have hypersensitivity pneumonitis. Some small studies found this condition to be slightly more common in women.

Screening and Prevention - Hypersensitivity Pneumonitis

Currently, there are no screening methods to determine who will or will not develop hypersensitivity pneumonitis. you avoid common environmental sources of substances known to cause this condition. If you are at risk for hypersensitivity pneumonitis, your doctor may recommend you avoid common environmental sources of substances known to cause this condition.

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  • Diagnosis will explain tests and procedures that your doctor may use to diagnose hypersensitivity pneumonitis.
  • Living With will discuss some additional medical care or lifestyle changes that your doctor may recommend to prevent hypersensitivity pneumonitis from recurring, getting worse, or causing serious complications.

Signs, Symptoms, and Complications - Hypersensitivity Pneumonitis

Signs and symptoms vary between acute, subacute, and chronic types of hypersensitivity pneumonitis. If your condition is not diagnosed or well controlled by treatment, it can lead to irreversible lung damage and other potentially fatal complications.

Signs and Symptoms

The following are common signs and symptoms of acute, subacute, and chronic hypersensitivity pneumonitis. 

table signs and symptoms hp by type
Signs and symptoms of acute, subacute, and chronic hypersensitivity pneumonitis may include flu-like illness including fever, chills, muscle or joint pain, or headaches; rales; cough; chronic bronchitis; shortness of breath; anorexia or weight loss; fatigue; fibrosis of the lungs; and clubbing of fingers or toes.

While some signs and symptoms occur in several types of hypersensitivity pneumonitis, they may vary in severity. The exact signs and symptoms you experience also may vary.

Learn about how exposure history affects whether you show signs and symptoms of acute, subacute, or chronic hypersensitivity pneumonitis.


- Hypersensitivity Pneumonitis

Hypersensitivity pneumonitis may cause the following potentially fatal complications if the condition is not diagnosed or well controlled by treatment.

  • Irreversible lung damage and permanently reduced lung function because of severe fibrosis and impaired ability to oxygenate the blood during normal breathing.
  • Pulmonary hypertension due to damage of blood vessels in the lungs.
  • Heart failure because inflammation makes it harder for the heart to pump blood to and through the lungs.

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- Hypersensitivity Pneumonitis

  • Diagnosis will explain tests and procedures that are used to detect signs of hypersensitivity pneumonitis and help rule out other conditions.
  • Treatment will discuss possible treatment-related complications.

Diagnosis - Hypersensitivity Pneumonitis

To diagnose hypersensitivity pneumonitis, your doctor will collect your medical history to understand your symptoms and see if you have an exposure history to possible causative substances. Your doctor will perform a physical exam and may order diagnostic tests and procedures. Based on this information, your doctor may able to determine whether you have acute, subacute, or chronic hypersensitivity pneumonitis.

Diagnostic tests and procedures

To diagnose hypersensitivity pneumonitis, your doctor may order:

  • Blood tests to detect high levels of white blood cells and other immune cells and factors in your blood that indicate your immune system is activated and causing inflammation somewhere in your body.
  • Bronchoalveolar lavage (BAL) to collect fluid from your lungs that can be tested for high levels of white blood cells and other immune cells. High levels of these cells mean your body is making an immune response in your lungs, but low levels do not rule out hypersensitivity pneumonitis.
  • Computed tomography (CT) to image the lungs and look for inflammation or damage such as fibrosis. CT scans, particularly high-resolution ones, can help distinguish between types of hypersensitivity pneumonitis.
  • Inhalation challenge tests to see if a controlled exposure to a suspected causative substance triggers your immune system and the onset of common signs and symptoms such as an increase in temperature, increase in white blood cell levels, rales that are heard during a physical exam, or reduced lung function. A positive test can confirm an inhaled substance triggers your immune system. A negative test does not rule out that you have hypersensitivity pneumonitis, because it may mean a different untested environmental substance is causing your condition. Before having this test, talk to your doctor about the benefits and possible risks of this procedure.
  • Lung biopsies to see if your lung tissue shows signs of inflammation, fibrosis, or other changes known to occur in hypersensitivity pneumonitis.
  • Lung function tests to see if you show signs of restriction such as reduced breathing capacity or abnormal blood oxygen levels and check if you have obstructed airways. These tests help assess the severity of your lung disease and when repeated they can help monitor whether your condition is stable or worsening over time. Lung function tests may be normal between acute flares.
  • Precipitin tests to see if you have antibodies in your blood that recognize and bind to a causative substance. While a positive test means that you have been exposed to a substance, it cannot confirm you have hypersensitivity pneumonitis. This is because some people without this condition also have antibodies in their blood to these substances. If you have antibodies to a substance, your doctor may have you perform an inhalation challenge test to see if a new exposure to the same substance can activate your immune system and cause a new acute flare.
  • Chest x-rays to image the lungs and look for inflammation or damage such as fibrosis in your lungs.

Learn how these diagnostic tests and procedures can help distinguish between types of hypersensitivity pneumonitis.

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Chest x-ray and CT scans, lung biopsies, and lung function tests may help your doctor diagnose the type of hypersensitivity pneumonitis that you have.

  • Chest x-rays and CT scans may show little to no inflammation in your lungs during the early phase of acute disease, but they will show obvious signs of inflammation and tissue damage such as fibrosis in subacute and chronic hypersensitivity pneumonitis.
  • Lung function tests may detect breathing difficulties, small airflow blockages, or reduced lung function in acute hypersensitivity pneumonitis. These problems may worsen and progress to severe lung restriction in subacute and chronic hypersensitivity pneumonitis.
  • Lung biopsies will reveal inflammation, fibrosis, and possibly granulomas in subacute and chronic hypersensitivity pneumonitis. They are not usually performed for acute hypersensitivity pneumonitis.

Your medical and exposure histories can help your doctor diagnose acute hypersensitivity pneumonitis and possibly identify the substance causing your condition. This is because the exposure will have occurred right before your acute symptoms started.

Is it hard to diagnose this condition?

It can take months or even years for your doctor to diagnose hypersensitivity pneumonitis in you or your child. Learn why hypersensitivity pneumonitis can be hard to diagnose.

  • There are no clear exposure histories to potential causative substances before having symptoms. This occurs in up to 50 percent of patients who are later diagnosed with hypersensitivity pneumonitis. Despite hypersensitivity pneumonitis being a common childhood interstitial lung disease, children are often diagnosed late after the condition has progressed to chronic disease. This is because children tend to be exposed to small amounts of causative substance over long periods of time, which does not trigger obvious acute symptoms and makes it very difficult to determine their exposure history.
  • Other conditions may cause similar signs and symptoms. Before diagnosing hypersensitivity pneumonitis, your doctor must rule out: unintentional effects of medicines such as bleomycin, methotrexate, or nitrofurantoin; lung infections such as pneumonia or the flu (influenza); smoking-related lung disease; connective tissue disease; bleeding in the lungs; idiopathic pulmonary fibrosissarcoidosis; and lung cancer.
  • Diagnostic features seen in chest x-rays, CT scans, and lung biopsies may differ between children and adults. Even when a person’s exposure history is known or hypersensitivity pneumonitis is suspected, doctors look for diagnostic features in chest x-rays, CT scans, and lung biopsies that are indicators of the disease in adults. More research is needed to help map diagnostic features for children with this condition.


  • Return to Risk Factors to review family history, lifestyle, or other environmental factors that increase your risk of developing hypersensitivity pneumonitis.
  • Return to Signs, Symptoms, and Complications to review differences between acute, subacute, and chronic hypersensitivity pneumonitis.

Treatment - Hypersensitivity Pneumonitis

Treatments for hypersensitivity pneumonitis usually include avoidance strategies and medicines. Occasionally, lung transplants are used to treat severe chronic disease in some patients.

Avoidance strategies

If your doctor is able to identify the environmental substance that causes your hypersensitivity pneumonitis, he or she will recommend that you adopt the following avoidance strategies.

  • Remove the causative substance if possible
  • Replace workplace or other products with available alternatives that do not contain the substance responsible for your condition
  • Alter work processes so you don’t continue to breathe in the causative substance
  • Stay away from known sources of your causative substance


If avoidance strategies do not work for your condition, your doctor may prescribe corticosteroids or other Immunosuppressive medicines to treat your condition. The choice, dose, and duration of these medicines will depend on your condition and medical history. Acute and subacute types of hypersensitivity pneumonitis usually respond well to these treatments.

Depending on your condition, your doctor also may prescribe some of the following supportive therapies.

  • Oxygen therapy as needed for low levels of oxygen in the blood.
  • Bronchodilators to relax the muscles in the airways and open your airways to make breathing easier.
  • Opioids to control shortness of breath or chronic cough that is resistant to other treatments. Regular (e.g. several times a day, for several weeks or more) or longer use of opioids can lead to physical dependence and possibly addiction.

Lung transplants

If your condition is not adequately controlled by avoidance strategies or medicines and you develop serious complications, you may be a candidate for a lung transplant. During this procedure, healthy donor lung will be transplanted into you to replace the damaged lung. Two important things to know:

  • This procedure is not a cure. This is because your immune system will be the same after the procedure. This means that if you are exposed again to the substances that triggers your immune system, new inflammation may damage the transplanted donor lung tissue.
  • This procedure is not for everyone. Even if you are a candidate for this procedure, it may be difficult to find a matching organ donor. Lung transplants are serious medical procedures with their own risks. Talk to your doctor about what procedures are right for you.

Tell me more

Treatment is more successful when hypersensitivity pneumonitis is diagnosed in the early stages of the disease, before permanent irreversible lung damage has occurred. As new data emerges, doctors are becoming more aware of the unique treatment needs for children with hypersensitivity pneumonitis.

Look for

  • Research for Your Health will explain how NHLBI is using current and advancing new research to treat people with hypersensitivity pneumonitis.
  • Living With will discuss some additional medical care or lifestyle changes that your doctor may recommend to prevent your condition from recurring, getting worse, or causing serious complications.

Living With - Hypersensitivity Pneumonitis

If you have hypersensitivity pneumonitis, you can take steps to control the condition and prevent complications by receiving routine follow-up care, monitoring your condition, preventing new acute flares and complications, and learning about and preparing for serious complications.

Receive routine follow-up care

In addition to treatments you are using to control your condition, your doctor may recommend other medical care to improve your quality of life, vaccines to prevent lung infections, and lifestyle changes such as physical activity and quitting smoking to improve overall health and avoid some complications.

  • Other medical care: Your doctor may evaluate how your condition is affecting your activity level and mental health.  To improve your quality of life, your doctor may recommend other treatments to address pain, fatigue, or mental health concerns that you may have.
  • Vaccines: Remember that your condition causes you to have reduced lung function, particularly if you have subacute or chronic hypersensitivity pneumonitis. Your doctor may recommend that you receive routine pneumococcal and flu (influenza) vaccines to avoid lung infections that can further impair your reduced lung function.
  • Physical activity: Patients with hypersensitivity pneumonitis benefit from regular exercise. Before starting any exercise program, ask your doctor about what level of physical activity is right for you.
  • Quitting smoking: If you smoke, quit. Although smoking does not increase the risk of developing hypersensitivity pneumonitis, some studies suggest smoking can worsen disease and shorten survival for people with chronic hypersensitivity pneumonitis compared to nonsmokers with chronic hypersensitivity pneumonitis. Another study reported lung cancer in patients who smoked and had chronic hypersensitivity pneumonitis.

Monitor your condition

If you have been diagnosed with subacute or chronic hypersensitivity pneumonitis, your doctor may recommend follow-up testing to see how well your treatment is working and if your disease is improving, stable, or worse. To monitor your condition, your doctor may recommend repeating tests used earlier to diagnose hypersensitivity pneumonitis such as chest x-rays, computed tomography (CT) scans, or lung function tests.

Your doctor may determine your disease is worse if you have new or more severe fibrosis or lung function problems. High-resolution CT scans may be more informative than lung function tests at assessing disease progression.

There is a growing recognition that disease tends to be worse, such as greater lung fibrosis, if it starts in childhood or early adult life. Therefore, more careful monitoring may be required for younger patients with hypersensitivity pneumonitis.

Prevent new acute flares and serious complications over your lifetime

To help prevent new acute flares and complications, your doctor may recommend tests to identify the substances causing your condition, as well as additional screening tests to prevent potentially fatal complications.

  • Identification of substances causing your condition: If you do not know the environmental substances causing your condition, your doctor may recommend diagnostic precipitin and inhalation challenge tests. Identification can help avoid the environmental sources of the substances causing your condition. Successful avoidance strategies can help you live a longer, prevent new acute flares, and slow or stop progression to chronic disease with serious complications.  
  • Screening for serious complications:  If you have been diagnosed with chronic hypersensitivity pneumonitis, your doctor may recommend echocardiography and right-heart catheterization to evaluate pulmonary artery pressure and screen for pulmonary hypertension. Pulmonary hypertension can occur in people who have chronic hypersensitivity pneumonitis, particularly in patients with more severe disease who have poorer lung function and reduced exercise capacity

Learn the warning signs of serious complications and have a plan

Always notify your doctor if your symptoms suddenly worsen. Your doctor will need to rule out other causes including infection and order repeat chest imaging tests. If these chest imaging tests show new findings without evidence of another cause, your doctor may modify your hypersensitivity pneumonitis treatment plan to better control your condition. Talk to your doctor and agree on a clinical decision plan to help you know when to seek urgent medical care.


  • Return to Signs, Symptoms, and Complications to review signs, symptoms, and complications by type of hypersensitivity pneumonitis.
  • Return to Diagnosis to review tests and procedures your doctor may recommend to monitor your condition or help identify the substances causing your condition.
  • Return to Treatment to review avoidance strategies and other treatments.



Research for Your Health - Hypersensitivity Pneumonitis

The NHLBI is part of the U.S. Department of Health and Human Services’ National Institutes of Health (NIH)—the nation’s biomedical research agency that makes important scientific discoveries to improve health and save lives. We are committed to advancing science and translating discoveries into clinical practice to promote the prevention and treatment of heart, lung, blood, and sleep disorders including hypersensitivity pneumonitis. Learn about the current and future NHLBI efforts to improve health through research and scientific discovery.

Improving health with current research

Learn how we are translating current research into improved health for people with hypersensitivity pneumonitis.

  • NIH Center for Human Immunology, Autoimmunity, and Inflammatory Diseases. We continue to support the NIH Center for Human Immunology as it promotes research on normal immune system processes and the problems that lead to diseases caused by the immune system that cause heart, lung, blood and sleep disorders including hypersensitivity pneumonitis.
  • NIH National Center for Advancing Translational Sciences (NCATS) Rare Diseases Clinical Research Network. We continue to support the NCATS Rare Diseases Clinical Research Network, which is focused on translating scientific discovery into new interventions such as diagnostics, treatments, and procedures that improve health. Our contributions to the larger effort focus on rare lung disease including hypersensitivity pneumonitis. 

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Learn about some of the pioneering research contributions we have made over the years that have improved clinical care.

  • NIH Respiratory Disease Task Force. In 1972, we helped this Task Force develop the first national report on interstitial lung diseases, a family of disorders that includes hypersensitivity pneumonitis.
  • NHLBI-sponsored clinical trials. We supported the Epidemiology of Interstitial Lung Disease study, which helped estimate how many total and new cases of hypersensitivity pneumonitis occur each year in the United States.
  • NHLBI Specialized Center of Research in Occupational and Immunologic Lung Disease. Our center collected data that have helped us control and prevent acute and chronic lung damage due to inhalation of environmental and occupational agents. Visit Specialized Center of Research in Occupational and Immunologic Lung Disease for more information.
  • NHLBI Office of Rare Diseases workshop on hypersensitivity pneumonitis. In 2004, we led this working group to identify scientific needs and opportunities for hypersensitivity pneumonitis. Read the full report from this workshop for more information, and learn about the recommendations we support to help improve the health of people with hypersensitivity pneumonitis.

Advancing research for improved health

In support of our mission, we are committed to advancing research on interstitial lung diseases including hypersensitivity pneumonitis.

  • We fund research. The research we fund today will help improve our future health. Our Division of Lung Disease, specifically the Immunology/Fibrosis program of the Lung Biology and Disease Branch, oversees research we fund on interstitial lung disease including hypersensitivity pneumonitis. Search the NIH Research Portfolio Online Reporting Tools (RePORT) to learn about research NHLBI is funding on hypersensitivity pneumonitis or other interstitial lung diseases. The NHLBI Strategic Vision highlights ways we may support research over the next decade.
  • We stimulate high-impact research. The NHLBI Trans-Omics for Precision Medicine (TOPMed) Program has the potential to help study rare chronic lung diseases such as hypersensitivity pneumonitis by integrating large genetic, environmental, imaging, clinical and other data sets from large patient cohorts.

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Our TOPMed program has the potential to address recommendations made at our 2013 Prevention of Chronic Lung Diseases meeting and that remain relevant to hypersensitivity pneumonitis research. For example, the recommendations called for research to define lung health across the lifespan so we can more precisely determine when disease begins. Through TOPMed and other collaborations, we hope to advance science so health professionals will be able to better predict who will develop hypersensitivity pneumonitis, identify subtypes of disease, and deliver personalized interventions to preempt, prevent, and treat disease.

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Participate in NHLBI Clinical Trials

The National Heart, Lung, and Blood Institute (NHLBI) leads or sponsors many studies aimed at preventing, diagnosing, and treating heart, lung, blood, and sleep disorders.

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