Hypersensitivity pneumonitis is caused by repeated exposure to environmental substances that cause inflammation in the lungs when inhaled. These substances include certain:
Learn more about common environmental sources of substances that may cause your condition.
Common environmental sources of substances that can cause hypersensitivity pneumonitis are:
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.
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.
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.
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.
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.
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.
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.
Some viral infections later in life may increase the risk of developing hypersensitivity pneumonitis.
Men and women can have hypersensitivity pneumonitis. Some small studies found this condition to be slightly more common in women.
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.
Sign and symptom 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.
The following are common signs and symptoms of acute, subacute, and chronic hypersensitivity pneumonitis.
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.
Acute hypersensitivity pneumonitis is the most common form of this condition. It is thought to occur as a result of a short period of exposure to a large amount of causative substance. Symptoms usually occur within 9 hours of being exposed again to a substance that triggers your immune system. If an additional exposure does not occur, symptoms usually resolve after a few days. Subacute and chronic forms of hypersensitivity pneumonitis occur after multiple or continuous exposures to small amounts of causative substance. Approximately 5 percent of patients develop chronic disease.
Hypersensitivity pneumonitis may cause the following potentially fatal complications if the condition is not diagnosed or well controlled by treatment.
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.
To diagnose hypersensitivity pneumonitis, your doctor may order:
Learn how these diagnostic tests and procedures can help distinguish between types of hypersensitivity pneumonitis.
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.
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.
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.
Treatments for hypersensitivity pneumonitis usually include avoidance strategies and medicines. Occasionally, lung transplants are used to treat severe chronic disease in some patients.
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.
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.
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:
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.
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.
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.
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.
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.
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.
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.
Learn how we are translating current research into improved health for people with hypersensitivity pneumonitis.
Learn about some of the pioneering research contributions we have made over the years that have improved clinical care.
In support of our mission, we are committed to advancing research on interstitial lung diseases including hypersensitivity pneumonitis.
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.
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.
After reading our hypersensitivity pneumonitis Health Topic, you may be interested in additional information found in the following resources.