Hypersensitivity Pneumonitis


What is hypersensitivity pneumonitis?

Hypersensitivity pneumonitis (HP) is a lung disease causing inflammation (swelling and sensitivity) of the lung tissue. This inflammation makes breathing difficult. It can lead to irreversible lung scarring over time.

HP results from breathing in specific environmental allergens. Allergens are substances that cause an allergic reaction in the body. These allergens may be present at home, at work, or in nature. Bird feathers and droppings, and household mold are examples of common allergens.

Who is likely to have hypersensitivity pneumonitis?

People working in certain occupations are more likely to develop HP. These occupations include:

  • Farmers, including those working with dairy cattle or vegetables
  • Animal handlers, including veterinarians and bird or poultry handlers
  • People who process and load grains or flour
  • Individuals working in lumber mills or who strip wood
  • Wallboard and paper manufacturers
  • Certain individuals in the electronics, plastic manufacturing, and painting industries

Other activities that expose you to allergens can increase your likelihood of developing HP. These include:

  • Keeping pet birds and breathing allergens from bird droppings or feathers (known as bird fancier’s lung)
  • Breathing allergens from humidifiers, heating systems, or air conditioners, especially if they are not cleaned properly or well-maintained (humidifier lung)
  • Inhaling bacteria found in hot tub water vapor (hot tub lung)

Is hypersensitivity pneumonitis contagious?

HP is not contagious.

Symptoms and Causes

What causes hypersensitivity pneumonitis?

HP results from inhaling certain allergens into the lungs. Over 300 substances are known to cause HP, including:

  • Bacteria
  • Molds and fungi
  • Some chemicals
  • Certain proteins

These substances cause inflammation of the lung tissue when inhaled. In many cases, the lungs remain inflamed over time, with repeated exposure to an allergen.

Many cases of HP occur acutely (suddenly). In general, acute (or sudden) HP symptoms occur 4- 6 hours after allergens are inhaled. These cases usually are treated successfully.

Some people develop chronic hypersensitivity pneumonitis. You are more likely to develop chronic hypersensitivity pneumonitis if you are exposed to low levels of allergens constantly over an extended period. The symptoms of chronic HP develop over months or years.

What are the symptoms of hypersensitivity pneumonitis?

The symptoms of HP depend on whether the disease is acute or chronic. In general, symptoms of acute, or sudden, HP last between 12 hours and several days. Symptoms of acute HP may include:

  • Shortness of breath
  • Dry cough
  • Chest tightness
  • Chills
  • Fatigue
  • Fever

The symptoms of chronic HP may include:

  • Shortness of breath, especially with exertion or activity
  • Cough
  • Fatigue
  • Weight loss

In rare cases, chronic HP leads to irreversible, permanent scarring of the lung tissue. This condition is called pulmonary fibrosis. This serious disease causes symptoms resulting from too little oxygen reaching body tissues.

Pulmonary fibrosis symptoms may include:

  • Shortness of breath
  • Fast, shallow breathing
  • Dry, hacking cough
  • Weight loss
  • Fatigue
  • Finger and toe clubbing (rounding and widening)

Diagnosis and Tests

How is hypersensitivity pneumonitis diagnosed?

Your doctor diagnoses HP by taking a detailed personal history and performing a physical examination.

During your examination, your doctor listens to your lungs with a special instrument called a stethoscope. If you have HP, your doctor may hear abnormal crackles, pops, or other lung sounds.

Your doctor also uses a device called a pulse oximeter to measure the oxygen level in your blood. This device is placed over the end of one of your fingers.

If necessary, your doctor uses other diagnostic tests to confirm your diagnosis. These tests may include chest X-rays, CT scans and lung function tests.

Additional tests like bronchoscopy can help diagnose HP. During this procedure, your doctor uses a small, flexible tube passed through your nose or mouth to view the interior of your lungs, and collect samples. In some cases, when a larger sample is required, doctors perform a surgical lung biopsy to surgically remove a sample of your lung tissue for further testing.

Management and Treatment

How is hypersensitivity pneumonitis treated?

Treatment for HP begins with avoiding any allergens that cause your symptoms. Your doctor may recommend staying away from sources of allergens, including pets or farm animals.

Your doctor may suggest changing the way you work so you do not breathe in allergens. For example, you may need to wear an air-purifying respirator, which prevents you from inhaling allergens.

If allergens are unavoidable, your doctor may prescribe corticosteroids like Prednisone. These medications help reduce inflammation, but you may need to take them for several months.

Your doctor may also prescribe immunosuppressive medications like mycophenolate (Myfortic®, CellCept®) or azathioprine (Imuran®). These medications help prevent your immune system from reacting to allergens you inhale. They can decrease inflammation within your lungs.

In severe cases, doctors prescribe supportive therapies to help make breathing easier. Supportive therapies may include:

  • Bronchodilators to help relax your airways so they open and make breathing easier
  • Oxygen to raise oxygen levels in your blood

What complications are associated with hypersensitivity pneumonitis?

In severe cases, HP causes permanent, irreversible scarring of the lung tissue from repeated, long-term exposure to allergens. This condition is known as pulmonary fibrosis. It makes breathing difficult and prevents enough oxygen from reaching body tissues.

If you have chronic hypersensitivity pneumonitis, you are more at risk for pulmonary hypertension. Pulmonary hypertension is extremely high blood pressure inside the arteries that supply blood to the lungs. This condition requires thorough management by your doctor.


Can hypersensitivity pneumonitis be prevented?

The best way to prevent pneumonitis is by avoiding exposure to allergens causing lung inflammation. If it is impossible to avoid allergens completely, protective masks or other air filters can reduce your exposure.

In some cases, pneumonitis can be prevented by avoiding water-damaged areas at home or work. Thoroughly cleaning wet ventilation systems, moist carpets, and other environments can help. In these spots, allergens like mold can grow and flourish.

Outlook / Prognosis

What is the prognosis (outlook) for people with hypersensitivity pneumonitis?

Many people with acute HP recover by avoiding allergens. For more severe cases, taking medications like corticosteroids often reverses the inflammation.

Living With

When should I call my doctor if I have hypersensitivity pneumonitis?

If you have symptoms of hypersensitivity pneumonitis, or if your symptoms suddenly get worse, notify your doctor immediately. With your doctor’s help, it is possible to manage the symptoms of HP and prevent complications.

Last reviewed by a Cleveland Clinic medical professional on 08/09/2018.


  • American Lung Association. Hypersensitivity Pneumonitis. (http://www.lung.org/lung-health-and-diseases/lung-disease-lookup/hypersensitivity-pneumonitis/) Accessed 8/13/2018.
  • American Lung Association. Symptoms of Pulmonary Fibrosis. (http://www.lung.org/lung-health-and-diseases/lung-disease-lookup/pulmonary-fibrosis/introduction/symptoms.html) Accessed 8/13/2018.
  • Chest Foundation. Hypersensitivity Pneumonitis. (https://foundation.chestnet.org/patient-education-resources/hypersensitivity-pneumonitis/) Accessed 8/13/2018.
  • Merck Manual Professional Version. Hypersensitivity Pneumonitis. (https://www.merckmanuals.com/professional/pulmonary-disorders/interstitial-lung-diseases/hypersensitivity-pneumonitis) Accessed 8/13/2018.
  • National Heart, Lung, and Blood Institute. Hypersensitivity Pneumonitis. (https://www.nhlbi.nih.gov/health-topics/hypersensitivity-pneumonitis) Accessed 8/13/2018.

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