Extrinsic Allergic Alveolitis

What Is Extrinsic Allergic Alveolitis? (Hypersensitivity Pneumonitis)

Extrinsic allergic alveolitis, also known as hypersensitivity pneumonitis (HP), is a type of allergic reaction from exposure to certain substances that cause inflammation in the lungs. Typically, people with HP are exposed to specific allergens at work or with certain hobbies. Bacteria, fungi, animal and plant proteins, and other allergens can cause HP (there are over 300 known allergens). People with HP experience inflammation in the small air sacs (alveoli) of the lungs due to exposure to certain allergens. The most common symptoms of HP are coughing and shortness of breath.

Symptoms may occur immediately with exposure (acute) or develop over time and remain ongoing (chronic). HP causes a unique immune reaction in the body, different from pet and pollen allergies. Unlike common allergies that cause asthma and hay fever, repeated exposure to allergens that cause HP can lead to permanent lung damage. Treatment centralizes on avoiding exposure and reducing inflammation.

Signs and Symptoms

Symptoms of HP can be acute or chronic. Sometimes people with HP may feel like they have the flu. If it is chronic HP, you may notice that shortness of breath happens easier and more frequently, or you may notice increased coughing and fatigue. There are several symptoms that occur with acute and chronic HP. Symptoms include:


  • Shortness of breath
  • Dry cough
  • Chest tightness
  • Fever or chills
  • Muscle aches
  • Fatigue 


  • Shortness of breath, especially with exertion or exercise
  • Cough
  • Fatigue
  • Weight loss
  • Finger or toe clubbing (changes to the ends of your toes or fingers, or to the areas around or under them)


HP develops when allergens get into the small air sacs (alveoli) of the lungs and cause inflammation. The inflammation makes it difficult for oxygen to pass through the lungs and into the bloodstream. There are over 300 known allergens or irritants that may cause HP. The most common allergens include:

  • Bacteria
  • Airborne mold and fungi particles (“farmer’s lung” or “hot tub lung”)
  • Certain chemicals and metals
  • Animal and plant proteins
  • Bird feathers and excrement
  • Certain drugs (certain chemotherapy medications, antibiotics, heart medications, or an overdose of aspirin)
  • Radiation therapy 

Risk Factors

There are several risk factors that contribute to the development of HP. Risk factors are associated with exposure to certain allergens. Common risk factors include:

  • Farming
  • Bird handling
  • Using hot tubs
  • Using humidifiers
  • Chemotherapy and radiation treatments 


Diagnosing HP requires your doctor to evaluate your symptoms, take a thorough medical history, and conduct a physical examination. During your physical examination, your doctor will listen to your lungs with a stethoscope and check your blood oxygen saturation with a pulse oximeter. Your doctor may also ask about your work and living environments to assess for possible allergen exposure. Additionally, there are several tests that will be used for diagnosing.

Tests include:

  • Blood tests. This test detects specific allergen reactions in the blood for HP (hypersensitivity pneumonitis panel)
  • Imaging tests. Chest X-rays and CT scans allow doctors to get a detailed image of your lungs and determine any lung problems or abnormalities.
  • Pulmonary function tests. These tests often include breathing into a tube to measure your lung health. Tests may also include blood tests and exercise tests to assess lung functioning.
  • Bronchoscopy. This test uses a camera attached to a small flexible tube to view the inside of the lungs and collect samples. The tube is inserted through the nose or mouth and down into the lungs.
  • Surgical lung biopsy. This test is used to collect lung tissue samples when a bronchoscopy is not able to reach certain sections of the lungs, or when the sample area is too large. 


The primary treatment for HP is to avoid exposure to the allergens that are causing the symptoms. If that is not possible, or if your symptoms continue even after avoiding exposure, your doctor may recommend other treatment options.

Other treatments include:

  • Corticosteroids. This type of medication helps to reduce inflammation in your lungs by suppressing your immune system. Using these medications long-term can lead to a greater risk of infection and osteoporosis.
  • Oxygen therapy. If a person is struggling to breathe, it may be recommended to use an oxygen machine to deliver oxygen throughout the body. The machine delivers oxygen through a mask or plastic tubing with prongs that fit into the nostrils. Some people may only need this while sleeping or exercising.
  • Pulmonary rehabilitation. This consists of physical therapy or breathing exercises that may help improve breathing.
  • Anti-fibrotic drugs. These are medications that help reduce lung scarring. 


The lung damage that occurs with chronic HP is not reversible. The best way to prevent HP is to avoid environments that expose you to harmful allergens. If it is impossible to avoid these environments, it is recommended to use appropriate PPE (personal protective equipment) while in environments with HP-causing allergens.

Additional prevention measures include:

  • Keeping hot tubs clean and in good condition
  • Keeping humidifiers and cooling systems clean and in good condition
  • Avoid using feather-filled bedding
  • Keeping pets’ living spaces clean, especially bird cages (wear a mask while cleaning these areas) 


Hypersensitivity pneumonitis that goes untreated can lead to permanent lung damage. Serious complications include:

  • Lung scarring (pulmonary fibrosis)
  • High blood pressure between your heart and lungs (pulmonary hypertension)

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