Hypersensitivity pneumonitis (extrinsic allergic alveolitis)

Hypersensitivity pneumonitis (extrinsic allergic alveolitis) 

Hypersensitivity pneumonitis (HP), or extrinsic allergic alveolitis, is an inflammatory syndrome of the lung caused by repetitive inhalation of antigenic agents in a susceptible host. The syndrome varies in intensity, clinical presentation, and natural history depending on the inciting agent, as well as the intensity of exposure. In most cases, the disease can be reversed with prompt diagnosis followed by identification and removal of exposure risks. As such, the prognosis is generally very good.


Hypersensitivity pneumonitis usually occurs in people who work in places where there are high levels of organic dusts, fungus, or mold. Long-term exposure can lead to lung inflammation and acute lung disease. Over time, the acute condition turns into long-lasting (chronic) lung disease.

Hypersensitivity pneumonitis may also be caused by fungi or bacteria in humidifiers, heating systems, and air conditioners found in homes and offices. Exposure to certain chemicals, such as isocyanates or acid anhydrides, can also lead to hypersensitivity pneumonitis.

Examples of hypersensitivity pneumonitis include:

  • Bird fancier's lung: This is the most common type of hypersensitivity pneumonitis. It is caused by repeated or intense exposure to proteins found in the feathers or droppings of many species of birds.
  • Farmer's lung: This type of hypersensitivity pneumonitis is caused by exposure to dust from moldy hay, straw, and grain.


Hypersensitivity pneumonitis is typically divided into two types based on how long you have been affected and how severe your symptoms are.

Acute hypersensitivity pneumonitis 

An attack of acute hypersensitivity pneumonitis usually occurs four to six hours after a short period of intense exposure to the substance you are allergic to. You may feel: 

  • flu 
  • Fever
  • Chills
  • muscle aches
  • Headache
  • cough 

These symptoms may last for as little as 12 hours to a few days and will resolve if further exposure is avoided.

Symptoms of Chronic hypersensitivity pneumonitis 

Chronic hypersensitivity pneumonitis develops after numerous or continuous exposures to small amounts of the allergen. Symptoms may include: 

  • Shortness of breath
  • Tiredness
  • Coughing
  • weight loss 

Over time, some people with chronic hypersensitivity pneumonitis develop clubbing of fingers and toes and irreversible pulmonary fibrosis


Your healthcare provider will diagnose hypersensitivity pneumonitis based on your medical history, symptoms, physical exam, and results from lung tests and blood tests. You may also need one of the following tests:

  • Inhalation challenge tests show whether you develop symptoms when you breathe in a specific substance.
  • Precipitin tests check for antibodies in your blood that recognize and bind to a specific substance. If you have antibodies to a substance, your healthcare provider may use an inhalation challenge test to confirm that this substance is causing your symptoms.

General Rating Formula for Interstitial Lung Disease (diagnostic codes 6825 through 6833): 

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