Hypersensitivity pneumonitis

Alternative names
Extrinsic allergic alveolitis; Farmer’s lung; Mushroom picker’s disease; Humidifier or air-conditioner lung; Bird breeder’s lung

Hypersensitivity pneumonitis is an inflammation in the lungs caused by exposure to an allergen (foreign substance), usually organic dust. This dust may come from animal dander, molds, or plants.

Causes, incidence, and risk factors

Hypersensitivity pneumonitis is usually an occupational disease in which exposure to organic dusts, fungus, or molds leads to acute and then, over time, chronic lung disease. Exposure may also occur in the home, from fungus present in humidifiers, heating systems, and air conditioners. Some people may have hobbies that can lead to exposure, such as owning birds.

Acute illness may occur 4 to 6 hours after the exposure, once the person has left the area where the allergen is present. Chronic illness with changes seen on chest X-ray may develop with continued exposures. The chronic form of this disease may lead to pulmonary fibrosis (a scarring of the lung tissue that is often not reversible).


Acute hypersensitivity pneumonitis may have the following symptoms:

  • Cough  
  • Fever  
  • Chills  
  • Shortness of breath  
  • Malaise (feeling ill)

Chronic hypersensitivity pneumonitis may have the following symptoms:

  • Breathlessness, especially with exertion  
  • Cough, often dry  
  • Loss of appetite  
  • Unintentional weight loss

Signs and tests

Crackles (rales) may be heard when a stethoscope is used to examine the chest.

Tests include the following:

  • Chest X-ray  
  • Pulmonary function tests  
  • CBC  
  • Hypersensitivity pneumonitis antibody panels  
  • Aspergillus precipitins test  
  • High-resolution CT scan of the chest  
  • Bronchoscopy with washings or transtracheal biopsy

Treatment seeks to identify the offending allergen and avoid further exposure to it. A change of occupations may be necessary if future worksite exposure is unavoidable. In chronic forms of the disease, treatment with glucocorticoids (a type of steroid drugs) can be tried because this may decrease inflammation.

Expectations (prognosis)

Most symptoms resolve after exposure to the allergen is limited.


Pulmonary fibrosis is a possible complication of hypersensitivity pneumonitis.

Calling your health care provider
Call your health care provider if symptoms of hypersensitivity pneumonitis develop.

The chronic form can be prevented by avoiding further exposure after the offending substance has been identified.

Johns Hopkins patient information

Last revised: December 4, 2012
by Harutyun Medina, M.D.

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