Learn About Hypersensitivity Pneumonitis

Hypersensitivity pneumonitis is caused when you breathe in a specific substance (allergen) that triggers an allergic reaction in your body. These allergens are often naturally occurring—such as bird feathers or droppings, household mold and animal dander.

Key Facts

  • Hypersensitivity pneumonitis is a rare reaction to an allergen that can be present at home, work or outdoors.
  • Cases vary in severity and are normally categorized as either acute or chronic. Acute hypersensitivity pneumonitis is the most common. 
  • Only about 5% of people with acute hypersensitivity pneumonitis develop chronic forms of the condition. 
  • Primary treatment is to eliminate exposure to the allergen, with medication being administered if symptoms don’t subside.
  • Sometimes hypersensitivity pneumonitis can cause permanent lung scarring (also called pulmonary fibrosis) if it is not detected early and you continue to be exposed to the allergen.

What Causes Hypersensitivity Pneumonitis?

There are more than 300 known substances have been known to cause hypersensitivity pneumonitis. Some commonly seen problems are given specific names related to the source of the allergen, including:

  • Farmer's lung: seen in farmers and cattle workers, this condition is caused by breathing in mold that grows on hay, straw and grain.
  • Bird fancier's lung: (also called pigeon breeder's disease) caused by breathing particles from feathers or droppings of many species of birds.
  • Humidifier lung: can develop by breathing in fungus growing in humidifiers, air conditioners and heating systems, particularly if they are not well maintained.
  • Hot tub lung: may develop by breathing in bacteria that may be found in the water vapor coming from indoor hot tubs.

Who Is at Risk for Hypersensitivity Pneumonitis?

If you work in certain occupations, then you might be at an increased risk of developing hypersensitivity pneumonitis. This includes farmers, bird breeders and poultry handlers, veterinary workers and animal handlers, grain and flour processors, woodworkers and workers in some chemical and electronics industries. However, most people who work in these occupations, around 85-95%, will never develop hypersensitivity pneumonitis.

Genetics may be a factor, as hypersensitivity pneumonitis commonly runs in families. Unfortunately, at this time, there is not much information about the genes that can put a person at increased risk for developing hypersensitivity pneumonitis. Many patients who are diagnosed with this condition are between the ages of 50 and 55, but it has been known to affect people of all ages

Reviewed and approved by the American Lung Association Scientific and Medical Editorial Review Panel.

Page last updated: February 27, 2020

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