Learn About Hypersensitivity Pneumonitis | American Lung Association

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Learn About Hypersensitivity Pneumonitis

Hypersensitivity pneumonitis is a disease of the lungs in which your lungs become inflamed as an allergic reaction to inhaled dust, fungus, molds or chemicals.

Key Facts

  • Hypersensitivity pneumonitis is caused by an allergy to certain dusts (allergens) that you inhale.
  • This allergy causes inflammation in your lungs
  • If detected early, and you avoid the allergy-producing substance, the inflammation can be reversed.
  • Sometimes hypersensitivity pneumonitis can cause lung scarring if it is not detected early and you continue to be exposed to the substances.

What Is Hypersensitivity Pneumonitis?

Hypersensitivity pneumonitis is caused by an allergy to certain dust (allergens) that you breathe in. Commonly, these allergens are naturally occurring (organic). These allergens may be present at home, work or in the environment. Commonly, these dusts contain fungus spores from moldy hay or bird droppings.

How Does Hypersensitivity Pneumonitis Affect Your Body?

When you breathe in the dust that you are allergic to, you will not notice any problems the first time. Some individuals may develop symptoms after a large exposure to the dust or after repeated smaller exposures. The tiny air sacs in the lungs become inflamed as their walls fill with white blood cells and, occasionally, the air sacs may also fill with fluid. The inflammation gets better within a few days if you no longer breathe in those dusts. If you breathe in those dusts repeatedly, the inflammation in the lung continues. This may lead to some portions of the lung developing scar tissue. When your lungs have scar tissue, you may have trouble breathing normally.

How Serious Is Hypersensitivity Pneumonitis?

Hypersensitivity pneumonitis can become a serious condition for some individuals whose lungs develop scarring. Lung scarring (also called pulmonary fibrosis) may occur in the later stages of the disease. The lung scarring is permanent. Unfortunately, there is no cure or effective treatment for chronic (or long-standing) hypersensitivity pneumonitis. The good news is that if the disease is caught in the early stages and if you avoid the dust, then it can be completely reversed. That is why it is very important to catch this disease in the early stages to prevent permanent lung damage.


    This content was developed in partnership with the CHEST Foundation, the philanthropic arm of the American College of Chest Physicians.


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