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What Causes Hypersensitivity Pneumonitis?

Repeatedly breathing in foreign substances can cause hypersensitivity pneumonitis (HP). Examples of these substances include molds, dusts, and chemicals. (Mold often is the cause of HP.) These substances also are known as antigens.

Over time, your lungs can become sensitive to antigens. If this happens, your lungs will become inflamed, which can lead to symptoms and may even cause long-term lung damage.

Antigens may be found in the home, workplace, or in other settings. Antigens can come from many sources, such as:

  • Bird droppings
  • Humidifiers, heating systems, and hot tubs
  • Liquid chemicals used in the landscaping and florist industries
  • Moldy hay, straw, and grain
  • Chemicals released during the production of plastics and electronics, and chemicals released during painting
  • Mold released during lumber milling, construction, and wood stripping
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Hypersensitivity Pneumonitis Clinical Trials

Clinical trials are research studies that explore whether a medical strategy, treatment, or device is safe and effective for humans. To find clinical trials that are currently underway for Hypersensitivity Pneumonitis, visit

May 09, 2014 Last Updated Icon

The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.