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Hypersensitivity pneumonitis is inflammation of the lungs due to breathing in a foreign substance, usually certain types of dust, fungus, or molds.

Extrinsic allergic alveolitis; Farmer's lung; Mushroom picker's disease; Humidifier or air-conditioner lung; Bird breeder's or bird fancier's lung

Hypersensitivity pneumonitis usually occurs in people who work in places where there are high levels of organic dusts, fungus, or molds.

• Bird fancier's lung 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 is caused by exposure to dust from moldy hay, straw, and grain.

These exposures can lead to lung inflammation and acute lung disease. Over time, this acute condition may turn 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.

Symptoms of acute hypersensitivity pneumonitis may occur 4 - 6 hours after you have left the area where the foreign substance is found, making it difficult to find a connection between your activity and the disease.

Symptoms may include:

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

Symptoms of chronic hypersensitivity pneumonitis may include:

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

Your doctor may hear abnormal lung sounds called crackles (rales) when listening to your chest with a stethoscope.

Lung changes due to chronic hypersensitivity pneumonitis may be seen on a chest x-ray. Other tests may include:

• Aspergillus precipitins test
• Bronchoscopy with washings, biopsy, and bronchoalveolar lavage
• CBC
• High-resolution CT scan of the chest
• Hypersensitivity pneumonitis antibody panels
• Krebs von den Lungen-6 assay (KL-6)
• Pulmonary function tests
• Video-assisted or open-lung biopsy

First, the foreign substance must be identified. Treatment involves avoiding this substance in the future. Some people may need to change jobs if they cannot avoid the substance at work.

If you have a chronic form of this disease, your doctor may recommend that you take glucocorticoids (powerful anti-inflammatory medicines). Sometimes treatments used for asthma can help people with hypersensitivity pneumonitis.

Most symptoms go away when you avoid or limit your exposure to the material that caused the problem.

The chronic form of this disease may lead to pulmonary fibrosis (a scarring of the lung tissue that often is not reversible).

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

The chronic form can be prevented by avoiding the material that causes the lung inflammation.

Rose CS, Lara AR. Hypersensitivity pneumonia. In: Mason RJ, Broaddus VC, Martin TR, et al, eds. Murray and Nadel's Textbook of Respiratory Medicine. 5th ed. Philadelphia, Pa: Saunders Elsevier; 2010:chap 66.

Samet JM. Occupational pulmonary disorders. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 93.