Hypersensitivity Pneumonitis

Medically Reviewed on 3/16/2023

What is hypersensitivity pneumonitis?

Hypersensitivity pneumonitis (HP) is an inflammation of the lung (usually of the very small airways) caused by the body's immune reaction to small air-borne particles. 

What triggers hypersensitivity pneumonitis?

Small airborne particles that trigger hypersensitivy pneumonitis include bacteria, mold, fungi, or even inorganic matter.

What is acute hypersensitivity pneumonitis?

Acute hypersensitivity pneumonitis tends to occur 4-12 hours after exposure (usually heavy exposure) to the particles.

What are early signs of pneumonitis?

Symptoms of acute hypersensitivity pneumonitis include:

Chest X-ray may show diffuse small nodules in the lungs. Typically, the symptoms will subside hours to days after exposure (provided there are no repeated exposures). The abnormalities on chest X-ray abnormalities will also disappear. The patient's condition can deteriorate after exposure; therefore it is imperative that a medical evaluation be performed to determine the best course of treatment.

What is chronic hypersensitivity pneumonitis?

Chronic (long-term) hypersensitivity pneumonitis causes lung scarring (fibrosis).

What are the symptoms of chronic hypersensitivity pneumonitis?

Symptoms of chronic hypersensitivity pneumonitis include:

  • Shortness of breath
  • Cough

Chronic disease is believed to occur after prolonged low grade exposure to the offending particles. It is sometimes quite surprising that individuals with a passion for their hobbies or occupation will continue to allow exposure to lung damage (if the offending particles are related to the hobby or job) despite the knowledge that it is harmful.

What are examples of hypersensitivity pneumonitis?

Examples of hypersensitivity pneumonitis include:

  • Farmer's lung disease from exposure to mold spores in hay
  • Pigeon breeder's disease from exposure to protein particles in pigeon droppings
  • Sauna takers' disease from exposure to mold growing in wet containers
  • Mushroom workers' disease from exposure to moldy compost
  • Bagassosis from exposure to moldy sugar cane
  • Winemaker's lung from exposure to a fungus on grapes called Botrytis cinerea
  • An unusual case was published involving a case of hypersensitivity to Canadian goose droppings. The individual was a physician who was exposed to both indoor and outdoor antigens while living in a suburban Illinois community. One can only imagine the tremendous detective work necessary to make this diagnosis.

A more detailed analysis is listed in the table, which includes the types of compounds, bacteria, and molds known to cause hypersensitivity pneumonitis.

Some Types, Antigens and Exposures That Have Been Identified
Disease Name Antigens Exposure
Bagassosis Bacteria (Thermophilic actinomycetes) Moldy bagasse (pressed sugarcane)
Mushroom worker lung Bacteria (Thermophilic actinomycetes) Mushroom compost
Metalworking fluids HP Bacteria (Mycobacterium immunogenum) Mist from metalworking fluids
Hot tub HP Bacteria (Mycobacterium avium complex) Mist from hot tubs
Lifeguard lung Bacteria (Endotoxin) Indoor swimming pool
Farmer's lung Bacteria (Thermophilic actinomycetes)
Fungus (Aspergillus species)
Moldy hay
Humidifer lung Bacteria (T. candidus, Bacillus subtilis, B. cereus, Klebsiella oxytoca)
Fungus (Aureobasidium pullulans)
Amoebae (Naegleria gruberi, Acanthamoeba polyhaga, Acanthamoeba castellani)
Mist from standing water
Compost HP Fungus (Aspergillus) Compost
Malt worker lung Fungus (Aspergillus clavatus) Moldy barley
Peat moss HP Fungi (Monocillium sp, Penicillium citreonigrum) Peat moss
Suberosis Fungus (Penicillum frequentans) Moldy cork dust
Maple bark HP Fungus (Cryptostroma corticale) Moldy wood bark
Wood pulp worker lung Fungus (Alternaria species) Moldy wood pulp
Wood trimmer lung Fungus (Rhizopus species) Moldy wood trimmings
Tree cutter lung Fungi (Penicillium (three species), Paecilomyces sp.,
Aspergillus niger, Aspergillus sp., Rhizopus sp.)
Wood chips from living maple and oak trees
Dry rot HP Fungus (Merulius lacrymans) Moldy rotten wood
Sequoiosis Fungi (Graphium species, Pullularia species) Moldy wood dust
Japanese summer-type HP Fungus (Trichosporon cutaneum) Damp wood and mats
Cheese washer lung Fungus (Pencillum casei or P.roqueforti) Cheese casings
Tobacco worker lung Fungus (Aspergillus sp.) Moldy tobacco
Greenhouse HP Fungi (Aspergillus sp., Penicillium sp., Cryptostroma corticale) Moldy soil
Esparto grass HP Fungus (Aspergillus fumigatus) Moldy esparto used to produce ropes, canvas, sandals, mats, baskets, and paper paste
Soy sauce brewer lung Fungus (Aspergillus oryzae) Fermentation starter for soy sauce
Bird breeder lung Avian proteins Bird droppings and feathers
Mollusc shell HP Aquatic animal proteins Mollusc shell dust
Animal handler lung Animal proteins urine, serum, fur
Wheat weevil HP Wheat weevil (Sitophilus granarius) Infested flour
Silk production HP Silk worm larvae proteins Silk worm larvae
Isocyanate HP TDI, HDI, MDI Paints, resins, polyurethane foams
TMA HP Trimellitic anhydride Plastics, resins, paints
Rose, CS, Lara AR. Hypersenstivity pneumonia In: Mason RJ, Broadus VC, Martin TR, et al. Eds. Murray and Nadel's Textbook of Respiratory Medicine. 5th ed. Philadelphia, Pa; Saunders Elsevier; 2010; Chap 66.


Allergies: 10 Ways to Reduce Mold Allergies See Slideshow

How is hypersensitivity pneumonitis diagnosed?

Many people with episodes of hypersensitivity pneumonitis are probably unrecognized and undiagnosed. Some cases believed to be viral pneumonias may actually be hypersensitivity pneumonitis. The patient's history of repeated episodes of typical symptoms, hours after exposure to certain environments are important in establishing the diagnosis.

Most patients with this disorder have symptoms of shortness of breath and/or cough. Quite often a chest X-ray may show a variety of abnormalities, but primarily increased lung markings. A specialist in lung disease will primarily depend on a history of exposure either by occupation or hobby. With the abnormal chest X-ray, a high resolution cat scan of the chest is often reviewed. This study shows a detailed image of the appearance of the lung tissue. Lung function tests that examine the lung volumes and the ability for gases to move through the lungs (diffusing capacity) are then performed. Not only are the CT scan and pulmonary function tests useful in diagnosis but they are also useful in following response to therapy.

Blood antibody tests and skin tests against certain offending molds, bacteria, or particles are available, but their results are usually inconclusive. Other more recent tests such as the serum KL-6 (Krebs von den Lungen-6; this test may act as a marker for activity seen in diffuse lung disease) may be used. Infectious causes as well as collagen vascular diseases and cancer must be excluded as a cause of the symptoms and signs, and this may involve a biopsy. Unfortunately the small biopsies obtained with a bronchoscope or a needle are usually inadequate. It is often necessary to perform Video Assisted Thoracoscopic Surgery (a.k.a. VATS), which is an open lung biopsy technique performed by thoracic surgeons under general anesthesia. This procedure allows for a larger sample size and more accurate diagnosis.

The interpretation of these tests is complicated, and is best performed by a doctor experienced in hypersensitivity pneumonitis, often with the help of a pathologist experienced in lung diseases. The most accurate determination of the cause of hypersensitivity pneumonitis is almost always a detailed exposure history. It is not uncommon, however, to be unable to find the causative agent.

What is the treatment for hypersensitivity pneumonitis?

The most important treatment of hypersensitivity pneumonitis is avoidance of repeated exposures to the offending particles. With early diagnosis and prevention, the prognosis is good. 

Is hypersensitivity pneumonitis serious?

Prolonged, repeated exposures can lead to permanent lung damage, scarring, and potentially significant disability.

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Medically Reviewed on 3/16/2023
Medically reviewed by William Fineman, MD; American Board of Internal Medicine with Subspecialty in Pulmonary Disease


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

Saltoun, et. al. Hypersensitivity pneyonitis resulting from community exposure to Cadada goose droppings when an exteran environmental antigen becomes an indoor environmental antigen. Annals of Allergy, Asthma and Immunology. Vol 84 Issue 1; Pgs 84-86 January 2000.

Takahashi, et. al. Serum Kl-6 Concentrations in Dairy Farmers. Chest. August 2000, vol. 118 no. 2 445-450.