Hypersensitivity Pneumonitis (Extrinsic Allergic Alveolitis)

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What is hypersensitivity pneumonitis?

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The term hypersensitivity pneumonitis (also known as extrinsic allergic alveolitis) refers to a group of lung diseases in which your lungs become inflamed as an allergic reaction resulting from exposure to dusts of animal and vegetable origin. Although complicated, the name “extrinsic allergic alveolitis” describes the origin and nature of these diseases.

  • "extrinsic"-- cause originating outside the body
  • "allergic"-- caused by the allergic reaction of the body to a specific substance or condition
  • "alveolitis"-- an inflammation in the inner part of the lungs (alveoli are the small air sacs in the lungs)

What causes hypersensitivity pneumonitis?

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Intense or prolonged (long-term) exposure to animal or vegetable dusts can result in hypersensitivity pneumonitis. The dust particles must be 5 microns or smaller to get into the alveoli (the small air sacks in the lungs). Animal and vegetable dusts are complex mixtures that come from many different sources, such as husks, bark, wood, animal dander, and microorganisms, including bacteria and fungi. The microorganisms produce toxic chemicals that form part of the mixture. Insects and insect fragments, bird droppings, and dried urine of rats may also be found in the dusts. Mouldy hay, straw, grain, and feathers are other dust sources.

How does hypersensitivity pneumonitis develop?

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Hypersensitivity pneumonitis does not develop on the first day of exposure to animal and vegetable dusts. Repeated and prolonged exposure is necessary. Even then, only some workers develop allergic reactions to the dusts. Ten to forty percent (10 - 40%) of exposed people do not show any symptoms.

The allergy is triggered by complicated reactions of the body's natural defence system that normally protects the lungs from foreign substances.  The body's overreaction to the presence of dust in the lungs is called sensitization.

What are the symptoms of hypersensitivity pneumonitis?

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Once a person is sensitized, hypersensitivity pneumonitis can show two different types of responses: acute (intense) response and chronic (long-term) response. Signs and symptoms will vary between people.

The acute attack begins with heavy exposure to the trigger. It starts with fever, muscular aches and a general, unwell feeling or malaise. These symptoms are accompanied by tightness in the chest, a dry cough, and shortness of breath. 

About 5% of people develop a chronic form of the disease.  It is marked by increasing cough, chronic bronchitis, shortness of breath, weight loss, and lung fibrosis. The individual suffers permanent lung damage (lung scarring).

How is hypersensitivity pneumonitis recognized and treated?

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In diagnosis, the best evidence for hypersensitivity pneumonitis is the patient's occupation and a history of exposure to animal or vegetable dusts. Although the doctor may want to do some tests, such as allergy tests, lung X-rays, blood tests, or lung function tests, these are not specific and may not distinguish between hypersensitivity pneumonitis (extrinsic allergic alveolitis) and other lung problems.

Following diagnosis, the person must avoid future exposure to animal and vegetable dusts. This action alone results in improvement. For serious cases, patients need medications that make breathing easier.

What are the occupations at risk?

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Hypersensitivity pneumonitis occurs in many diverse occupations.

The following table lists several examples and the related occupations. Note that this list is not a full list of all occupations at risk. 

Examples of Hypersensitivity Pneumonitis
DiseaseExposurePreventive Maintenance
Air conditioner/humidifier lungHumidifier waterMaintenance of air and water handling systems.
Animal handlers' lungDust of dander, hair particles, dried urine of ratsGood exhaust ventilation.
BagassosisMouldy sugar caneApplication of propionic acid to bagasse. Good exhaust ventilation. Keeping moisture content above 20%. Enclosure of processes.
Bird fanciers' lungProteins in  droppings and feathersGood exhaust ventilation. Water spraying of droppings while cleaning.
Cheese washers' lungCheese mouldWrapping the cheese in foil during aging.
Farmers' lungMouldy hay, straw, grainSee the prevention section below for dust control.
Hot tub lungBacteria in mist from hot tubMaintain disinfectant level for water. Regular hot tub cleaning. Good exhaust ventilation.
Maltworkers' lungMouldy maltApplication of mechanical methods in the malting process.
Maple bark strippers' diseaseMouldy maple barkSpraying of logs during debarking. Remote control of some operations.
Mushroom workers' lungMouldy mushroom compostGood exhaust ventilation.
SequoiosisMouldy sawdustGood exhaust ventilation. Enclosure of processes.
Sewage sludge diseaseDust of heat-treated sludgeGood exhaust ventilation. At outside facilities, stand upwind of storage piles.
Wheat weevil lung / Miller’s lungMouldy grain, flour, dustSee the prevention section below for dust control.
SuberosisMouldy cork dustGood exhaust ventilation.
Wood pulp workers' diseaseMouldy wood chipsGood exhaust ventilation. Remote control of some operations.

How can we help prevent hypersensitivity pneumonitis?

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The ways to reduce dust exposure (dust control) include using the hierarchy of controls - engineering controls, administrative controls, and personal protective equipment. 

Methods of engineering control include local exhaust ventilation, general ventilation, process enclosure and process isolation (separating the worker from the dusty process). 

On farms, prevention of particle release and control of dust cloud formation are achieved by well-designed, leakproof ducting, and enclosed conveyor systems for grains and feeds. Buildings should have local ventilation systems in areas frequented by workers engaged in egg handling and feed storage and preparation. Within enclosed livestock units, temperature and relative humidity should be monitored. Adequate ventilation and sufficient fresh, replacement air should be provided. For field operations, tractors or combine harvesters with enclosed cabs that provide filtered air should be used.

Administration controls include education and training programs that emphasize the significance of animal and vegetable dust in causing diseases. Managers and workers should learn about methods of storing materials to prevent mould formation and reduce dust. In addition, develop and implement maintenance programs to, for example, keep humidifiers, hot tubs, and heating and cooling systems in good condition. 

Personal protective equipment should be considered as the last resort for respiratory protection. Personal protective equipment should not be a substitute for proper dust control. Respirators, including dust masks, should only be used:

  • when engineering or administrative controls are not technically feasible,
  • when engineering controls are being installed or repaired, or
  • when emergencies or other temporary situations arise (e.g., maintenance operations).

If respiratory protective equipment is needed for the job, then a full respiratory program should be put in place that includes selection, use, and care of respirators plus training and education for the worker. Because respirators provide different levels of protection, it is very important to assess the airborne contaminant before selecting the specific type of respirator.

  • Fact sheet last revised: 2023-09-14