Hypersensitivity pneumonitis is an inflammation of the lungs that usually affects the tiny air sacs (alveoli). It is most often caused by an immune system reaction to tiny organic (living) dusts or vapors in the form of bacteria, mold or fungi. Repeated exposure to the particles sensitizes the person to the allergen. In some cases, the source of these particles can also be inorganic (not derived from animals or plants).

Widespread inflammation of the primary lung tissues (parenchyma) and airways is the chief feature of hypersensitivity pneumonitis.

There are two categories of hypersensitivity pneumonitis:

• Acute hypersensitivity pneumonitis. The sensitization period to the allergen varies from months to years. Following sensitization, the symptoms usually occur within four to 12 hours after exposure to an allergen. Symptoms may persist for up to 18 hours or longer, but usually begin to resolve soon after exposure to the allergen ceases.
  
  
• Chronic hypersensitivity pneumonitis. A result of several bouts of the acute form of the disease, it can lead to a scarring of the lung tissue (pulmonary fibrosis) that often is not reversible.

Hypersensitivity pneumonitis is often found in people who keep birds as pets. People also commonly contract the condition on the job after being repeatedly exposed to certain allergens. Occupations with a high risk for the disorder include:

• Bird handlers
  
• Farmers and cattle workers
  
• Ventilation workers
  
• Veterinarians and animal handlers
  
• Grain and flour processors and loaders
  
• Lumber mill workers and paper and wallboard manufacturers
  
• Plastic manufacturers, painters and electronics industry workers
  
• Textile workers

In addition, hypersensitivity pneumonitis may be contraced through fungi and bacteria in air conditioners, heating systems, humidifiers and hot tubs.