Hypersensitivity pneumonitis is most often triggered by exposure to organic (living) dusts or vapors in the form of bacteria, mold or fungi. It is repeated exposure over a period of time, which can vary from months to years, that triggers the sensitization to the allergen.

Thermoactinomyces bacteria are a major source of the disease. They are found in damp hay and other vegetation, and in air-conditioning and humidifying systems. When these substances vaporize, they can circulate and trigger a form of the disease called ventilation pneumonitis.

Variations of hypersensitivity pneumonitis and their antigens include:

• Farmer’s lung. Saccharopolyspora rectivirgula.
  
• Bagassosis. Thermoactinomyces sacchari.
  
• Grain handler’s lung. S rectivirgula, Thermoactinomyces vulgaris.
  
• Humidifier/air-conditioner lung. S rectivirgula, T vulgaris.
  
• Pigeon breeder lung. Avian or animal proteins.
  
• Cheese worker’s lung. Penicillium casei.
  
• Malt worker’s lung. Aspergillus clavatus.
  
• Paprika splitter’s lung. Paprika dust.
  
• Wheat weevil. Sitophilus granarius.
  
• Mollusk shell hypersensitivity. Sea snail shells.
  
• Chemical worker’s lung. Trimellitic anhydride, diisocyanate, methylene diisocyanate.