Hypersensitivity pneumonitis is usually an occupational disease in which exposure to organic dusts, fungus, or molds leads to acute lung disease. Over time, this acute condition may turn into chronic lung disease. Exposure may also occur in the home from fungus present in humidifiers, heating systems, and air conditioners. Some hobbies, such as owning birds, can lead to exposure. Acute illness may occur 4 - 6 hours after the exposure, once the person has left the area where the allergen is present. Chronic illness with changes seen on chest x-ray may develop with continued exposures. The chronic form of this disease may lead to pulmonary fibrosis (a scarring of the lung tissue that is often not reversible).
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