8 Respiratory Conditions Beyond Asthma and Allergies

8. Hypersensitivity Pneumonitis - The Environmental Lung Disease

Hypersensitivity pneumonitis, formerly known as extrinsic allergic alveolitis, represents an inflammatory lung disease caused by repeated inhalation of organic dusts or chemicals that trigger an immune-mediated response in susceptible individuals. This condition differs from typical allergic reactions in that it involves the deep lung tissue (alveoli and interstitium) rather than the airways, and can progress to irreversible pulmonary fibrosis if exposure continues. Common causes include bird proteins from pet birds or poultry farming (bird fancier's lung), moldy hay or grain (farmer's lung), contaminated humidifiers or air conditioning systems, and various occupational exposures to organic dusts. The disease can present in acute, subacute, or chronic forms depending on the intensity and duration of exposure. Acute episodes typically occur 4-8 hours after heavy exposure and present with fever, chills, cough, and shortness of breath that may be mistaken for pneumonia. Subacute forms develop with lower-level ongoing exposure and present with gradually worsening cough, dyspnea, and fatigue. Chronic hypersensitivity pneumonitis results from prolonged low-level exposure and can lead to irreversible pulmonary fibrosis with progressive respiratory failure. Diagnosis requires a high index of suspicion based on exposure history, combined with characteristic imaging findings on high-resolution CT scans showing ground-glass opacities and nodules. Serum precipitating antibodies against specific antigens can support the diagnosis but are not definitive. Bronchoalveolar lavage may show increased lymphocytes, while lung biopsy reveals characteristic inflammatory changes. The most crucial treatment is complete avoidance of the offending antigen, which can lead to significant improvement in acute and subacute cases. Corticosteroids may be used for severe acute episodes or to slow progression in chronic cases.