Table 2 Classification and diagnostic criteria of hypersensitivity pneumonitis

Exposure to causal antigen

Intermittent high-level exposure

Continuous low-level exposure

Onset of symptoms

2–9 h after exposure; may evolve to gradually increasing symptoms over days to weeks

Insidious, over weeks to months

Nature of symptoms

Cough and dyspnea, but predominantly influenza-like symptoms

Progressive symptoms (dyspnea, cough, and weight loss), sometimes punctuated by intermittent attacks of symptoms or slowly increasing

Physical signs

Fever

Inspiratory crackles; cyanosis; digital clubbing; cor pulmonale

Outcome

Symptoms peak within 6–24 h after exposure, lasting hours to days. Symptoms recur on re-exposure and may progress to severe dyspnea

End-stage fibrotic disease and/or emphysema. Exacerbations may occur despite avoidance of exposure