Hypersensitivity pneumonitis or extrinsic allergic alveolitis is a lung disease due to T cell and macrophage activation with IgA, IgG or IgE immunocomplex tissue lesions following extrinsic exposure to organic or inorganic agents. We report a case of hypersensitivity pneumonitis (pigeon protein sensitized) with a second nosocomial exposure to Aspergillus fumigatus proteins from a contaminated oxygen water humidifier: the second extrinsic exposure induced significant acute respiratory failure with ARDS. A pre-existing COPD syndrome requiring prolonged oxygen therapy (7 days) involved lung disease with delayed clinical diagnosis and therapy. Microbiological and mycological analysis of oxygen water humidifiers should be considered, especially for hypersensitivity pneumonitis patients, when a new inexplicable clinical impairment occurs.