A 46-year-old man visited our hospital with a
fever and
cough. The symptoms had started two months after continued use of an ultrasonic
humidifier. He had
hypoxemia on admission and late inspiratory
crackles in both lungs on physical examination. The laboratory findings showed an increased white blood cell count and a
C-reactive protein level, and his serum KL-6 level was slightly elevated, at 674 U/mL. Chest computed tomography showed diffuse ground-glass opacities, and histological examination of a transbronchial lung biopsy showed alveolitis without
granulomas. The
humidifier inhalation challenge test result was positive. Therefore, we diagnosed the patient with
humidifier lung. His symptoms gradually improved after avoiding the
humidifier without taking medication. The
humidifier water was contaminated by various bacteria and fungi, as well as Mycobacterium gordonae and a high concentration of
endotoxin. Unlike in those with typical
hypersensitivity pneumonitis, the elevation of serum KL-6 levels in
humidifier lung patients is mild, and
granulomas are not apparent on histological examination, similar to our case. Furthermore, the
endotoxin identified from the
humidifier is one of the known pathogens of
humidifier lung. Thus,
humidifier lung seems to have different characteristics compared to other
hypersensitivity pneumonitis phenotypes. The mechanism driven by the high concentration of
endotoxin could be one of the main causes of
humidifier lung.