A case of humidifier lung; the key diagnosis is detailed medical history taking

A 74‐year‐old woman was admitted with a dry cough and dyspnea that had persisted for 2 weeks at the beginning of winter. Chest computed tomography revealed bilateral diffuse non‐segmental ground‐glass opacities without centrilobular nodules. Bronchoalveolar lavage fluid revealed a marked increase in the lymphocyte ratio. Her condition and chest radiographic findings improved spontaneously after admission. An additional interview conducted after admission revealed that the patient had started using a contaminated humidifier approximately 2 weeks before the onset of symptoms. Thus, the diagnosis of humidifier lung was established. Humidifier lung is a rare phenotype of hypersensitive pneumonitis that often occurs during dry winter when the use of humidifiers increases. Humidifier lung is an important differential diagnosis of bilateral pneumonia during dry winter, and detailed history‐taking regarding the use of humidifiers, assuming a humidifier lung, is crucial for its diagnosis.