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Oral dryness is a common complication during noninvasive ventilation (NIV). We measured the oral dryness of patients and performed a bench study to investigate factors related to humidification during NIV.Patients were randomly assigned into 2 groups: medium (Med group) and maximum (Max group) heated humidifier (HH) settings. Oral moistness was measured using an oral moisture-checking device, and the feeling of oral dryness was evaluated using a 0 to 10 numerical rating scale (NRS) at 0, 12, and 24 hours from the beginning of NIV and at 12 and 24 hours after NIV was discontinued. A bench study was performed to assess the effects of positive end-expiratory pressure (PEEP), the fraction of inspired oxygen (FIO2), and air leaks on absolute humidity. We evaluated 3 HH settings: no HH, HH at the medium setting, and HH at the maximum setting. The temperature in the outlet chamber was 31°C to 32°C for the medium HH setting and 38°C to 41°C for the maximum HH setting.In the clinical study, 12 patients were assigned to the Med group and 11 to the Max group. In the Med group, oral moistness decreased and NRS increased at 12 and 24 hours compared with 0 hours (P < .05). In the Max group, neither the oral moistness nor the NRS changed throughout the study period, whereas in the bench study, high FIO2, high PEEP, and air leak decreased the absolute humidity for both HH settings (P < .01). However, it is not clear to what extent these factors affected the patients' oral dryness because the ranges of FIO2 and PEEP were narrow.Oral dryness was a common problem in our patients. The HH setting significantly affected humidification and oral dryness during NIV.