BACKGROUND: Noninvasive positive-pressure ventilation (NPPV) delivers air at a high flow, which is associated with airway mucosal drying and impaired airway functioning. OBJECTIVES: To examine the effects of mechanical ventilation parameters on relative humidity and absolute humidity during NPPV, and to evaluate the effect of a heated passover humidifier on relative humidity, absolute humidity, and ventilator performance during NPPV. METHODS: We performed a bench study to assess the effects of inspiratory positive airway pressure (IPAP) of 10 cm H(2)O, 15 cm H(2)O, and 20 cm H(2)O, respiratory rates of 12 breaths/min and 24 breaths/min, and inspiratory-expiratory ratios of 1:2 and 1:3 on relative and absolute humidity. The measurements were obtained on room air and with a heated humidifier at medium and maximum heater settings. RESULTS: Without humidification, the relative humidity in the NPPV circuit (range 16.3-26.5%) was substantially lower than the ambient relative humidity (27.6-31.5%) at all ventilatory settings. Increasing the IPAP decreased the relative humidity (Spearman's rho = 0.67, p < 0.001). Changing the respiratory rate or inspiratory-expiratory ratio had no significant effect. Both relative and absolute humidity increased with humidification, and the air was fully saturated at the maximum heater setting. Delivered IPAP was reduced by 0.5-1 cm H(2)O during humidification. CONCLUSIONS: NPPV delivers air with a low relative humidity, especially with high inspiratory pressure. Addition of a heated humidifier increases the relative and absolute humidity to levels acceptable for nonintubated patients, with minimal effect on delivered pressure. Consideration should be given to heated humidification during NPPV, especially when airway drying and secretion retention are of concern.