The liquid needed for oxygen humidifier would be sterile water.
When administering a high dose of oxygen(>6lpm) it is important to use a humidifier so the air that enters the lungs is humid enough so it won't dry the airway. Sterile water is best because there is a chance that bacteria from water will enter the airway. Impurities in water are not harmful to the body but it might damage the machine, so distilled water would be best.
