Distilled water 1 gallon
Because distilled water is so pure, it is a common tool used in the medical field
When using a CPAP/BIPAP/OXYGEN CONCENTRATOR humidifier it's important to only use distilled water. Even though tap water straight from the sink is probably more convenient for many CPAP users to access. However, CPAP manufacturers and clinicians recommend that patients only use distilled water in their humidifier chambers for important reasons.
Filtered water may remove some of the minerals but may not remove living organisms or other chemicals.