[dreamcompiler]

A CPAP humidifier is a distiller. You could use muddy swamp water in it and the vapor that goes into your lungs would still be pristine.

However, that would make the machine prone to bacterial and scale buildup, and hard to clean. Ease of cleaning is the real reason to use only distilled water in CPAP machines.

[bordakt]

You should use distilled water to avoid volatiles in tap water, like chlorine.