Cool pass-over style humidifiers allow the air flow to pass over a pool of water and subsequently pick up water vapor by free evaporation. Typically the larger the surface area of the pool of water the more water vapor the air stream can pick up.  Heated pass-over style humidifiers are much more efficient at providing humidification simply by increasing the water temperature, which increases water vapor production vs. free humidification.   By increasing the water temperature, additional energy is added to the individual molecules, allowing more of them to transform from the liquid state. As a result, the heated humidifier has the capability to provide additional water vapor to the air stream vs. a typical cool pass-over design.

• Bacteria and the Chamber

Heated humidification produces molecular water vapor (.0001 microns in size) that is too small to transport bacteria into the air stream that is delivered to the patient. If small amounts of bacteria are present in the chamber there is little risk to the patient. Bacteria requires a carrier and molecular water vapor cannot provide this mechanism.

Note: The humidifier chamber does offer good conditions for colonization by micro-organisms such as bacteria. This has been demonstrated over the years with other respiratory devices. This fact highlights the importance of proper and frequent cleaning of the humidifier chambers.

• Distilled water vs. tap water

Most tap water contains various minerals. When a pool of water is heated and evaporated, these minerals are left behind and may cause discoloration to the aluminum plate and plastic housing of the humidifier chamber. It may adversely affect the longevity of the chamber. Distilled water has been purified and will not generate mineral residue upon evaporation and may extend the usable life of the chamber.