Humidifiers add water vapour to inspired gas, taking the place of normal body mechanisms which are bypassed or impeded by invasive and non-invasive ventilation. Maintaining adequate humidity of inspired gas is important in:

  • Reducing metabolic load
    Humidification of inspired gas accounts for ~15% of basal heat expenditure.
  • Maintaining function of the mucociliary elevator
    Inspiration of dry gas increases viscosity of mucous.
  • Reducing water loss
    Water will be absorbed from mucosa to humidify gas.

Humidifiers can be classified into active or passive.

Passive Humidifiers

Passive humidifiers:

  • Do not require power
  • Do not require water

The Heat and Moisture Exchange (HME) filter is the classic passive humidifier:

  • Placed between the patient and the patient Y-piece
  • Consists of:
    • A moisture exchange layer
      Pleated, hygroscopically coated foam or paper.
      • Expired gas cools as it passes, condensing onto the foam, with condensation promoted by hygroscopic coating (usually this is NaCl)
      • The latent heat of vapourisation results in a decreased temperature of expired gas
    • A filter layer
      Typically a electrostatic or hydrophobic material.
  • Expired gas is cooled and dried
  • Inspired gas is then heated and humidified
  • An HME takes up to 20 minutes to be fully effective, and can achieve a relative humidity up to 70%
  • Efficacy depends upon the patient's core temperature and the condition of the airway


  • Cheap
  • Lightweight
  • Straightforward
  • May contain anti-bacterial filter


  • May be blocked with vomit and secretions
  • Increase airway resistance
  • Increase dead space
  • Not as effective as powered active systems
  • Only last 24 hours
  • Takes 15-20 minutes to become fully effective

Active Humidifiers

Active Humidifiers:

  • Require either:
    • Power
      Unpowered humidifiers are typically less effective, and only operate well at lower flow rates.
    • Water
    • (Or both)
  • Consist of:
    • A water bath
      Typically sterile water.
    • A heating element
      To heat the water bath.
    • A gas pipe
      Inspired gases are bubbled through the water bath to humidify them.
    • A water trap
      To trap condensed water. Should be changed regularly to minimise infection risk.


  • Greater humidification
  • Appropriate for long-term ventilation


  • Bulky
  • Expensive
  • Require power
  • Infection risk from water bath


  1. McNulty G, Eyre L. Humidification in anaesthesia and critical care. Continuing Education in Anaesthesia Critical Care & Pain, Volume 15, Issue 3, 1 June 2015
Last updated 2019-07-18

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