Reviewed and revised 5 August 2015


  • Humidity describes the amount of water vapour in a gas
  • The upper airway warms, moistens and filters inspired gas
  • With normal nasal breathing the temperature in the upper trachea is between 30°C and 33°C, with a relative humidity of approximately 98%, providing a water content of 33 mg/L
  • Clearance of surface liquids and particles from the lung depends on  beating cilia, airway mucus and transepithelial water flux are all are very sensitive to temperature and humidity
  • When the upper airway is bypassed by an tracheal tube, the normal heating and humidification functions of the upper airways must be provided artificially



  • absolute humidity (mg/L) = the mass of water vapour (g) present in a given volume of air (m3)
  • relative humidity (%) = the ratio of the mass of water vapour in a given volume of air to the mass required to fully saturate that volume of air at a given temperature (37°C) = actual vapour pressure / saturated vapour pressure
  • humidity may also be expressed as the pressure exerted by water vapor in a gas mixture (mmHg)

Normal values

  • at the carina: absolute humidity 44g/m3, relative humidity 100% (this is the maximum absolute humidity of saturated air at 37°C)
  • optimal function requires: an absolute humidity > 33g/m3 or relative humidity of > 75%

There is no universal agreement on the optimal humidity required to prevent pathological changes — there are (at least) two schools of thought as to what humidity should be targeted in mechanically ventilated patients:

(1) full saturation at 37°C (44g/m3)
Rationale: prevent drying of secretions, decrease incidence of ETT blockage and limit cilia dysfunction

(2) 75% saturation (33g/m3)
Rationale: concerns that full saturation may also contribute to cilial dysfunction and may increase risk of nosocomial pneumonia


Inadequate humidification leads to:

  • up to 250 mL/day water loss from breathing dry air
  • increased mucous viscosity causing slow mucociliary transport, eventually stopping completely
  • depressed ciliary function
  • cytological damage to tracheobronchial epithelium
  • microatelectasis from obstruction of small airways and reduced surfactant leading to decreased lung compliance and shunt
  • tenacious sputum and airway obstruction (including the endotracheal tube) and increased airways resistance
  • body heat loss

Excessive humidification resulting in an increased water load can contribute to:

  • ciliary degeneration and paralysis
  • pulmonary edema
  • altered alveolar-arterial oxygen gradient
  • decreased vital capacity and compliance
  • decrease in haematocrit
  • hyponatraemia


It would have the following features:

  • inspired gas delivered into the trachea at 32-36° C with a water content of 30-43g/m3
  • set temperature remains constant
  • humidification and temperature remain unaffected by a large range of fresh gas flows
  • device simple to service
  • humidification can be provided for air, oxygen or any mixture of inspired gas
  • humidifier can be used with spontaneous or controlled ventilation
  • safety mechanisms: alarms against overheating, over hydration and electrocution
  • resistance, compliance and dead space do not adversely effect the spontaneously breathing patients
  • sterility of inspired gas is not compromised


There are numerous sources of humidity (e.g. moistened breathing tubes, rebreathed gas, water released by carbon dioxide reacting with an absorbent), however specialised humdifiers or HME filters are generally used.

  • HME filters (Heat and Moisture Exchanger) are used for passive, rather than active, humidification
  • Humidifiers (also called a vaporizer or vaporizing humidifier) can be either either heated or unheated and passes a stream of gas over water (passover), across wicks dipped in water (blow-by) or through water (bubble or cascade).

Cold Water/ Unheated Humidifiers

  • simple
  • inexpensive
  • inefficient (cannot deliver more than about 9 mg H2O/L)
  • source of microbiological contamination

Hot Water/ Heated Humidifiers

  • ‘blow by” or ‘bubble’ through
  • more efficient
  • can be automatically adjusted based on sensors
  • can decrease the amount of condensation
  • problems: over heating, water intoxification, rain out, airway burns, impaired mucociliary clearance, and an unsubstantiated theoretical risk of infection (heated humidification actually appears to decrease pneumonia rates)


  • Both active and passive humidification decrease prevalence of artificial airway occlusion, mortality and pneumonia
  • Heated humidification and HME are more effective in maintaining inspired air temperature and absolute tracheal humidity than cold humidification
  • No significant difference of HME and heated humidification on mucus properties and cilia transport over 72 hours in mechanically ventilated patients
  • Heated humidification and HME have do not negatively impact respiratory function i.e. atelectasis, pneumothorax, changes in tidal volume, minute ventilation or tracheal aspirations
  • HME reduces total respiratory heat loss and evaporative heat exchange without increasing WOB in tracheostomised patients —> passive humidification is effective


References and Links


Journal articles

  • Al Ashry HS, Modrykamien AM. Humidification during mechanical ventilation in the adult patient. Biomed Res Int. 2014;2014:715434. [pubmed]
  • Esquinas Rodriguez AM, Scala R, Soroksky A, et al. Clinical review: humidifiers during non-invasive ventilation–key topics and practical implications. Crit Care. 2012;16:(1)203. [pubmed]
  • Kelly M, Gillies D, Todd DA, Lockwood C. Heated humidification versus heat and moisture exchangers for ventilated adults and children. Cochrane Database Syst Rev. 2010 Apr 14;(4):CD004711. PMID: 20393939.
  • Wilkes AR. Humidification: its importance and delivery. BJA CEPD Reviews (2001) 1 (2): 40-43. doi: 10.1093/bjacepd/1.2.40 [Free Fulltext]

CCC 700 6

Critical Care


Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of three amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.