Define humidity and give an outline of the importance of humidification

Humidification describes the amount of water vapour present in air:

  • Absolute Humidity is the amount of water vapour in a given volume of air (g.m-3)
  • Relative Humidity is the ratio between the amount of water vapour in a sample of air (absolute humidity) and the amount of water required to fully saturate that sample at its current pressure and temperature
  • Moisture is the water produced by condensation when relative humidity exceeds 100%.
  • Humidification of inspired air is important to avoid drying out mucosa and sputum, which leads to tissue damage and failure of the mucociliary elevator
  • Optimal function requires a relative humidity of greater than 75%


The nose is:

  • Optimised for humidification
    The septum and turbinates increase contact of air with mucosal surfaces by:
    • Increasing surface area
    • Generating turbulent flow
  • The preferred orifice for breathing unless airways resistance becomes a significantly limiting factor
    This is relevant in:
    • Airway obstruction (e.g. polyps)
    • At high minute ventilations (> 35L.-1)
  • Humidifies inspired gas to 90%, compared to 60% for the mouth

Method of humidification:

  • Fluid lining the airway acts as a heat and moisture exchanger
  • In inspiration:
    • Relatively dry air is evaporates water from the airway lining
    • Relative humidity is increased to 90% in the nasopharynx and 100% BTPS by the second generation of bronchi
      This gives a water vapour pressure of 47mmHg at BTPS, with an absolute humidity of 44g.m-3.
  • In expiration:
    • Air cools in the upper airway
      As cooler air has a lower saturated vapour pressure, moisture condenses on the airway.
    • Moisture is reabsorbed
      This reduces potential water losses from the airway from to


  1. Lumb A. Nunn's Applied Respiratory Physiology. 7th Edition. Elsevier. 2010.
  2. WeatherFaqs. Absolute and Relative Humidity.
  3. CICM September/November 2012
Last updated 2018-09-21

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