F10iv: Define humidity and give an outline of the importance of humidification

Principles of Humidification

  • H2O consists of molecules
  • Molecules have variable kinetic energy (E)
  • Temp of H2O determined by mean kinetic E of these molecules
  • At liquid/gas interface, the molecules with enough E overcome attractive forces of H2O in liquid & escape into the gas as a vapour
  • Molecules escaping have the most kinetic E
  • When they leave, the mean kinetic E of the liquid H2O ↓ (& ∴so does temp)
  • The molecules in vapour form will exert a partial pressure within the gas
  • In a sealed container at constant temp., equal no’s of molecules are going liq ⮂ vapour
  • At steady state the pressure being exerted ka SATURDATED VAPOUR PRESSURE


  • Latent heat of vaporisation: the heat required to convert 1g of a substance from liquid phase to gas phase at a given temp
  • Humidification: the addition of water vapour to a gas
  • Absolute humidity (g/m3): the mass of H2O vapour (g) present in a given volume of air (m3)
        • g /m3 = mg / L 
  • Relative humidity (%): a ratio; of the mass of H2O vapour in a given volume of air vs. the mass required to fully saturate that volume of air
Relative Humidity
  • Saturated vapour pressure (mmHg): the vapour pressure (pressure exerted) when the liquid & vapour phase are in equilibrium

Normal Values

  • PSVP = 44mmHg at 1ATM
  • At 20°C, 1m3 of air in fully saturated with 17g H2O
  • At 37°C, 1m3 of air is fully saturated with 44g H2O (i.e. at carina)
  • ∴As temp ↑, relative humidity
  • e. 1m3 of air with 17g water @ 20°C
  • At 37°C, 1m3 of air will have relative humidity  (because saturated is 44g at 37°C)
  • relative humidity at ↑temp

SVP depends on temp

  • ↑temp. of liquid
  • ↑kinetic E of molecules
  • More molecules can escape the liquid phase
  • Exerting a greater saturated vapour P

Physiological Humidifcation


  • Nose breathing → nasal turbinates ↑SA of nasal epithelium
  • Air is warmed by heat from radiant blood supply
  • As air is warmed ↑T = ↑SVP
  • Moisture on epithelium evaporates → joins dry air to ↑humidity to 90%
  • NOTE: mouth breathing ↓relative humidity to ~70%
  • Humidification continues in similar way down pharynx
  • Just below carina is ISOTHERMIC SATURATION BOUNDARY (ISB) = anatomical part where inspired gas becomes fully saturated

→ Distal to ISB, temp. & humidity is constant

→ Proximal to ISB the anatomy is acting as a HME

  • @ Carina
    1. Relative H = 100%
    2. Absolute H = 44g/m3 (37°C)


  • Expired gas transfers heat back to cooler trachea & nasal mucosa
  • As gas cools it holds less vapour (↓T = ↓SVP)
  • ∴Condensation occurs on nasal mucosa → some H2O reabsorbed
  • ↓potential H2O loss from 300 → 150mL/day

Importance of Humidification

  • ↓H2O loss
  • Ciliary function
  • ↓Mucous viscosity
  • ↓Risk sputum plugging
  • Without humidity, cilia die → tracheal epithelium becomes keratinised, ulcerated & necrosed
  • Excess humidity also bad → ciliary dysfunction, APO, ↓Na+

The Ideal Humidifier

  • Delivers gas at temp 32 – 36°C with water content 43g/m3
  • Set temp. remains constant
  • Not affected by gas flow
  • Simple to service
  • Humidification can be provided for air, O2 & other gases
  • Can be used with SPV/controlled ventilation
  • Safety → prevents overheating & electrocution
  • Doesn’t affect DS, Resistance, Compliance
  • Sterility uncompromised
  • Cost effective

Methods & Devices in ICU

Passive: HME → no E/H2O source

Active: H2O baths, bubble humidifiers, Passover, nebulisers


  • Does not require external E/H2O to function
  • Plastic casing with inlet & outlet which forms a closed unit
  • Material with low thermal conductivity (paper) is impregnated with hygroscopic (H2O retaining) salts i.e. calcium
  • On EXPIRATION → warm, moist air passes through the device → gas is cooled → H2O condenses
  • The element/matrix is warmed by the latent heat of H2O condensing on it & by exhaled gas
  • On INSPIRATION → moisture retained evaporates, humidifying the inspired gas & warming the air


      1. Air reaching trachea has H2O vapour of 34g/m3
      2. Reaches humidity 25g/m3
      3. HME Relative Humidity = 60 – 70%


  • Cheap & simple
  • No power
  • 60 – 70% humidification
  • Retains heat & moisture
  • Improves ciliary function & mucous clearance


  • ↑DS
  • R 0.1 – 2cm H2O
  • Bulk
  • Can become occluded
  • ↓efficiency at ↑MV/VT
  • Can take 10 – 20 mins to reach full efficiency (a relative humidity of 70%)

HMEF = variant with bac filter to isolate infected patients from breathing systems & protect patients at risk of infection

Heated H2O Bath Humidifer (Passover Humidifer)


  • Heater base & water filled chamber
  • Water is heated → ↑T = ↑SVP
  • Fresh gas passes through & collects H2O vapour
  • Carries humidified gas to patient
  • Insp. limb may contain a heating wire so ↓H2O loss as it goes to patient (because ↓temp along INSP LIMB)


  • Less infection risk cf aerosols because vapour cannot carry microbes
  • No ↑R/DS


  • Condensation build up in INSP LIMB
  • If thermostat fails → scaling of patient
  • Bac/fungal colonisation of H2O reservoir

Bubble Humidifier


  • Simplest active humidifier
  • Fresh gas flow is passed through H2O reservoir
  • Bubbles absorb H2O vapour as they pass to the surface of the reservoir


  • Compact
  • Cheap
  • ↑humidifier cf. HME


  • Not v. efficient
  • ↑Absolute H ~20g/L
  • Bac growth in bath
  • H2O aerosols can transmit infection

Nebulised Humidifiers

  • Relies on Bernoulli Effect
  • BERNOULLI: if the pressure along a tube is measured at the narrowest point of the tube, the pressure will be lower than elsewhere & often below atmosphere
  • Flowing fluid contains 2 energies:
  • POTENTIAL (pressure)
  • KINETIC (flow)
  • Law of Conservation of Energy → total E must remain constant
  • Therefore, as gas move through a narrow point of tubing
    1. Gas Velocity increases (Kinetic E)
    2. Pressure decreases (Potential E)

So that the total E remains constant

The Bernoulli effect allows entrainment of air/fluid around constricted site


  • A nebuliser is a device that converts liquid into aerosol droplets
  • High velocity gas driven by the nebuliser through a tube sitting in a reservoir of H2O
  • As gas jets through the narrowest part of the -ve P draws H2O into tube
  • The high velocity gas breaks the H2O droplets
  • May contain an ANVIL which sits in path of entrained droplets
  • When liquid hits ANVIL it disperses into even smaller objects


  • ↑Humidity
  • ↓R/DS


  • oedema
  • Bac/viral route
  • Expensive
  • Needs sterile H2O

Droplet Size Matters

  • 20 μm → nuisance → forms pools of H2O in tubing
  • 5 μm → falls into trachea
  • <1 μm → pass into alveoli → extremely stable ^ can be INSP/EXPIRED again (this is the size you want to deliver drugs)

ULTRASONIC → a vibration plate of 2 – 3MH2 positioned in a H2O reservoir transmits vibrations to H2O → to create H2O droplets

Measuring Humidity

Humidity measured by HYGROMETER

Hair Hygrometer

  • Human hair linked to spring gauge
  • The hair length ↑as it absorbs H2O
  • ∴Moves pointer across non-linear scale
  • Most accurate to measure R humidity 30 – 40%

PRX & Wet Bulb

  • 2 standard mercury thermometers
  • One bulb exposed to air → Ambient T
  • One bulb kept wet by a wick which is submerged in H2O
  • H2O evaporates → bulb cools due to LATENT HEAT of VAPORISATION
  • Wet Bulb temp < Dry Bulb temp
  • ∆temp is related to the rate of evaporation of H2O which depends on humidity
  • A scale is referenced to obtain R Humidity

Dew Point Hygrometer

Dew Point = the temp at which H2O vapour condenses to form liquid H2O

Dew Point occurs when Relative Humidity reaches 100% ∴H2O vapour begins to condense. When dew forms → a table is used to correspond the humidity to the ambient temp

Renault’s Hygrometer

  • A type of Dew Pt Hygrometer
  • A mirrored tube (so you can see condensation)
  • Filled with Ether (highly volatile)
  • Air is blown through the ether, cooling it
  • As ether evaporates → tube is cooled
  • Temp at which dew forms on the outside of the tube noted as Dew Pt
  • Table used to correspond that temp to humidity