Why humidify oxygen? 

Medical oxygen is dry. When delivered at higher flows or for prolonged periods, dry oxygen can: 

Dry nasal and tracheobronchial mucosa 
Impair mucociliary function 
Cause discomfort, epistaxis, thick secretions 
Increase airway resistance and atelectasis risk (especially with artificial airways) 

Humidification adds water vapour to inspired gas to approximate normal airway conditions. 

Normal airway humidification (physiology) 

Upper airway normally warms gas to 37°C 
Provides 100% relative humidity 
Absolute humidity ≈ 44 mg H₂O/L 
Bypassing or overwhelming this system (high flow, intubation) necessitates artificial humidification. 

When is humidification required? 

Generally NOT required:

Low-flow oxygen (≤4 L/min) via nasal cannula 
Short-term oxygen therapy 
Patients without mucosal irritation 

Recommended / Required:

Oxygen flow ≥4–5 L/min via nasal cannula 
Face mask oxygen at moderate–high flows 
High-flow nasal cannula (HFNC) 

Mechanical ventilation 

Patients with: 
Thick secretions 
Artificial airways (ET tube, tracheostomy) 
Prolonged oxygen therapy 

Methods of humidification 

1. Bubble humidifier 
Cold water system 
Oxygen bubbles through sterile water 

Pros 
Simple 
Cheap 

Cons 
Minimal humidification at high flows 
Ineffective above ~6 L/min 
Infection risk if poorly maintained 

Used with: Low–moderate flow nasal cannula 

2. Heated humidifier 

Actively heats water and gas 

Pros 
Delivers high absolute humidity 
Prevents mucosal drying 
Ideal for ventilated patients 

Cons 
More complex 
Risk of condensation (“rainout”) 
Requires temperature monitoring 

Used with: 
Mechanical ventilation 
HFNC 
Long-term high-flow oxygen 

3. Heat and Moisture Exchanger (HME) – green filter 

Also called “artificial nose” 

Mechanism 
Traps heat and moisture from exhaled gas 
Returns it during inspiration 

Pros 
Simple 
No power needed 
Low infection risk 

Cons 
Less effective with: 
High minute ventilation 
Thick secretions 
Adds dead space 

Used with: Short-term mechanical ventilation (long term ventilation requires ‘wet circuit’ (HME removed as can quickly become water logged) 

High-Flow Nasal Cannula (HFNC) 

Delivers heated, humidified oxygen 
Flow up to 60 L/min 
Near 100% relative humidity 

Clinical benefits 
Improved comfort 
Better secretion clearance 
Reduced work of breathing 

Complications of inadequate humidification 
Nasal dryness, pain 
Epistaxis 
Thickened secretions 
Mucus plugging 
Atelectasis 
Endotracheal tube obstruction 

Key points: 

Humidification is essential when bypassing the upper airway 
Bubble humidifiers ≠ adequate at high flows 
HME increases dead space 
HFNC must always be heated and humidified 
Ventilated patients require active or passive humidification