“Cannula or mask?” sounds like a comfort choice, but it is part of the oxygen prescription. A standard nasal cannula, simple face mask, Venturi mask and non-rebreather mask do not deliver equivalent oxygen at the same flow. Before comparing them, identify exactly which mask is being discussed.
For most stable home-oxygen users at low flow, the nasal cannula wins on comfort and practicality. In acute care, a mask may be chosen when a cannula is not tolerated, is not achieving the target, or when a controlled or high oxygen concentration is required.
Quick comparison
| Question | Nasal cannula | Simple oxygen mask |
|---|---|---|
| Common source flow | 1–6 L/min for a standard cannula | 5–10 L/min |
| Approximate FiO₂ | Variable; roughly 24–44% across 1–6 L/min | Variable; roughly 35–60% |
| Eating and drinking | Usually easy | Mask must be removed |
| Talking | Easy | Muffled and less comfortable |
| Sleeping | Usually best tolerated | Often dislodges; not ideal for routine home sleep |
| Claustrophobia | Less likely | More likely |
| Nasal obstruction | Can reduce predictable delivery | Covers nose and mouth |
| CO₂ rebreathing risk at low flow | No mask reservoir | Yes if a simple mask is run below its minimum |
The percentages are estimates, not a prescription table. Breathing rate, tidal volume, nasal patency, mask fit and leaks change the actual inspired concentration.
Why the nasal cannula is the home-oxygen default
Two small prongs sit just inside the nostrils while the face remains open. That seemingly minor design difference matters over 15 or more hours per day: the patient can eat, drink, speak, cough and sleep without repeatedly removing the interface.
A standard low-flow cannula mixes source oxygen with room air. The familiar estimate is an increase of roughly four percentage points in FiO₂ for each additional litre per minute from 1 to 6 L/min, but it is only a rule of thumb. A person taking large, fast breaths entrains more room air and receives a lower percentage than a calm person taking smaller breaths at the same setting.
The flow written on the prescription should not be “corrected” using the rule-of-thumb table. It is titrated against the patient’s oxygen saturation, symptoms and sometimes arterial blood gas under the conditions that matter — rest, walking and sleep.
What “oxygen mask” can mean
There are three commonly confused masks:
- A simple face mask delivers a moderate, variable concentration and is typically used at 5–10 L/min.
- A Venturi mask uses a calibrated adapter to provide a selected concentration. See the Venturi colour and FiO₂ chart.
- A non-rebreather mask has a reservoir bag and provides high-concentration oxygen at high flow. See Venturi versus non-rebreather.
Moving from a cannula to “a mask” therefore does not define a dose. The mask type, adapter, flow and target must all be documented.
Mouth breathing and blocked noses
Mouth breathing does not make a nasal cannula instantly useless. Oxygen flowing into the nose can collect in the nasopharynx between breaths and still be carried into the lungs. But delivery becomes less predictable when the nose is severely blocked, the patient is breathing rapidly through the mouth, or inspiratory demand is high.
The correct test is not whether the mouth is open. It is whether the prescribed interface maintains the target saturation without excessive work of breathing. Persistent failure needs clinical reassessment, not adhesive tape over the mouth.
Why a simple mask needs a minimum flow
Exhaled gas occupies the space inside a simple mask. Fresh oxygen flow must wash that gas out through the side holes before the next breath. If flow is set too low, carbon dioxide can accumulate in the mask and be inhaled again.
Guidelines commonly specify at least 5 L/min for a simple mask, with many hospital protocols using 5–10 L/min. Follow the actual mask instructions. A 2 L/min prescription belongs on a low-flow cannula, not on a conventional simple mask.
Comfort, dryness and skin problems
A cannula can dry or irritate the nose, particularly at higher flows and in air-conditioned or dry rooms. It can also cause pressure injury over the ears or at the nostrils. Practical measures include correct sizing, soft ear protectors, water-based nasal moisturiser if approved by the treating team, and regular replacement of stiff or discoloured tubing. Avoid petroleum jelly around oxygen because petroleum products are combustible.
Masks avoid nasal prong pressure but create their own problems: facial pressure, sweating, claustrophobia, difficulty eating and aspiration risk in someone who is vomiting or has reduced consciousness.
Humidification is not automatically required for every low-flow user. If a bubble bottle is prescribed, fill, clean and replace it exactly as directed; do not add tap-water mixtures, fragrances or medication.
When to change the interface
An interface change is reasonable when:
- the target saturation is not maintained despite checking the source, tubing and prescribed flow;
- the cannula cannot be tolerated because of nasal trauma or obstruction;
- a known, controlled FiO₂ is needed;
- a higher concentration is temporarily needed in acute care; or
- the clinical problem requires ventilation rather than oxygen alone.
Changing interface without reassessment can hide deterioration. A patient who suddenly needs substantially more oxygen, becomes drowsy or develops increasing work of breathing needs urgent evaluation.
Takeaway
A nasal cannula is usually the practical choice for stable low-flow home oxygen. A simple face mask can provide a moderate concentration when a cannula is unsuitable, but it must run at its minimum safe flow and remains a variable-delivery device. Venturi and non-rebreather masks are separate categories with different purposes.
Use the interface and flow on the prescription. If the prescribed setup no longer meets the person’s saturation target, contact the treating service rather than switching masks or increasing flow without a plan.
Primary references: British Thoracic Society oxygen guideline; BTS home-oxygen equipment appendix; AARC adult acute-care oxygen guideline.