The visual test is simple: no bag, simple mask; reservoir bag, non-rebreather. The clinical difference is larger. A simple face mask provides a moderate oxygen concentration. A non-rebreather uses stored oxygen and valves to reduce room-air dilution, making it a high-concentration emergency interface.
Calling both an “oxygen mask” loses the information that matters. A handover or prescription should name the exact interface.
Side-by-side comparison
| Feature | Simple face mask | Non-rebreather mask |
|---|---|---|
| Reservoir bag | No | Yes |
| One-way valves | No | Usually |
| Common flow | 5–10 L/min | 10–15 L/min |
| Approximate FiO₂ | About 35–60%, variable | Usually above 60%, variable |
| Typical role | Moderate short-term oxygen | Severe acute hypoxaemia/critical illness bridge |
| Safe at very low flow | No | No |
| Routine chronic home use | Rare | No |
Actual FiO₂ depends on source concentration, mask fit, breathing pattern and flow. A mask connected to an oxygen concentrator also cannot outperform that source’s rated continuous flow and purity.
How a simple oxygen mask works
Oxygen enters near the bottom of the mask and mixes with room air drawn through the side openings. Exhaled breath also exits through those openings. Because there is no reservoir, the amount of oxygen available during a fast inspiration is limited by the source flow and by the oxygen already present inside the mask.
This makes the simple mask a variable-performance interface: two patients at the same 8 L/min can inhale different oxygen concentrations if one is breathing quietly and the other is tachypnoeic.
The mask needs a minimum source flow to clear exhaled carbon dioxide. British Thoracic Society guidance uses 5–10 L/min for a simple face mask. Do not put a conventional simple mask on a 2 or 3 L/min setting just because that was the patient’s nasal-cannula flow.
How a non-rebreather works
The reservoir bag fills continuously during exhalation and between breaths. During the next inspiration, the patient draws oxygen-rich gas from the bag rather than relying only on the instant flow arriving through the tubing. One-way valves reduce mixing with exhaled gas and room air.
The name “non-rebreather” describes the design goal, not a perfect seal. Real devices still allow some room-air entrainment around the face and may be supplied with only one side valve to reduce suffocation risk if the oxygen source fails. Delivered FiO₂ is therefore high but not precisely fixed.
The reservoir-bag setup check
Before applying the mask:
- Connect it to a capable oxygen source.
- Occlude the valve between mask and bag briefly if required by the device instructions.
- Fill the reservoir substantially.
- Apply the mask and watch the bag during several breaths.
The bag should remain at least partly inflated. If it empties on inspiration, check the source flow, kinks, connections, bag and valves immediately. A collapsing bag means the patient’s inspiratory demand is exceeding the available reservoir supply.
Do not tape over safety ports or modify valve discs to make the mask appear “more sealed.”
Why a non-rebreather is not a ventilator
A reservoir mask increases inspired oxygen. It does not create positive airway pressure, deliver a tidal volume or guarantee a breath. A patient can remain in ventilatory failure with an acceptable-looking saturation while carbon dioxide rises.
Increasing drowsiness, exhaustion, shallow breathing or worsening acidosis calls for urgent assessment of ventilation. Depending on the cause, the next interface may be BiPAP/NIV or invasive ventilation, not a different oxygen mask.
Which one is used at home?
Neither is the usual chronic home interface. Stable long-term oxygen is generally delivered by nasal cannula because it permits eating, talking and sleeping. A simple mask may appear in a short-term clinician-directed plan when cannula delivery is inadequate or poorly tolerated.
A non-rebreather at home should be part of an explicit emergency or palliative plan with a source capable of the required flow. It should not be bought as a casual “stronger mask” for a 5 L/min concentrator. If a home patient unexpectedly needs it to maintain their target, seek urgent medical advice.
Frequent errors
- Simple mask below minimum flow: increases rebreathing risk.
- Non-rebreather bag left empty: delays high-concentration delivery at the moment it is needed.
- Reservoir collapse ignored: indicates inadequate supply relative to demand.
- Missing valve discs: lowers performance.
- Flowmeter and source assumed equivalent: 15 L/min printed on a hospital wall outlet does not mean a 5 L/min concentrator can provide it.
- Saturation treated as the whole assessment: work of breathing, mental state and blood gases can reveal deterioration that the oximeter misses.
Takeaway
A simple mask is a moderate-concentration, no-reservoir interface used at 5–10 L/min. A non-rebreather is a high-concentration reservoir interface used at 10–15 L/min, with the bag pre-inflated and maintained during inspiration. The presence of the reservoir changes the device’s role; it does not turn it into breathing support.
New severe breathlessness, confusion, cyanosis, chest pain or a rapidly falling saturation is an emergency. Use the patient’s emergency plan and local emergency services rather than experimenting with masks.
Primary references: British Thoracic Society oxygen guideline; BTS oxygen-equipment appendix; European Respiratory Society device review.