A Venturi adapter normally shows two numbers: a percentage and a flow in L/min. The percentage is the intended oxygen concentration. The L/min is the minimum source flow that makes the adapter entrain room air at the designed ratio.
The safest one-line rule is: read the adapter, not the colour. Colour systems are helpful inside one manufacturer’s kit but are not sufficiently universal to prescribe or set oxygen by colour name alone.
Common Venturi settings
The table below shows a frequently encountered hospital colour convention. Treat it as orientation only.
| Common adapter colour | Nominal FiO₂ | Frequently printed minimum source flow |
|---|---|---|
| Blue | 24% | 2–3 L/min |
| White | 28% | 4 L/min |
| Orange | 31% | 6 L/min |
| Yellow | 35% | 8 L/min |
| Red | 40% | 10 L/min |
| Green | 60% | 15 L/min |
Some kits use different colours, offer 50% rather than 60%, or specify different minimum flows. Adjustable Venturi barrels may use a rotating percentage selector rather than separate jets. The marking on the device and its instructions always override a generic chart.
FiO₂ is not the flowmeter setting
FiO₂ is the fraction of oxygen in the gas the patient inhales. Room air is about 21% oxygen. A 28% adapter aims to deliver a gas mixture containing approximately 28% oxygen.
The flowmeter might be set to only 4 L/min because the Venturi jet uses those four litres of oxygen to entrain a much larger volume of room air. The total flow delivered toward the mask can therefore be several times the oxygen-source flow.
This is why comparing a “4 L/min Venturi” with “4 L/min nasal cannula” is misleading. The Venturi number powers an air-entrainment system; the cannula number is simply the continuous oxygen entering the nose.
Why turning the flow up does not normally change the percentage
The adapter geometry fixes the air-to-oxygen entrainment ratio. When source flow rises, oxygen jet flow and entrained-air flow rise together. The total flow increases while the mixture stays close to the printed FiO₂.
That extra total flow matters in a tachypnoeic patient. If respiratory rate is above 30 breaths per minute, British Thoracic Society guidance permits increasing source flow by up to 50% above the adapter’s stated minimum to better meet inspiratory demand. This should be done within the clinical protocol; it does not mean selecting a percentage by trial and error.
If a higher oxygen concentration is required, change to the prescribed higher-percentage adapter or another interface rather than assuming the flow knob converts one jet into another.
Why Venturi masks are used for controlled oxygen
A nasal cannula or simple mask delivers a variable FiO₂ because room-air mixing changes with breathing pattern and mask fit. A Venturi system provides a more predictable concentration as long as:
- source flow meets or exceeds the printed minimum;
- entrainment ports are open;
- the tubing is not kinked;
- the mask and adapter are assembled correctly; and
- total flow is adequate for the patient’s inspiratory demand.
Controlled oxygen is particularly important in people at risk of hypercapnic respiratory failure. A common provisional approach is 24% or 28% Venturi oxygen with a prescribed saturation target and blood-gas reassessment. The percentage alone is not treatment: the saturation response, mental state, breathing effort and carbon dioxide must also be evaluated.
For the choice between controlled and high-concentration masks, see Venturi versus non-rebreather.
Common errors that break the calibration
Selecting by colour alone. A loose blue adapter from one kit may not represent the same setting as blue in another. Check percentage and flow.
Obstructing the entrainment ports. Bedding, clothing, a hand or tape over the side ports prevents the device from drawing the designed amount of air and makes FiO₂ unpredictable.
Running below minimum flow. The jet cannot generate the intended total flow and entrainment performance.
Using the mask on an incompatible source. A concentrator must be capable of the continuous flow required at acceptable oxygen purity. Many home 5 L/min concentrators cannot run a 10 or 15 L/min adapter.
Assuming exact FiO₂ despite very high breathing demand. When patient inspiratory flow exceeds device total flow, additional room air enters around the mask and dilutes the mixture.
Ignoring deterioration because the “right colour” is fitted. Rising oxygen requirement, drowsiness, exhaustion or abnormal blood gases requires urgent reassessment.
Can a Venturi mask be used at home?
It can be, but only within a clinician-directed controlled-oxygen plan. Most chronic home oxygen uses a nasal cannula because it is easier for eating, speaking and sleeping. If a Venturi is prescribed at home, the plan should specify:
- adapter percentage;
- minimum source flow;
- target saturation range;
- when to recheck saturation;
- what counts as treatment failure; and
- whom to call or when to seek emergency care.
The source also needs adequate rated flow. Do not attach a high-flow adapter to a lower-capacity concentrator and assume the printed percentage will still be delivered.
Takeaway
On a Venturi adapter, percentage means FiO₂ and L/min means minimum oxygen-source flow. Increasing flow above that minimum usually raises total gas flow without changing the selected percentage. Because colour conventions vary, always read the printed percentage and flow on the actual adapter.
Oxygen is a prescribed medicine. Do not change a patient’s FiO₂ or target range from a generic internet chart; use the prescription, the adapter instructions and the treating team’s escalation plan.
Primary references: British Thoracic Society oxygen guideline; BTS summary recommendations; European Respiratory Society oxygen-device review.