Updated November 5, 2026 · By Andy Leleux, sleep technologist
Never block the holes in your mask
Putting tape, foam or anything else over the holes in your CPAP mask is something you should never do. ResMed and Philips both say it in their manuals: the vent must stay clear. Otherwise the air you breathe out stays in the mask, and you breathe it back in.
What is the intentional leak?
Illustration being redrawn
CPAP mask cross-section: pressurized air (blue arrow) comes in, exhaled air carrying CO2 (red arrows) leaves through the vent
fuite-calibree-co2-evacuation · 900×500px · medical-diagram
Almost all CPAP masks are "vented": they have a vent, a cluster of small holes in the cushion, frame or elbow. The machine constantly pushes fresh air through it, which flushes out the air you just breathed out.
How much air flows through the vent depends on your pressure. For the ResMed AirFit N20 nasal mask, the manual lists about 20 L/min at 4 cmH₂O, 35 L/min at 11 cmH₂O and 49 L/min at 20 cmH₂O. This isn't a manufacturing flaw. It is a safety feature.
What happens inside your mask:
When you breathe in
Your CPAP pushes room air, under pressure, into the mask and down into your airway.
When you breathe out
The air you breathe out, loaded with CO₂, has to leave the mask before your next breath. That is the vent's job.
Without the vent (never do this)
The air you breathe out stays in the mask and you breathe it back in. CO₂ can then build up in your blood. This is called hypercapnia.
Never wear the mask with the machine off
Same logic. ResMed says to put the mask on only when the machine is running. Without airflow from the machine, the vent can't clear the air, so you end up breathing in what you breathed out. If this goes on for several minutes, it can, in some circumstances, lead to suffocation. The manual also notes that some rebreathing may happen at low pressures.
Why some people block the holes, and why it is dangerous
The vent makes a soft whooshing sound, louder at higher pressures. Bothered by the noise, some people cover the holes with tape, foam or glue. That is a serious mistake: the sound you hear is your exhaled air leaving the mask.
Headaches
A possible result of CO₂ building up, especially on waking.
Sleepiness
Sleep that feels less restful, even though the machine is running.
Hypercapnia
In severe cases: confusion and breathing distress. This is a medical emergency.
If the noise bothers you: choose a mask with a diffuser vent, which is quieter (ResMed makes QuietAir versions). If your mask allows it, turn the elbow so the airflow points away from your bed partner. If the noise got louder after a settings change, tell your sleep center, and don't change the pressure yourself. But never block the vent.
Intentional vs. unintentional leak: the key difference
Your machine measures the air that escapes. The intentional leak is expected: it depends on your pressure and your mask. Unintentional leak, on the other hand, varies: it is air escaping between the mask and your skin, or through your mouth.
| Intentional leak | Unintentional leak | |
|---|---|---|
| Where it comes from | The mask vent | Poor fit, worn cushion, open mouth |
| Pattern | Constant, rises with pressure | Comes and goes, in bursts |
| Sound | Steady whoosh | Irregular hissing, air blowing in your eyes |
| On a ResMed report | Already subtracted | This is the number you see |
| Alert threshold | None: it is normal | Above 24 L/min at the 95th percentile (ResMed) |
| Should you fix it? | No, never | Yes: adjust or change the mask |
On your CPAP report
Brands don't report the same thing. A ResMed machine shows unintentional leak: the intentional leak has already been subtracted, and ResMed sets the alert threshold at 24 L/min at the 95th percentile. ResMed mask manuals also tell you which mask type to select on the machine. A Philips machine shows total leak, which includes the intentional leak, so the 24 L/min threshold doesn't apply. How to read your CPAP report →
Non-vented masks: a tightly limited exception
Some masks have no vent and are called "non-vented" (the ResMed AirFit F20 NV, for example). They are used with certain ventilators or bilevel machines whose circuit has its own exhalation valve. In that setup, the circuit clears the exhaled air, not the mask.
A non-vented mask on a CPAP is an immediate danger. The AirSense 11 manual says to use vented masks only, to allow normal breathing and prevent suffocation. Always check the compatibility listed by the manufacturer.
Frequently asked questions
Sources
- ResMed, AirFit N20 User Guide: keep the vent clear, wear the mask only with the machine running, vent flow by pressure (copy of the official guide). PDF
- ResMed, AirSense 11 User Guide: vented masks only (copy of the official guide). PDF
- Philips Respironics, DreamWear Nasal Mask, Instructions for Use: do not block or seal off the exhalation ports. documents.philips.com
- ResMed, What does "good mask seal" mean?: under 24 L/min of leak at the 95th percentile. resmed.com
- Adaptive Servo-Ventilation: A Comprehensive Descriptive Study in the Geneva Lake Area, 2020: unintentional leak on ResMed devices, total leak on Philips devices. PubMed Central
