Level X6 · Breathing endurance
20 TO 30 MINUTES
Critical comfort — major breathing constraint
New respirator · Within the manufacturer's stated conditions of use
What it means
The breathing constraint is major.
At this level the breathing constraint is major. Wear is brief and exceptional — twenty to thirty minutes — with verified medical fitness and supervision. A mask at this level reaches the physiological tolerance ceiling during prolonged sustained effort; it is unsuitable for prolonged wear and for sensitive groups.
Brief, exceptional wear, in the order of twenty to thirty minutes. Verified medical fitness and supervision required. Unsuitable for prolonged wear and for sensitive groups.
- Specific resistance
- > 200 Pa/(L·s⁻¹)
- Comfort qualification
- Critical comfort — major breathing constraint
- Reference state
- New respirator, unworn, taken out of its original packaging for the test, measured on a constant-flow bench
Where the boundary comes from
No threshold is set by convention.
Above 200 Pa/(L·s⁻¹) a mask consumes, at 105 L/min of minute ventilation, 96 % of the physiological margin allowed by ISO 16976-4. The absolute bound of the scale, 242, is the most permissive approval ceiling on record: beyond it no use is conceivable.
Same level, four situations
The level does not change with effort; what you feel does.
Specific resistance is invariant, so a mask keeps its level whatever the activity. But the pressure the wearer actually overcomes grows with the breathing flow, and the work of breathing grows with its square: a mask that is comfortable at rest becomes demanding in fast walking without having changed nature.
| Profile | Situation | Bench flow | Δp at X6 (Pa) |
|---|---|---|---|
| P1 | Rest, seated | 30 L/min | > 100 |
| P2 | Light to moderate activity | 60 L/min | > 200 |
| P3 | Sustained occupational effort | 95 L/min | > 317 |
| P4 | Intense, sporting effort | 140 L/min | > 467 |
What the level does not say
How long you can actually wear the mask is the smallest of several limits. This level sets one of them.
- Breathing enduranceSet by the resistance of the device. This is the object of the XRS scale.
- Clogging of the filterResistance rises as particles accumulate, at a rate that depends on the working atmosphere.
- Exhaled moistureLoads the media, alters the fit and favours microbial growth.
- Surface hygieneThe outer surface is contaminated by deposit and by handling.
- Fit over timeDegrades with handling and the loosening of the straps.
- The manufacturer's instructionsPrevail over everything else — notably the NR marking limiting use to a single work shift.
The XRS level qualifies breathing comfort. It does not qualify filtration efficiency: the protection of a mask is declared separately, under the applicable standard. The level describes a new mask; it never authorizes use beyond the manufacturer's conditions.
The other levels