Level X4 · Breathing endurance
1 TO 2 HOURS
Degraded comfort — distinct breathing effort
New respirator · Within the manufacturer's stated conditions of use
What it means
Breathing takes a distinct effort.
At this level breathing takes a distinct effort. The mask is worn in sequences of one to two hours, each followed by a recovery break, and it is reserved for healthy adults. It is a legitimate choice for a task that demands it — not a mask for a whole day.
Wear in sequences of one to two hours, followed by a recovery break. Reserved for healthy adults.
- Specific resistance
- 100 – 140 Pa/(L·s⁻¹)
- Comfort qualification
- Degraded comfort — distinct breathing effort
- 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.
The X4/X5 boundary, at 140 Pa/(L·s⁻¹), is the resistance ceiling admitted for the most widespread intermediate filtration class. A mask at this level is at the upper limit of what the standards tolerate in ordinary use.
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 X4 (Pa) |
|---|---|---|---|
| P1 | Rest, seated | 30 L/min | 50 – 70 |
| P2 | Light to moderate activity | 60 L/min | 100 – 140 |
| P3 | Sustained occupational effort | 95 L/min | 158 – 222 |
| P4 | Intense, sporting effort | 140 L/min | 233 – 327 |
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