Standards cap resistance
EN 149, ASTM F3502 and NIOSH 42 CFR 84 set breathing-resistance ceilings attached to a protection class. They delimit what is physiologically tolerable, not what is comfortable.
XRS™ scale · technical reference XRS 001:2026
The XRS scale — breathing comfort classification of a mask from breathing resistance
Six levels, X1 to X6, derived from a single bench-measurable quantity: specific resistance. Each threshold is anchored on a measured or published reference, and the document states in advance what would refute it.
- XRS 001:2026(E)
- First edition
- 2026-08
- CC BY-ND 4.0
XRS 001:2026 is a proprietary technical reference published under the sole responsibility of XALPES LLC. It is not a standard, has no regulatory value, does not classify filtration efficiency and does not authorize any regulated use. Human-panel validation (clause 14) has not yet been conducted.
Why
Standards cap, literature correlates, nobody grades.
Literature correlates
The link between resistance and perceived effort is established. The detection threshold of added inspiratory resistance lies between 58.8 and 74.5 Pa/(L·s⁻¹). No study goes as far as a scale.
Nobody grades
No reference grades comfort or wearing duration. There is no invariant quantity to compare masks measured at different flows, and no scale linking a resistance level to a use.
The quantity
One input: specific resistance.
A pressure drop in pascals means nothing on its own: it depends on the flow at which it was measured. The XRS scale is built on the pressure drop divided by the flow that produced it.
Rs = Δp / Q
- Δp
- pressure drop across the mask, in pascals (Pa)
- Q
- test flow, in litres per second (L/s)
- Rs
- specific resistance, in Pa/(L·s⁻¹) — the only input of the scale
- Flow invariance verified to better than ± 11 % on the normative values (Table 3 of the reference)
- Declared uncertainty band: ± 12 % on Rs
- A measurement within 12 % of a boundary is recorded as lying in the overlap band; the least favourable level is attributed
- A device carries one level only — never a pair, an interval or an "in-service" level
The scale
Six levels, X1 to X6.
Table 7 of XRS 001:2026, reproduced in full. The capitalized wording that opens each breathing-endurance cell is the normative short form, taken up word for word by any marking.
| Level | Rs Pa/(L·s⁻¹) | Comfort qualification | Breathing endurance |
|---|---|---|---|
| X1 | ≤ 30 | Neutral comfort — resistance not perceptible | NO LIMIT — Continuous wear without time limit, including for sensitive groups (children, elderly people, people with reduced respiratory capacity) and during sleep. |
| X2 | 30 – 60 | High comfort — at the limit of perception | ALL DAY — Continuous wear over a full day without imposed breaks: office, school, retail, transport, urban travel. |
| X3 | 60 – 100 | Standard comfort — perceptible, without discomfort | SEVERAL HOURS — Wear of several hours interspersed with natural breaks: active standing work, light handling, brisk walking. |
| X4 | 100 – 140 | Degraded comfort — distinct breathing effort | 1 TO 2 HOURS — Wear in sequences of one to two hours, followed by a recovery break. Reserved for healthy adults. |
| X5 | 140 – 200 | Constrained comfort — established breathing discomfort | UNDER 1 HOUR — Wear in short sequences, under one hour, with mandatory breaks. Trained subject, supervised task. |
| X6 | > 200 | Critical comfort — major breathing constraint | 20 TO 30 MINUTES — 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. |
Note 1 — Comfort and filtration are two independent dimensions: the breathing-endurance column carries no reference to a protection class. A mask can be X2 and highly filtering, or X5 and poorly filtering.
Note 2 — The capitalized short forms are taken up word for word, without paraphrase, rounding or conversion, by any document or marking stating an XRS level, always under the heading "breathing endurance". The level applies to a new mask, within the conditions of use set by the manufacturer.
French text is authoritative; this translation is provided for information. See XRS 001:2026(F).
Origin of each boundary
No threshold is set by convention.
Table 8 of the reference, condensed. Each boundary refers to a measured or published external reference.
- X1 / X230Half of the detection floor. Safety margin for groups whose perception threshold is lowered: children, elderly people, reduced respiratory capacity.
- X2 / X360Floor of the measured detection threshold (58.8 Pa/(L·s⁻¹)), rounded conservatively. Boundary between the imperceptible and the perceptible — the best-supported boundary of the scale.
- X3 / X4100Exit of the detection band (74.5 Pa/(L·s⁻¹)) plus the uncertainty margin, and vicinity of the most demanding published breathability ceiling (103.8 Pa/(L·s⁻¹)). Beyond, resistance is no longer merely perceived: it is endured.
- X4 / X5140Resistance ceiling admitted for the most widespread intermediate filtration class. A mask at this level is at the upper limit of what references tolerate in everyday use.
- X5 / X6200Ceiling of the highest filtration class. Independently confirmed by the standardized physiological limits: Rs ≤ 218 by peak pressure and Rs ≤ 208 by work of breathing at the highest sustained work rate (clause 13).
- Beyond X6242Most permissive absolute approval ceiling on record. Serves as a safety bound: beyond it, no use is conceivable.
Four use profiles
One scale, four readings.
Since Rs does not depend on flow, one scale generates four application tables. A mask keeps its level whatever the profile; only the measured Δp and the work of breathing change, because they describe what the wearer actually experiences in that situation.
| Profile | Bench flow | Equivalent minute ventilation | Physiological situation |
|---|---|---|---|
| P1 | 30 L/min | ≈ 10 L/min | Rest, seated position |
| P2 | 60 L/min | ≈ 19 L/min | Light to moderate activity |
| P3 | 95 L/min | ≈ 30 L/min | Sustained occupational effort |
| P4 | 140 L/min | ≈ 45 L/min | Intense, athletic effort |
Table 9 — Profile P1, rest / sedentary
Test flow 30 L/min · equivalent minute ventilation ≈ 10 L/min| Level | Rs Pa/(L·s⁻¹) | Measured Δp (Pa) | WoB/VT (J/L) | Comfort qualification |
|---|---|---|---|---|
| X1 | ≤ 30 | 0 – 15 | 0.00 – 0.02 | Neutral comfort — resistance not perceptible |
| X2 | 30 – 60 | 15 – 30 | 0.02 – 0.05 | High comfort — at the limit of perception |
| X3 | 60 – 100 | 30 – 50 | 0.05 – 0.08 | Standard comfort — perceptible, without discomfort |
| X4 | 100 – 140 | 50 – 70 | 0.08 – 0.11 | Degraded comfort — distinct breathing effort |
| X5 | 140 – 200 | 70 – 100 | 0.11 – 0.16 | Constrained comfort — established breathing discomfort |
| X6 | > 200 | > 100 | > 0.16 | Critical comfort — major breathing constraint |
Table 10 — Profile P2, moderate activity
Test flow 60 L/min · equivalent minute ventilation ≈ 19 L/min| Level | Rs Pa/(L·s⁻¹) | Measured Δp (Pa) | WoB/VT (J/L) | Comfort qualification |
|---|---|---|---|---|
| X1 | ≤ 30 | 0 – 30 | 0.00 – 0.05 | Neutral comfort — resistance not perceptible |
| X2 | 30 – 60 | 30 – 60 | 0.05 – 0.09 | High comfort — at the limit of perception |
| X3 | 60 – 100 | 60 – 100 | 0.09 – 0.16 | Standard comfort — perceptible, without discomfort |
| X4 | 100 – 140 | 100 – 140 | 0.16 – 0.22 | Degraded comfort — distinct breathing effort |
| X5 | 140 – 200 | 140 – 200 | 0.22 – 0.31 | Constrained comfort — established breathing discomfort |
| X6 | > 200 | > 200 | > 0.31 | Critical comfort — major breathing constraint |
Table 11 — Profile P3, sustained effort
Test flow 95 L/min · equivalent minute ventilation ≈ 30 L/min| Level | Rs Pa/(L·s⁻¹) | Measured Δp (Pa) | WoB/VT (J/L) | Comfort qualification |
|---|---|---|---|---|
| X1 | ≤ 30 | 0 – 48 | 0.00 – 0.07 | Neutral comfort — resistance not perceptible |
| X2 | 30 – 60 | 48 – 95 | 0.07 – 0.15 | High comfort — at the limit of perception |
| X3 | 60 – 100 | 95 – 158 | 0.15 – 0.25 | Standard comfort — perceptible, without discomfort |
| X4 | 100 – 140 | 158 – 222 | 0.25 – 0.35 | Degraded comfort — distinct breathing effort |
| X5 | 140 – 200 | 222 – 317 | 0.35 – 0.50 | Constrained comfort — established breathing discomfort |
| X6 | > 200 | > 317 | > 0.50 | Critical comfort — major breathing constraint |
Table 12 — Profile P4, maximal / athletic effort
Test flow 140 L/min · equivalent minute ventilation ≈ 45 L/min| Level | Rs Pa/(L·s⁻¹) | Measured Δp (Pa) | WoB/VT (J/L) | Comfort qualification |
|---|---|---|---|---|
| X1 | ≤ 30 | 0 – 70 | 0.00 – 0.11 | Neutral comfort — resistance not perceptible |
| X2 | 30 – 60 | 70 – 140 | 0.11 – 0.22 | High comfort — at the limit of perception |
| X3 | 60 – 100 | 140 – 233 | 0.22 – 0.37 | Standard comfort — perceptible, without discomfort |
| X4 | 100 – 140 | 233 – 327 | 0.37 – 0.51 | Degraded comfort — distinct breathing effort |
| X5 | 140 – 200 | 327 – 467 | 0.51 – 0.73 | Constrained comfort — established breathing discomfort |
| X6 | > 200 | > 467 | > 0.73 | Critical comfort — major breathing constraint |
Note — The same mask keeps its XRS level whatever the profile, since Rs is invariant. The Δp and WoB/VT columns change from one table to the next because they describe what the wearer actually experiences in the situation considered. WoB/VT values are analytical estimates (clause 11), not measurements.
French text is authoritative; this translation is provided for information. See XRS 001:2026(F).
Calculator
From a bench measurement to a level.
Design aid only — a level may only be declared under the conditions of XRS 002 and with XALPES LLC's written authorization. Nothing is stored or transmitted.
Enter a pressure drop and a test flow. The specific resistance Rs = Δp / Q is computed with Q converted to litres per second.
Specific resistance
–
Level–
Comfort qualification–
Breathing endurance–
New mask · within the conditions of use set by the manufacturer. Human-panel validation not yet conducted.
Geometry & form factor
Surface is a stronger lever than media.
What crosses the filtration media is not a flow but a velocity. Two masks cut from the same media can show resistances in a ratio of one to three from geometry alone.
u = Q / A
- u
- face velocity through the media, cm/s
- A
- useful filtering surface, cm²
φ = A_ref / A
- A_ref
- 236 cm², reference duckbill surface measured in the literature
- φ
- form factor; Rs ≈ φ × Rs_reference — φ > 1 degrades breathability, φ < 1 improves it
| Geometry | Surface A | Factor φ | Status of the value |
|---|---|---|---|
| Flat three-fold | ≈ 150 cm² | 1.57 | XALPES estimate, to be confirmed by measurement |
| Rigid moulded cup | ≈ 180 cm² | 1.31 | XALPES estimate, to be confirmed by measurement |
| Duckbill | 236 cm² | 1.00 | Published measured value — reference of the scale |
| Fish type / folded boat | ≈ 300 cm² | 0.79 | XALPES estimate, to be confirmed by measurement |
| Large-surface pleated | 415 cm² | 0.57 | Published measured value |
To target a high XRS level under effort, increasing the developed surface is a more powerful lever than the choice of media. The three estimates are the first task of the measurement program (clause 14.2).
French text is authoritative; this translation is provided for information. See XRS 001:2026(F).
Level of proof
Five properties, each verifiable by a third party.
A heuristic cannot be proven perfect, and one that claimed to be would cease to be a heuristic. Table 1 of the reference lists the five properties that establish its level of proof and the clause where each is established.
Traceability
Every threshold refers to a measured or published external reference, not to an author's choice.
Clauses 7, 9 · check in 12
Reproducibility
A third party with the same test bench obtains the same classification, without interpretation.
Clauses 7, 11
Internal consistency
External references, placed back on the scale, order themselves without inversion or overlap.
Clause 12 · 15 independent references
External conformity
Confronted with physiological limits established independently of it, the scale proves compatible.
Clause 13 · independent convergence
Falsifiability
The document states in advance what, once measured, would invalidate it.
Clauses 14, 15
Position of XALPES LLC
The XRS scale is presented as fully traceable, reproducible, consistent, conforming to the standardized physiological limits and falsifiable — the maximum demonstrable for a tool of this nature. It is not presented as perfect, and clause 15 explicitly states the conditions under which it will have to be corrected. It is this assumed refutability that makes the reference defensible before an expert, and not the reverse.
Declared limits
Five factors deliberately excluded from the model.
Breathing resistance does not explain all of a mask's comfort. The XRS level is a classification of breathing comfort, never of overall comfort.
Thermal & humidity comfort
Heat and moisture build-up under the mask are a major cause of discomfort, independent of resistance.
Mechanical pressure & fit
Pressure points, strap tension and skin irritation bear no relation to pressure drop.
Dead space & CO₂ rebreathing
Depends on internal volume — hence, paradoxically, on the geometry that improves breathability.
Clogging in use
The level describes the reference state: a new mask. Resistance rises with dust and moisture; no "in-service" level exists.
Hygiene & surface contamination
Deposits, handling and microbial growth bound the duration of use without any relation to resistance.
The effective wearing duration of a device is the minimum of several independent ceilings — breathing endurance, clogging, humidity, hygiene, fit — and, first of all, the manufacturer's instructions, which prevail over everything else. A "no limit" breathing endurance means that resistance imposes no limit; it never authorizes use beyond the manufacturer's conditions.
Using the scale
The reference is free; the mark is licensed.
Free
- Read, cite and redistribute the reference in full (CC BY-ND 4.0)
- Internal use in design and technical specification, without licence
- Study, teaching and research, with full attribution
Recommended citation
XRS 001:2026, Échelle XRS, première édition, XALPES LLC
Licensed
Applying the XRS designation or a level X1 to X6 to a product requires the written authorization of XALPES LLC. Licensees carry the wording "XRS™ is a trademark of XALPES LLC, used under licence". Three declaration regimes:
A
Controlled self-declarationQualified internal laboratory of the manufacturer.
B
Declaration on third-party testIndependent qualified laboratory — the ordinary regime.
C
XALPES certificationTests conducted by the Testing Laboratory Division; the only regime allowing "XRS level certified by XALPES".
Principle: the reference is free, the mark is paid for and funds the control. Terms and procedure on scalexrs.com.
Challenge it
The reference explicitly calls for criticism. XALPES LLC solicits comments from any competent person on the position of the thresholds, the model's hypotheses, the values flagged as estimates and the validation protocol. A reference whose refutation criteria are stated in advance gains from being examined.
Send a comment or a challengeGovernance
Editions and authoritative version.
The first edition is the establishing edition, published openly. A second edition will follow the validation program of clause 14: measured developed surfaces, work of breathing measured on a breathing simulator, extension to cumulative breathing loads, and the threshold corrections the data impose.
Any change to a threshold, a qualification or a breathing endurance gives rise to a new indexed edition with its justification. Earlier editions are archived on scalexrs.com and marked as superseded. The authoritative version is the one published by XALPES LLC under the reference number on the cover; in case of divergence with a redistributed copy, it prevails.
Next step
Declare a level, or talk to the laboratory.
Manufacturers, laboratories and buyers: the Testing Laboratory Division answers questions on measurement, declaration regimes and the use of the mark.