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.
Evidence · XRS 001:2026, Clauses 5, 6 and 11 to 15
Traceable, reproducible, consistent, conformant — and falsifiable.
A heuristic cannot be proven perfect, and one that claimed to be would cease to be a heuristic. The reference document claims the highest level of proof such a tool can reach, establishes it clause by clause, and states in advance what would refute it.
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.
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.
Internal consistency · Clause 12
Fifteen independent markers, no inversion.
A scale built on a few anchors could be consistent with those alone. The decisive test is to place every available marker back on it — including those that served to fix no boundary — and to check that they order themselves correctly. Fifteen markers from five distinct sources were converted to specific resistance. No adjustment was applied.
| Rs computed | Level | Marker |
|---|---|---|
| 15,9 | X1 | Most breathable cloth mask measured on the market |
| 20,8 | X1 | Least resistant laboratory prototype (29,4 Pa at 85 L/min) |
| 34,6 | X2 | Ceiling of the most demanding published breathability level |
| 41,5 | X2 | Intermediate laboratory prototype (58,8 Pa at 85 L/min) |
| 42,2 | X2 | Commercial N95, measured value (6,1 mm H₂O) |
| 58,8 | X2 / X3 | Lower bound of the psychophysical detection threshold |
| 62,3 | X3 | "No further benefit beyond" ceiling (88,2 Pa at 85 L/min) |
| 71,3 | X3 | Least breathable KN95 measured on the market (10,3 mm H₂O) |
| 74,5 | X3 | Upper bound of the psychophysical detection threshold |
| 103,8 | X4 | Ceiling of the least demanding published breathability level |
| 120,0 | X4 | Resistance ceiling of the lowest filtration class |
| 140,0 | X4 / X5 | Resistance ceiling of the intermediate filtration class |
| 173,1 | X5 | Exhalation approval ceiling |
| 200,0 | X5 / X6 | Resistance ceiling of the highest filtration class |
| 242,3 | X6 | Inhalation approval ceiling — absolute bound |
No inversion
No commercial mask is classified more severely than a regulatory ceiling; no regulatory ceiling is classified more favourably than an optimised laboratory prototype.
The detection band frames one boundary
The 58,8–74,5 band served only to set the X2/X3 boundary at 60. That it overlays this boundary over its whole width, spilling into neither X1 nor X4, is not a mechanical consequence of the construction.
Products below, ceilings above
Real masks fall in X1 to X3; regulatory ceilings occupy X4 to X6. Standards are safety guard-rails set well above what products actually reach — which is why a comfort scale could not be derived from them directly.
This consistency establishes that the scale is well built and compatible with all available published data. It does not establish that the qualifications and endurances match what wearers actually feel: that is the object of the experimental validation of Clause 14, which remains to be conducted.
External conformity · Clause 13
Two reasonings with nothing in common designate the same boundary.
The X5/X6 boundary was set at 200 Pa/(L·s⁻¹) from the resistance ceiling of the highest filtration class — a purely regulatory route. ISO 16976-4, a standard of physiological tolerance built by an international committee from a literature the reference did not use, sets limits on work of breathing and peak pressure. Converted to specific resistance, its most constraining ceiling is 208 by work of breathing and 218 by peak pressure.
200 ≈ 208 ≈ 218
Boundary of the scale · ISO 16976-4 limit by work of breathing · by peak pressure, at 105 L/min minute ventilation. Gap of 4 % and 8 %, both on the prudent side.
| Profile | ISO class | WoB/VT at X6 floor | Limit | Margin left |
|---|---|---|---|---|
| P1 | 10 L/min | 0,16 kPa | 1,8 kPa | 91 % |
| P2 | 20 L/min | 0,33 kPa | 1,8 kPa | 82 % |
| P3 | 35 L/min | 0,58 kPa | 1,8 kPa | 68 % |
| P4 | 50 L/min | 0,82 kPa | 1,8 kPa | 54 % |
The whole scale, on its six levels and four profiles, lies inside the physiologically acceptable domain. The margin closes regularly with effort, which designates the maximal-effort profile as the one to measure first on a real product. A mask exactly at the X6 floor, at 105 L/min, consumes 96 % of the available margin without exceeding it.
Derived physiological scale (Clause 11): WoB/VT = Rs × Q̇peak × π / 2, under a sinusoidal flow, a purely resistive load and inhalation–exhalation symmetry. For a valved mask the analytical value is an upper bound.
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.
Falsifiability · Clause 15.3
What would refute the scale.
These observations, stated before any experiment, would constitute a total or partial refutation and trigger a revision. One cannot claim to be refutable and correct oneself quietly: every modification gives rise to an indexed edition with the data that motivated it.
| Observation | Consequence |
|---|---|
| Correlation between Rs and the Borg score below 0,50 | Refutation of the very principle of the scale: resistance would not suffice to predict comfort. Complete redesign. |
| Experimental detection threshold outside the 58,8–74,5 band | Displacement of the X2/X3 boundary and repositioning of the whole scale. |
| Inversion of two adjacent levels in the median scores | Merger of the two levels or redefinition of their boundary. |
| Rs variation above 25 % between 30 and 140 L/min on the tested samples | Abandonment of the single scale in favour of four independent scales, one per profile. |
| Observed wearing durations shorter by more than half than the announced ones | Revision of the breathing-endurance column, without questioning the thresholds. |
| Shape-factor regression with R² below 0,5 | Withdrawal of the shape factor from the predictive model, kept as indicative. |
| Work of breathing measured on the simulator exceeding the analytical value of 11.2 by more than 30 % | Replacement of the analytical relation by a law fitted on measurement, and re-examination of the Clause 13 conformity check. |
| A standardized physiological limit exceeded by a mask classified X5 or below | Lowering of all upper boundaries. The gravest refutation: the scale would declare comfortable a mask that is physiologically unacceptable. |
Experimental validation · Clause 14
A pre-registered protocol, with its success criteria fixed before the study.
Consistency and conformity being established, it remains to verify experimentally that the six levels correspond to six distinct perceptions. This is the link no publication has established, and the one the reference undertakes to provide. The criteria below are set before the study and will not be revised afterwards.
- Design
- Randomized, double-blind crossover: each subject wears the whole set of masks in a drawn order; masks unmarked; operators blind to the level.
- Subjects
- Sixty at least, stratified by sex, age group and fitness; exclusion of known respiratory or cardiac pathology and heavy active smoking.
- Samples
- At least two masks per level, twelve at minimum, of varied geometries so as to test the shape factor as well.
- Physical measurements
- Pressure drop at 30, 60, 95 and 140 L/min on every mask; work of breathing on a breathing simulator per ISO 16900-12; developed filtering area.
- Subjective measurements
- Borg CR10 for breathing discomfort and a 0–100 visual analogue scale for overall comfort, at t₀ then every twenty minutes for sixty minutes at the profile's effort; heart rate, breathing rate, SpO₂ and under-mask temperature recorded continuously.
| Analysis | Criterion |
|---|---|
| Resistance / discomfort relation (Spearman, Borg at 60 min) | ρ ≥ 0,70 |
| Separation of adjacent levels (paired comparisons, multiplicity-corrected) | At least 4 of the 5 boundaries separate two score populations |
| Position of the X2/X3 boundary (ROC on declared detection) | Optimal threshold within 58,8–74,5 |
| Monotonicity of median scores X1 → X6 | No inversion between adjacent levels |
| Announced durations (survival analysis on abandonment or Borg ≥ 5) | Observed medians bracket the Table 7 durations |
| Shape factor (regression of Rs on 1/area) | R² ≥ 0,8 |
Call for collaboration
Laboratories, universities and manufacturers wishing to take part in the validation programme — by contributing masks, bench time, a breathing simulator or a subject panel — are invited to contact the Test Laboratory Division. Data from licensed declarations are only used for the second edition after aggregation over at least five distinct licensees and full anonymisation.
Bibliography
Eighteen sources, all cited with their role.
The reference cites a small number of limit values established by international standards. These are factual data, systematically attributed, converted into another quantity and used for a purpose distinct from the texts they come from. The standards remain the only authoritative documents.
Normative references
- EN 149 — Respiratory protective devices: filtering half masks to protect against particles. Inhalation and exhalation resistances, classes FFP1, FFP2, FFP3.
- FFP protective mask — synthesis of the European standard requirements (cross-check of [1]).
- ASTM F3502-21 — Standard Specification for Barrier Face Coverings. Breathability levels 1 (≤ 15 mm H₂O) and 2 (≤ 5 mm H₂O) at 85 L/min.
- Overview of the ASTM F3502-21 Barrier Face Covering Standard — NIOSH Science Blog, CDC.
- ISO 16900-12:2016 — Respiratory protective devices — Methods of test and test equipment — Part 12: Determination of volume-averaged work of breathing and peak respiratory pressures.
- ISO 16976-4:2023 — Respiratory protective devices — Human factors — Part 4: Work of breathing and breathing resistance: physiologically based limits. Source of the Clause 13 conformity check.
- ISO 16976-1:2022 — Respiratory protective devices — Human factors — Part 1: Metabolic rates and respiratory flow rates.
Experimental literature
- Pressure drop of filtering facepiece respirators: how low should we go? — International Journal of Occupational Medicine and Environmental Health. Source of the 58,8–74,5 Pa/(L·s⁻¹) detection threshold and of the 88,2 Pa benefit ceiling. Central anchor of the scale.
- Impact of Low Filter Resistances on Subjective and Physiological Responses to Filtering Facepiece Respirators — PLOS ONE.
- Physiologically acceptable resistance of an air purifying respirator — Ergonomics, 2011.
- Comparison of ISO work of breathing and NIOSH breathing resistance measurements for air-purifying respirators. Correlation of 0,7 to 0,9 between the two quantities.
- Assessment of best-selling respirators and masks — American Journal of Infection Control.
- Work of Breathing for Respiratory Protective Devices: method implementation, variability and repeatability.
Geometry and filtration physics
- Filtration efficiency and pressure drop of nonwoven materials for respirator designs — Aerosol and Air Quality Research. Source of u = Q/A and of the measured areas of 236 cm² and 415 cm².
- The effects of face mask specifications on work of breathing and particle filtration efficiency.
Subjective measurement instruments
- Borg Rating of Perceived Exertion and Borg CR10 scale.
- Use of the modified Borg scale and numerical rating scale to measure chronic breathlessness — European Respiratory Journal.
- Detection and perception of inspiratory resistive loads in adults — Journal of Applied Physiology.
Full references with links are given in the Bibliography of XRS 001:2026, in every published edition.
Governance
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.