Dead space (harmful space) determination is a critical performance test for full-face positive pressure respirators, quantifying the amount of exhaled carbon dioxide that is re-inhaled from the interior volume of the facepiece. This article explains the test principle, applicable standards such as EN 136, EN 137, EN 13274-6 and GB/T 16556, sample requirements, measurement methods, and acceptance criteria used by accredited third-party laboratories. It is written for manufacturers, certification engineers, and safety professionals responsible for respiratory protective equipment.
Technical Overview
The harmful space (commonly referred to as dead space or effective dead volume) of a full-face positive pressure respirator is the interior volume of the facepiece in which exhaled gas, rich in carbon dioxide, can accumulate and be re-inhaled during subsequent breathing cycles. Unlike the purely geometric internal volume, harmful space is a functional parameter: it depends on the design of the inner orinasal mask, the exhalation valve, the internal gas flow paths, and the degree of flushing provided by the positive pressure air supply.
Excessive dead space causes rebreathing of carbon dioxide, which can produce headaches, dizziness, increased breathing resistance perception, impaired judgment, and, at severe levels, loss of consciousness. These effects are particularly hazardous in firefighting, chemical emergency response, and confined-space operations, where users already face high physical and thermal stress. Dead space determination is therefore a mandatory item in the type examination of full-face masks and complete positive pressure breathing apparatus.
The relevant technical frameworks include EN 136 for full face masks, EN 137 for self-contained open-circuit compressed air breathing apparatus, EN 13274-6 for the measurement of the carbon dioxide content of inhaled air, and GB/T 16556 for the corresponding Chinese national requirements for self-contained breathing apparatus. The result is expressed as the average CO2 concentration of the inhaled air under a standardized breathing pattern, and for full-face masks it is typically limited to 0.5% by volume.
Testing Samples
Full-Face Masks for Positive Pressure Self-Contained Breathing Apparatus (SCBA)
Complete facepiece assemblies, including the lens, inner mask, inhalation and exhalation valves, speech diaphragm, and head harness, submitted in new condition. Where the certification scheme requires system-level testing, the mask is tested together with the pressure demand valve and a regulated air supply at nominal cylinder pressure.
Full-Face Masks for Supplied-Air and Airline Respirators
Facepieces used with compressed-air line systems and combination apparatus, tested in the delivered configuration, including the hoses, couplings, and regulators specified by the manufacturer.
Escape and Combined-Use Apparatus
Full-face masks integrated into escape breathing devices or combined SCBA/airline units, evaluated in the configuration in which the wearer breathes from the apparatus during escape.
Sample Quantity and Condition
Test programs normally require several identical units (commonly three to five) so that measurements can be repeated in different states, such as as-received, after temperature and humidity conditioning, and after mechanical or storage-aging treatment, as defined by the applicable product standard.
Testing Items
- Average carbon dioxide concentration of the inhaled air under the standardized breathing pattern
- Peak (maximum) CO2 concentration recorded during the inhalation phase
- Effective dead space volume derived from the measured inhaled CO2 concentration, where required by the specification
- Repetition of the measurement at minute volumes representative of rest, work, and heavy work rates, where required
- Verification of results after pre-conditioning treatments such as heat, cold, humidity exposure, and mechanical handling
Testing Methods
Test Principle
A breathing simulator (artificial lungs) reproduces human respiration on a standard headform wearing the respirator. The exhaled gas is enriched with carbon dioxide at a physiological concentration, commonly about 4% to 5% by volume. A sampling probe located at the mouth position of the headform continuously feeds gas to a fast-response non-dispersive infrared (NDIR) CO2 analyzer. The CO2 concentration measured during the inhalation phases is averaged over several stable breathing cycles and compared against the acceptance limit.
Test Apparatus
- Breathing machine or metabolic simulator with adjustable breathing frequency and tidal volume
- Standard headform with a gas sampling point at the mouth and nose region
- CO2 gas supply with flow control for enriching the exhaled air
- NDIR carbon dioxide analyzer with rapid response time and data acquisition system
- Flow meters, pressure sensors, and timing instrumentation for verifying breathing parameters
Typical Test Procedure
- Condition the samples according to the applicable standard before testing
- Don the mask on the standard headform following the manufacturer instructions and verify correct seating and valve function
- Set the breathing parameters; common settings are 25 breaths per minute with a tidal volume of approximately 1.5 L to 2.0 L, corresponding to a minute volume of roughly 40 L/min to 50 L/min, with the exact values defined by the applied standard
- For positive pressure apparatus, supply breathing air from a regulated source at the specified operating pressure so that the demand valve functions as in real use
- Introduce CO2 into the exhaled air at approximately 5% by volume and operate the simulator until a steady state is reached
- Record the inhaled CO2 concentration continuously and calculate the average value over the specified number of breathing cycles
- Repeat the sequence at additional work rates or after conditioning treatments where the standard requires
Acceptance Criteria
For full face masks tested under EN 136, the average carbon dioxide content of the inhaled air shall not exceed 0.5% by volume. Other respiratory device standards may specify different limits, for example 1.0% average inhaled CO2 for certain filtering half masks under EN 149. Complete apparatus standards such as EN 137 and GB/T 16556 incorporate the corresponding facepiece requirement within their system-level test programs. The test report documents the breathing parameters, conditioning state, measured average and peak values, and a clear pass or fail statement against the limit.
Applications
Dead space determination supports the full life cycle of full-face positive pressure respirators, from design development to market surveillance. Typical applications include:
- Type examination and conformity assessment for product certification and market access under EN and GB standards
- Research and development, particularly the optimization of inner mask geometry, valve placement, and internal airflow distribution to minimize CO2 rebreathing
- Manufacturing quality control and periodic batch verification to confirm consistent production
- Procurement acceptance testing by fire services, petrochemical facilities, mining operations, maritime operators, and confined-space rescue teams
- Comparative benchmarking of respirator designs and verification of design changes during product upgrades
FAQ
What exactly is the harmful space of a respirator?
It is the interior volume of the facepiece in which exhaled gas remains and mixes with the incoming fresh air. It is assessed functionally: instead of simply measuring geometric volume, the test measures the carbon dioxide that is actually re-inhaled, which reflects the combined effect of internal geometry, valve behavior, and airflow flushing.
Does positive pressure eliminate the need for dead space testing?
No. A positive pressure respirator maintains slight overpressure during the inhalation phase, but during exhalation the pressure inside the mask falls toward ambient, and exhaled gas can still accumulate in the mask volume. Certification standards therefore still require the inhaled CO2 test on the full-face mask used in the apparatus.
Why is dead space evaluated by measuring CO2 rather than by measuring the internal volume?
Geometric volume alone does not correlate reliably with rebreathing. Two masks with identical internal volumes can behave very differently depending on the inner mask, exhalation valve design, and air supply flow pattern. Measuring the CO2 content of the inhaled air captures the real physiological effect on the wearer.
What breathing parameters are typically used?
A common configuration is 25 breaths per minute with a tidal volume of about 1.5 L to 2.0 L, giving a minute volume of approximately 40 L/min to 50 L/min, with the exhaled air enriched to roughly 5% CO2. The exact values depend on the standard applied, and some schemes add higher work-rate settings to represent demanding use conditions.
What are the consequences of exceeding the limit?
A result above the limit indicates excessive rebreathing, which can lead to CO2 retention, reduced work capacity, and impaired decision-making during critical tasks. The product fails the type examination, and the design, typically the inner mask or valve arrangement, must be revised and retested.
Where should this test be performed?
The test should be carried out by accredited third-party laboratories or in-house laboratories equipped with calibrated breathing simulators, standard headforms, and traceable CO2 analysis instrumentation, following the test methods specified in the applicable EN, GB, or ISO documents.