Determination of the exhalation resistance coefficient of full-face respirators quantifies the pressure drop a wearer must overcome when breathing out through the mask body and exhalation valve, making it one of the most decisive indicators of respiratory comfort, work-of-breathing, and physiological safety. This article reviews the technical principles, applicable standards such as EN 136, EN 13274-8, NIOSH 42 CFR Part 84 and the ISO 16900 series, typical test samples, laboratory procedures, and practical applications of exhalation resistance testing performed by accredited third-party laboratories.
Technical Overview
A full-face respirator (full-face mask) forms a gas-tight seal around the entire face, protecting both the respiratory tract and the eyes against particulates, gases, and vapors. During exhalation, the expired air stream must travel through the internal dead space, across the exhalation valve seat, past the valve membrane and any protective screens, and finally exit to the ambient atmosphere. Every element along this flow path contributes to a pressure drop, and the sum of these contributions is measured as the exhalation resistance.
Excessive exhalation resistance directly increases the work of breathing. At high work rates this can lead to premature fatigue, hypoventilation, elevated carbon dioxide retention in the mask dead space, visor fogging, and ultimately reduced wearer tolerance and compromised protection. For this reason, every major respiratory protective device standard imposes strict upper limits on exhalation resistance.
Test results are typically expressed in pascals (Pa), millibars (mbar), or millimeters of water column (mm H2O) at a standardized constant airflow. When the pressure drop is normalized by the test flow rate, the result is reported as an exhalation resistance coefficient, commonly stated in Pa per (L/min), which allows direct comparison of valve and mask performance across different flow conditions.
- EN 136 – Respiratory protective devices: full face masks, requirements, testing, marking
- EN 13274-8 – Respiratory protective devices: methods of test, determination of breathing resistance
- NIOSH 42 CFR Part 84 – Approval of respiratory protective devices (United States)
- GB 2890 and GB 2626 – Chinese national standards for filtering respiratory protective equipment
- ISO 16900 series – Respiratory protective device test methods (International Organization for Standardization)
Testing Samples
Complete Full-Face Mask Assemblies
The most common test object is the complete full-face mask as placed on the market, including the facepiece body, visor, head harness, internal nose cup, exhalation valve with its membrane and cover, and all sealing gaskets. Testing the assembled mask captures the true interaction between the exhalation valve and the internal flow geometry of the facepiece.
Exhalation Valve Units and Components
Valve manufacturers and mask assemblers frequently submit the exhalation valve alone as a discrete component sample. Component-level testing isolates the cracking (opening) pressure, membrane stiffness, and seat geometry from the influence of the mask body, which is valuable for supplier qualification and incoming quality control.
Configured Respirator Systems
Where a full-face mask is approved for use with specific filters, cartridges, canisters, or a supplied-air / self-contained breathing apparatus connection, the resistance measurement may be repeated in the complete configured system. Although exhalation resistance is dominated by the exhalation side, system-level data are required for certification dossiers and user instructions.
Sample Preparation and Conditioning
Samples are usually tested in as-new condition and, for certification purposes, again after standardized pre-conditioning such as temperature and humidity cycling, simulated mechanical handling, and storage aging defined by the applicable standard. Aged and conditioned samples reveal whether the valve membrane or seat degrades in a way that increases breathing resistance over the product life.
Testing Items
A typical exhalation resistance test program for full-face respirators covers the following items:
- Exhalation resistance at standardized constant flow rates, commonly 85 L/min, 95 L/min, and 160 L/min depending on the referenced standard
- Exhalation resistance under high-workload flow conditions, for example 300 L/min, where required by the product class
- Exhalation valve opening (cracking) pressure
- Exhalation valve leak tightness and back-pressure behavior
- Exhalation resistance coefficient, calculated as the measured pressure drop divided by the volumetric flow rate
- Repeatability of resistance over a defined number of breathing cycles to detect valve flutter, sticking, or drift
- Comparison of resistance values before and after environmental and mechanical pre-conditioning
Testing Methods
Test Principle and Apparatus
The measurement follows the constant-flow principle. A regulated airflow is drawn or pushed through the mask in the exhalation direction at a prescribed steady rate, while a calibrated differential pressure transducer measures the pressure difference between the interior of the mask (at a point near the mouth region of a headform or test fixture) and the ambient atmosphere. Once the flow and pressure readings stabilize, the recorded pressure drop is taken as the exhalation resistance.
A standard test bench comprises an air source with precise flow control, a calibrated flow meter or mass flow controller, a headform or rigid fixture that holds the mask in its natural donned geometry, a differential pressure sensor with suitable range and resolution, connecting tubing with minimal internal resistance, and data acquisition software. All instruments are traceably calibrated, and laboratory ambient conditions of temperature, pressure, and humidity are recorded for each test.
Conditioning and Pre-Treatment
Depending on the applicable standard, samples may be conditioned before measurement. Typical regimes include storage at elevated temperature, storage at low temperature, humidity exposure, and simulated rough handling, after which the exhalation resistance is re-measured. The purpose is to confirm that valve membranes and seats do not warp, stiffen, or deform in a manner that would materially increase the work of breathing during real-world service.
Measurement Procedure
- Inspect the sample visually, verify conformity to the manufacturer's drawings, and mount it on the calibrated headform or test fixture without artificial distortion of the exhalation valve.
- Confirm the sealing integrity of the mounting so that leakage does not falsify the pressure reading.
- Establish the specified constant exhalation flow rate and allow the flow and pressure signals to stabilize.
- Record the stabilized differential pressure as the exhalation resistance at that flow rate.
- Repeat the measurement at each required flow point and for the required number of samples, typically the smallest series defined by the standard.
- Repeat the sequence on pre-conditioned samples where the certification scheme requires it.
Calculation of the Resistance Coefficient
The exhalation resistance coefficient is obtained by normalizing the measured pressure drop by the corresponding volumetric flow rate, expressed as k = ΔP / Q, with the coefficient reported in Pa per (L/min). For example, a mask exhibiting a pressure drop of 245 Pa at 85 L/min corresponds to a coefficient of approximately 2.9 Pa per (L/min). Reporting the coefficient alongside the absolute resistance allows engineers to extrapolate expected pressure drops at other work rates and to benchmark valve designs independently of the specific test flow chosen by a given standard.
For acceptance, the measured values are compared against the limits of the applicable scheme. Under NIOSH 42 CFR Part 84, exhalation resistance for most air-purifying respirators must not exceed approximately 245 Pa (25 mm H2O) at 85 L/min. EN-based schemes for filtering devices commonly limit exhalation resistance to about 300 Pa at 160 L/min, while GB and ISO standards define their own flow rates and thresholds according to device class. The specific limit applied must always be stated together with the flow condition.
Applications
Exhalation resistance data for full-face respirators support a wide range of engineering, regulatory, and occupational health decisions:
- Product certification and market access, including CE marking under the EU PPE Regulation based on EN 136 test evidence, NIOSH approval in the United States, and conformity with GB standards for the Chinese market
- Design verification and optimization of exhalation valves, valve seats, membranes, and internal nose-cup flow paths during research and development
- Incoming component inspection and batch-by-batch quality control in mask and valve manufacturing
- Procurement verification, enabling safety managers to compare candidate respirators on objective breathing-comfort data before large-scale purchase
- Workplace respiratory protection programs, where resistance data feed into selection for high-workload tasks and duration-of-use assessments
- Failure analysis and research, for example investigating valve contamination, dust loading, membrane aging, or freezing effects on breathing effort
Manufacturers, regulators, and end-user organizations commonly rely on accredited third-party laboratories for these measurements, because certification bodies require test reports generated under recognized quality systems with traceably calibrated equipment.
FAQ
What flow rate is used to measure exhalation resistance?
The flow rate depends on the applicable standard. NIOSH-based procedures use a constant flow of 85 L/min, EN-based procedures for filtering devices commonly use 160 L/min, and certain schemes add a high-workload point such as 300 L/min. The report must always state the flow rate together with the measured resistance.
What is the difference between exhalation resistance and the resistance coefficient?
Exhalation resistance is the absolute pressure drop in pascals measured at one specific flow rate. The resistance coefficient normalizes that pressure drop by the flow rate, giving a value in Pa per (L/min) that characterizes the flow behavior of the mask and valve more generally and permits comparison across different test conditions.
Why must exhalation resistance be low?
High exhalation resistance increases the work of breathing, accelerates wearer fatigue, promotes carbon dioxide retention in the mask dead space, and encourages users to remove or break the face seal. All of these outcomes reduce both comfort and the level of protection actually delivered.
Do samples need conditioning before testing?
For certification testing, yes. Standards typically require measurements both on as-new samples and after pre-conditioning such as temperature, humidity, and mechanical treatment, in order to demonstrate that breathing resistance remains acceptable throughout the intended service life.
How is exhalation resistance different from inhalation resistance?
Inhalation resistance is measured with air flowing inward through the filter or air-supply path, while exhalation resistance is measured with air flowing outward through the exhalation valve. Both are reported separately because each is governed by different components and limited by different clauses of the standards.
Who can perform exhalation resistance testing?
Testing can be performed in manufacturers' in-house laboratories for design and quality control purposes, but certification-grade reports are generally issued by accredited third-party laboratories operating under recognized quality management systems with calibrated test benches conforming to EN 13274-8, NIOSH procedures, ISO 16900 methods, or the relevant GB standards.