Salt medium filtration efficiency testing measures how effectively masks, respirators, and filter media capture submicron sodium chloride (NaCl) aerosol particles, providing a critical indicator of protection against solid, non-oily airborne contaminants such as dusts, bioaerosols, and pathogens. This test is a core requirement under standards including GB 2626, GB 19083, EN 149, and NIOSH 42 CFR Part 84, and is typically performed by accredited third-party laboratories using automated filter efficiency testers. This guide covers the principle, sample types, testing items, methodology, acceptance criteria, and frequently asked questions related to NaCl filtration efficiency testing.
Technical Overview
Salt medium filtration efficiency testing quantifies the percentage of solid sodium chloride aerosol particles removed by a mask or its filter material under controlled laboratory conditions. Sodium chloride is the standard challenge agent for evaluating performance against non-oily (solid) particles because it can be nebulized into a stable, reproducible aerosol with a submicron size distribution centered near the most penetrating particle size (MPPS), typically in the range of 0.1 to 0.3 micrometers, where mechanical and electrostatic filtration mechanisms are least effective. The test result therefore reflects the true barrier capability of the filtration layers, most commonly melt-blown polypropylene electret media, against dust, pollen, smoke, and pathogen-laden particles.
- Challenge agent: sodium chloride (NaCl) solid aerosol particles
- Representative particle size: approximately 0.3 micrometers mass median diameter (0.075 micrometers count median diameter under NIOSH procedures)
- Standard test flow rates: 85 L/min (GB 2626, NIOSH 42 CFR Part 84) and 95 L/min (EN 149)
- Detection principle: light-scattering photometers or equivalent instruments measuring upstream and downstream particle concentrations
- Typical acceptance thresholds: greater than or equal to 95% for KN95/N95 class, greater than or equal to 94% for FFP2 class
Testing Samples
Filtering Facepiece Respirators
Disposable particulate respirators such as KN95, KN100, N95, N99, FFP2, and FFP3 filtering half masks are the primary category subject to salt medium filtration efficiency testing. Testing is performed on the complete respirator sealed into a test fixture so that edge leakage is eliminated and the intrinsic performance of the filter media is isolated and measured.
Medical and Surgical Masks
Medical protective masks evaluated under GB 19083 are verified with NaCl aerosol as part of their particulate barrier requirements. Surgical and medical face masks assessed under YY 0469, EN 14683, and ASTM F2100 are more commonly characterized by bacterial filtration efficiency, but salt-based particle filtration testing is frequently requested for research, benchmarking, and marketing claim substantiation.
Filter Media and Flat Sheet Materials
Melt-blown nonwoven fabric, composite laminates, activated carbon layers with particulate filtration function, and other flat sheet materials can be tested by cutting circular specimens and clamping them into a standardized sample holder, enabling incoming material inspection and process quality control before masks are assembled.
Other Respiratory Protective Equipment
Replaceable filter cartridges, powered air-purifying respirator (PAPR) filters, and other respiratory protective devices may also undergo NaCl filtration efficiency testing depending on the applicable product standard and certification scheme.
Testing Items
The core testing item is salt medium filtration efficiency; however, comprehensive evaluation programs commonly include related parameters that influence real-world performance.
- Initial NaCl filtration efficiency on as-received specimens
- Filtration efficiency after temperature and humidity pretreatment to simulate aging and storage degradation
- Loaded filtration efficiency maintained over a defined deposited mass of NaCl, where required by the certification scheme
- Filtration efficiency at multiple flow rates for flow-dependence characterization
- Complementary tests frequently requested alongside filtration efficiency: inhalation resistance, exhalation resistance, total inward leakage, exhalation valve leakage, and head strap strength
Testing Methods
Aerosol Generation and Characterization
A sodium chloride solution is atomized with clean compressed air, passed through a dryer, and charge-neutralized to produce a stable, polydisperse solid aerosol. Applicable standards define the particle size distribution, for example a count median diameter of 0.075 micrometers with a geometric standard deviation not exceeding 1.86 under NIOSH procedures, with comparable submicron distributions specified in GB 2626 and EN 13274-7. The aerosol concentration is maintained within the range permitted by the method, commonly up to 200 mg/m³, and verified with calibrated detection instruments before each test series.
Specimen Preparation and Conditioning
Specimens are equilibrated in a controlled environment prior to testing. Certain certification schemes additionally require pretreatment, such as 24 hours of thermal exposure at elevated temperature followed by conditioning at high humidity (for example, 85% relative humidity at 38°C), to simulate degradation before filtration efficiency is measured. Finished masks are mounted onto a test head form or sealed into a sample fixture with the perimeter completely sealed so that all challenge airflow passes through the filter media.
Measurement Procedure
The conditioned NaCl aerosol is drawn through the specimen at a constant volumetric flow rate, typically 85 L/min under GB 2626 and NIOSH protocols or 95 L/min under EN 149, representing high work-rate breathing conditions. Light-scattering photometers or equivalent detectors measure the particle concentration upstream and downstream of the specimen once equilibrium is reached. Where the standard requires it, efficiency is monitored continuously as a specified mass of salt deposits onto the filter to confirm that performance is maintained under loading.
Calculation and Reporting
Filtration efficiency is calculated as the ratio of captured particles to the total challenge, expressed as E = (1 − C_downstream / C_upstream) × 100%, where C represents the measured aerosol concentrations. Results are reported as a percentage and compared against the class threshold of the applicable standard, for example at least 95% for the KN95 class under GB 2626-2019 and at least 94% for the FFP2 class under EN 149.
Applications
- Product certification and market access: demonstrating conformity with GB 2626, GB 19083, GB/T 32610, EN 149, and NIOSH 42 CFR Part 84
- Factory quality control: routine batch release testing of melt-blown filter media and finished masks during production
- Research and development: comparing electret treatments, layer constructions, basis weights, and charging processes
- Supplier and incoming material verification: independent confirmation of filter media quality for mask converters, brand owners, and distributors
- Procurement and regulatory surveillance: post-market compliance checks and technical documentation support for tenders, customs clearance, and market supervision
FAQ
What is salt medium filtration efficiency testing?
It is a laboratory test in which a mask or filter material is challenged with a controlled sodium chloride aerosol, and the percentage of particles captured is measured. The result indicates the level of protection provided against solid, non-oily airborne particulates.
How does salt medium testing differ from oil medium testing?
NaCl aerosol represents solid, non-oily particles, whereas paraffin oil or DOP/PAO aerosols represent oily liquid particles that can progressively neutralize electret charges in filter media. Non-oil rated classes such as N and KN are certified using NaCl only, oil-resistant and oil-proof classes are additionally tested with oil aerosol, and EN 149 requires both NaCl and paraffin oil testing for all FFP classes.
What flow rate is used for the test?
GB 2626 and NIOSH procedures specify 85 L/min, while EN 149 specifies 95 L/min. Both levels simulate demanding breathing rates and are intentionally more severe than normal at-rest respiration.
What results are required to pass?
Typical minimum requirements include 95% for KN95 (GB 2626-2019), 95% for N95 (NIOSH), 94% for FFP2 and 99% for FFP3 (EN 149), 99.97% for KN100 and N100 classes, and 95% for medical protective masks under GB 19083.
How many samples are required?
Sample quantities depend on the applicable standard and test scope; filtration efficiency programs commonly require multiple specimens per condition, with additional conditioned subsets where pretreatment applies. Accredited third-party laboratories confirm the exact number of specimens during test planning.
Does salt filtration testing replace bacterial filtration efficiency testing?
No. Bacterial filtration efficiency tests use biological aerosols with a mean particle size near 3 micrometers and address a different performance aspect. For medical face masks, NaCl filtration testing and bacterial filtration efficiency testing are complementary, and both are typically included in a complete evaluation program.
Can salt testing detect degradation of electret filter media?
Yes. Because filtration of submicron NaCl particles relies heavily on electrostatic attraction in electret media, a decline in measured efficiency is a sensitive indicator of charge decay caused by aging, humidity exposure, or storage degradation.
