Over the past two years, the COVID-19 pandemic has created an unprecedented global health crisis. Masks, specifically, have drawn immense attention worldwide. People started to focus on performance evaluation of different face masks and learnt the critical factors as well as various international standards. Amongst different standards for medical devices and masks, the American Society for Testing and Materials (ASTM) standards, European Standard (EN), and Guobiao (GB) standards are commonly recognised standards in Hong Kong.

Hong Kong testing services

In early 2020, the Hong Kong Government launched the “Local Mask Production Subsidy Scheme” under the Anti-epidemic Fund and specified Level 1 of ASTM F2100 – Standard Specification for Performance of Materials Used in Medical Face Masks as one of the mandatory requirements of application. ASTM F2100 classifies medical face masks into three levels based on the following five tests:

• Differential pressure;
• Particle filtration efficiency (PFE);
• Bacteria filtration efficiency (BFE);
• Resistance to synthetic blood penetration; and
• Flammability.

With plenty of new face masks being manufactured, performance evaluation has become essential to ensure effectiveness to safely protect citizens. Unfortunately before the COVID-19 pandemic, no third-party laboratory in Hong Kong had yet developed capability in medical face mask testing.

SGS, one of the world’s largest testing, inspection and certification companies, decided to establish a mask testing laboratory in Hong Kong to support the Government and the local industry. However, SGS observed that only PFE testing equipment complying with the National Institute for Occupational Safety and Health (NIOSH), EN and GB for respirators was available in the open market, not for medical face masks under the ASTM standard. SGS decided to work with Ir Dr Aaron Tong to design and build PFE testing equipment according to ASTM F2299, which is a standard laboratory test method to measure the PFE of materials used in face masks with latex spheres.

Test method of PFE – ASTM F2299

The test method establishes procedures for measuring the initial particle filtration efficiency of materials and utilises light scattering particle counting in the size range of 0.1 to 5.0 μm and airflow test velocities of 0.5 to 25 cm/s.

Aerosol is defined as “a suspension of a liquid or solid particles in a gas with the particles being in the colloidal size range”. Aerosols include solid particles having a diameter of 0.1 to 5.0 μm suspended or dispersed in an airflow at concentrations of less than 102 particles/cm³.

Clause 7.2 of ASTM F2299 further states that the aerosol generator must be capable of a latex sphere count concentration output of 10⁷ to 10⁸ particles/m³.

The standard also specifies the extraction and transportation of aerosol samples (Clause 7.8). Geometrically and kinematically identical centreline probes, which have a radius of curvature (R) of 12 cm or R/D (Diameter) > 20:1 and present a cross-sectional area of less than 10% of the cross-sectional area of the test system ducting, should be used to extract representative aerosols. Additionally, the upstream probe should be located at least eight duct diameters downstream of the aerosol injection point and two duct diameters ahead of the material specimen. Downstream probe should be three duct diameters downstream of the filter medium specimen.

The sampling line flow should be maintained in the laminar flow regime ensuring the Reynolds number remains lower than 1000. A limitation exists of <100 cm, <2 m, and >12 cm for the horizontal sampling line length, the total sample transport line and all radius of curvatures, respectively.

Differences between ASTM F2299 and GB/T 38413

Some laboratories had claimed that their testing services fully complied with ASTM standards. Through investigation, it is figured out that they used PFE testers that complied with EN and GB standards for the ASTM method.

One critical point or focus is the particles to be used in PFE testing. The EN for medical face masks does not require PFE test GB/T standard which specifies sodium chloride (NaCl) and oily particles such as dioctyl phthalate (DOP) or di-ethyl-hexyl-sebacat (DEHS). Unlike GB/T standard, ASTM F2299 uses monodispersed latex spheres suspension and generated aerosol to challenge the medical face mask sample. As different physical properties between salt and latex particles would lead to different result in measurement, it is crucial to adopt equipment with correct design which complies with the specification of ASTM F2299.

Requirement on vertical placement

PFE testers make use of a mixture of latex beads, water and air in the test duct to perform the test. To minimise aerosol sedimentation loss, the test duct (tube) for aerosol transportation cannot be placed horizontally (Figure 1). When the test duct is kept vertical, the mixture will flow down at a constant rate under the influence of gravity, and the test duct shall have sufficient distance (length) to provide thorough mixing of latex aerosol. As a result, ignorable or insignificant difference should be found at the end of the tube.

Figure 1: Tube direction

Even though the direction requirement is indicated in various papers and guidelines (Figures 2 and 3), numerous PFE testers available in the market continue to adopt horizontal orientation.

Figure 2: US typical PFE test system (Nicholson, 1986)

Figure 3: GB typical PFE test system (Standardization Administration of the People’s Republic of China, 2019)

Requirement on laminar flow

To avoid turbulent flow, the radius of curvature of the sampling probe must be large enough so as not to cause significant pressure and velocity changes. In addition, ASTM F2299 also defines that the location to measure pressure drop of the pressure taps should be flush with the duct walls at a distance of one duct diameter upstream and downstream of the filter medium faces. Balance must be obtained to achieve all requirements.

Some world-renowned laboratories have shown on their websites their own PFE tester designs (Figure 4). Unfortunately, these photos can create misleading adverse influences.

Figure 4: Dwarf PFE tester

Engineers therefore could be misguided to design their PFE tester with a short tube as illustrated in Figure 4. Consequently, they are unable to fulfil other constraints, resulting in failure.

Unexpected Trend

Expected:

Unexpected actual:

Analysis:

The unexpected actual trend was significant enough to affect the stability of the test as the concentration of each second varied substantially. This increased difficulties in multiple areas including calibration. Root causes of those waves were identified and solved one by one, so stability was finally achieved.

Overcoming all the difficulties in amalgamating different constraints and limitations, SGS successfully designed PFE equipment that is fully compatible with ASTM F2299 and is currently operating in Hong Kong. With this PFE tester, SGS has become the first laboratory in Hong Kong to acquire and be in full compliance with both International Accreditation Service (IAS) and The Hong Kong Laboratory Accreditation Scheme (HOKLAS) accreditation.

As the first IAS and HOKLAS accredited ASTM complying PFE tester in Hong Kong and mainland China, it is believed that this achievement not only supports the administration of face masks but also revives the manufacturing industry in Hong Kong.

Figure 5: Final achievement

References

1. ASTM International. (2017). ASTM F2299/F2299M-03 Standard Test Method for Determining the Initial Efficiency of Materials Used in Medical Face Masks to Penetration by Particulates Using Latex Spheres. West Conshohocken, PA, United States.
2. Nicholson, R. (1986). A standard test method for initial efficiency measurements on flatsheet filter media. In Fluid Filtration: Gas Volume I. ASTM International.
3. Standardization Administration of the People’s Republic of China. (2019). GB/T 38413-2019 Textiles - Test Methods for Filtration of Particulate Matter. China.

Sources

1. International Accreditation Service. https://www.iasonline.org/?post_type=ias_certificate&orderby=org&order=ASC&s=&global=&service=13579&keyword=&number=&org=&city=&state=&country=HK&zip=&status
2. International Accreditation Service. https://www.iasonline.org/wpcontent/uploads/2018/11/TL-817-cert-New.pdf
3. Hong Kong Accreditation Service. The Hong Kong Laboratory Accreditation Scheme. https://www.itc.gov.hk/en/quality/hkas/doc/scopes/009.pdf
4. Hong Kong Accreditation Service. The Hong Kong Laboratory Accreditation Scheme. https://www.itc.gov.hk/en/quality/hkas/doc/scopes/003.pdf