What are the ASTM tests and what do they mean
, 3 min reading time
Face masks provide the user with protection against airborne particles, pathogens, secretions, and body fluids by physically filtering them from breathable air. According to the American Society of Testing and Materials (ASTM) F2100 standard, which specifies the performance requirements for materials used in medical face masks, five performance characteristics have been identified. These are particulate filtration efficiency (PFE), bacterial filtration efficiency (BFE), fluid resistance, differential pressure, and flammability.
Particulate Filtration Efficiency (PFE)
This test measures the filtration efficiency of face masks towards monodisperse particles under a constant airflow rate. While the F2299 standard allows consistent comparison of the PFE value of different materials used for face masks, it does not access the effectiveness of the overall design of the face mask, nor the quality of the mask’s seal to the wearer’s face.
Bacteria Filtration Efficiency (BFE)
This test quantifies the performance of the mask material in filtering out bacteria when challenged with an aerosol of Staphylococcus aureus, as recommended by the ASTM F2101 standard. S. aureus was chosen for its clinical relevance as one of the leading causes of nosocomial infections acquired in a hospital or healthcare facility. For surgical masks, a minimum BFE of 95% BFE is required.
Viral Filtration Efficiency (VFE)
The viral filtration efficiency (VFE) is another parameter used by mask manufacturers for marketing and in FDA 510(k) applications for certain N95 filtering face piece respirators, although it is not currently recognized as a standard test method by ASTM and hence is not a requirement for mask evaluation.
Fluid Resistance
Fluid resistance evaluates the mask’s ability to act as a barrier to the transfer of fluids from its outer to its inner layers due to spraying or splashing. According to the ASTM F1862 standard, 2 mL of synthetic blood, containing a red dye for visual detection and a thickening agent for stimulating blood flow properties, is dispensed against a complete medical mask specimen at different velocities. These velocities correspond to different blood pressures of 80 mmHg (Level 1, venous blood pressure), 120 mmHg (Level 2, arterial pressure), and 160 mmHg (Level 3, high pressures occurring during trauma or under surgical conditions with high-pressure irrigation), assuming the face mask is within 300 mm of the blood vessel puncture. The pass/fail determinations are based on visually detecting penetration of the synthetic blood to the inner layer. To simulate actual usage conditions, i.e., breathing, which creates high humidity (thus affecting fluid resistance), and mask material, the test specimens are also preconditioned at high relative humidity of 85% at 21°C.
Differential Pressure (DP)
This parameter, otherwise known as “delta P,” measures the ability of the mask material to restrict airflow through it, giving an objective indication of the mask’s breathability – this test is not used with respirators. Typically, it is determined by measuring the difference in air pressure on both sides of the mask material using a manometer at a constant airflow rate, and the difference in pressure is divided by the surface area of the sample, according to the MIL-M-36954 standard
As such, DP is usually expressed in units of mm H2O/cm2, where a lower value (i.e., smaller difference in pressure on both sides) indicates greater breathability, feels cooler to the wearer, and hence gives an overall better comfort level. ASTM requires that moderate and high barrier masks have a DP value of <5.0, while low barrier masks have <4.0. It is noteworthy that a trade-off exists between DP and fluid resistance for the same design and fit of the wearer: generally, an increase in resistance to synthetic blood penetration also results in a greater pressure drop across the mask layers and hence reduces breathability
Flammability
Hospitals contain numerous sources of ignition, such as heat, oxygen, and fuel sources. As the natural and synthetic fibres making up the mask materials are flammable, these can pose potential risks to the wearer due to the speed and intensity of flame spreading.
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