Explainer: Testing the Efficacy of Protective Face Masks
As we are encouraged to wear face masks as part of efforts to contain the spread of COVID-19, here are three standard tests that determine the efficacy of medical and surgical masks.
Image: Various tests to determine and understand the efficacy of medical and surgical masks
The Bacterial Filtration Efficiency (BFE) test
This test indicates how well a mask filters droplets containing biological agents such as bacteria or viruses. With SARS-CoV-2 thought to be transmitted through droplets, the BFE test is a proxy for the efficacy of a mask in preventing such transmission.
Studies indicate that a cough typically expels droplets between 5 and 20 microns in size. The BFE test measures how effectively a mask filters bacteria-containing droplets about 3 microns in size. (An average human hair is about 70 to 100 microns wide.)
The higher the BFE, the more effective a mask is in preventing bacteria-containing droplets from reaching the wearer. For instance, a mask with a BFE of 95 per cent – meeting the requirements for medical and surgical masks (ASTM F2100-19 Level 1 barrier protection) – blocks 95 per cent of droplets it is exposed to.
The Particle Filtration Efficiency (PFE) test
While BFE and VFE tests are designed to find out how well a mask filters biological aerosols, the PFE test measures how well it filters airborne particles such as pollen or dust. This is done by filtering particles about 0.1 micron in size.
The higher the PFE of a mask, the more particles it filters. If it can filter particles about 0.1 micron wide, it is likely to be able to filter droplets that are typically much larger, and which might contain SARS-CoV-2.
For example, a mask with a PFE of 95 per cent – meeting the requirements for medical and surgical masks (ASTM F2100-19 Level 1 barrier protection) – blocks 95 per cent of particles it is exposed to.
The Differential Pressure (DP) test
This test determines how breathable a mask is by measuring how easily air passes through from one side to the other. It does so by calculating the difference in pressure across the surface at a flow rate of 8 litres per minute.
The lower the DP of a mask, the more breathable it is. Breathability is important because it plays a role in determining if people will wear – and leave on – their masks.
Note: Standards referred to above are based on ASTM and European Standards for surgical and medical masks.
Here are three other tests that have been considered to understand the efficacy of medical and surgical masks.
The Viral Filtration Efficiency (VFE) test
This test determines how well a mask filters virus-containing droplets about 3 microns in size. It is similar to BFE in that it uses droplets of similar size, but uses a different challenge organism (bacteriophage instead of bacteria).
The higher the VFE, the more effective a mask is as a barrier against viruses, including coronaviruses. A mask with a VFE of 95 per cent, for example, blocks 95 per cent of virus-containing droplets it is exposed to.
The absorbent test
This test gives an indication of how well the inner layer of a mask absorbs water, indicating its ability to trap droplets emitted by the wearer.
The faster the material absorbs water, the more hydrophilic it is, and the more effective it is in collecting the wearer’s droplets and preventing the droplets from spreading to others.
The hydrophobic test
This test indicates how well the outermost layer of a mask repels water.
The more hydrophobic its outermost layer is, the better a mask is at protecting the wearer from droplets emitted by others that might contain bacteria or viruses.
Higher hydrophobic levels typically correspond with higher BFE and VFE levels.
For more discussions on the various methods of mask testing, please see Dr Xian Jun Loh and team from A*STAR's Institute of Materials Research and Engineering (IMRE)'s research paper on "Face Masks in the New COVID-19 Normal: Materials, Testing, and Perspectives".
Find out more about A*STAR’s R&D efforts in boosting nationwide mask production here.