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For medical face masks, we often see these characters such as PFE, BFE and VFE. What is the meaning of these indicators?
PFE - particulate filtration efficiency. The percentage of particulate matter filtered by filter element under specified conditions. This indicator is mentioned in the following standards, NIOSH 42 CFR Part 84, EN 149 (EN 143), ASTM F2299, GB 2626, GB / T 32610, GB 19083, etc.
BFF - bacterial filtration efficiency. The percentage of suspended particles containing bacteria filtered by mask or filter media under specified flow rate. This indicator is mentioned in the following standards, EN 14683, ASTM F2100 / 2101, YY 0469, YY / T 0969, etc.
VFE - viral filtration efficiency. The percentage of virus suspended particles removed by the test sample under the specified flow rate. The standard for this index is YY / T 1497.
2. Test Method
In PFE test, the requirements of aerosol, flow rate and particle size are different. Generally, NaCl, paraffin oil or DOP aerosols are used. The test flow rate is 95L / min in European standard and 85L/min in American Standard and Chinese standard. American Standard and Chinese standard test PFE@0.3μm normally, European standard test PFE@0.4μm normally. In addition, according to ASTM 2100 / 2299, polystyrene latex spheres are used, and test PFE@ 0.1μm.
The regulations of different standards for BFE test were basically the same. Staphylo-coccus aureus is used as test aerosol, the sampling flow rate is 28.3L/min, the bacterial concentration was (2200 ± 500) CFU, and the MPS of bacterial aerosol was (3.0 ± 0.3)μm。
According to Chinese standard, Phi-X174 bacteriophage is used for VFE test. The diameter of the virus is 15 ~ 500 nm normally, but the virus can not be transmitted alone, it needs to be carried on aerosol. Therefore, the MPS of bacteriophage aerosol specified in YY/T 1497 is (3.0 ± 0.3) μm. And we have not found VFE testing standards in the United States, the European Union and other countries. On the website of the testing organization affiliated to the University of Minnesota, the following detection methods are available.
3. The relationship between PFE, BFE and VFE
PFE test does not need to cultivate bacteria or bacteriophage, and the test period is shorter and safer. Therefore, many scholars have studied the relationship between PFE and BFE, PFE and VFE. The following are some related research results.
1) PFE and BFE
The correlation between PFE and BFE was studied by Beijing Institute of medical devices. The filtration efficiency of non oil particulate matter was measured by NaCl aerosol at a flow rate of 30L/min, test the PFE@0.3μm. BFE was tested according to the standard of medical surgical mask. The correlation between PFE and BFE is shown in the figure 2, in which the ordinate is PFE value and abscissa is BFE value.
2) PFE and VFE
Bernadette Fuhrer, a senior scientist from Austria, used the virus substitute bacteriophage Phi6 and human coronavirus 229E for determination of viral reduction effect in air and on surfaces.
Use the following testing equipment (Fig.3).
The PFE was tested with 0.9% NaCl aerosol, and the filtration efficiency of 0.2~1.22 μm particles was tested. The same testing equipment is used for VFE testing, and the testing steps and calculation methods are shown in the figure 4.
It is necessary to cultivate bacteriophage by microbial detection method (double layer method) using bacterial host strains, and then calculate VFE. The filtration efficiency of the material against NaCl aerosol, MS2 bacteriophage, PHI6 bacteriophage and bacteria is shown in the figure 5. It can be seen that when the PFE is 91.83%, the VFE of MS2 bacteriophage is 98.67%, and that of PHI6 bacteriophage is 99.19%.
There is a certain correlation between PFE and BFE. For medical face masks, with NaCl aerosol, flow rate 30L/min, when the PFE is higher than 90.0%, the BFE can be higher than 99.0%.
Compared with VFE, PFE, when the PFE reach 91.83%, the VFE can reach more than 98%, even higher than 99%.
Test aerosol of BFE and VFE are bacteria and bacteriophage respectively, and biological experiment is a very troublesome and complicated process. For mask manufacturers, these indicators are too complex as routine tests. Therefore, some indirect experiences are often used in practice.
Although it is impossible to calculate the actual value of BFE and VFE according to the test results of PFE. However, it is very feasible to use a large amount of test data as an auxiliary means of evaluation, which is used by some experienced manufacturers. It is simple and feasible to control BFE and VFE by controlling PFE index.
 Liu Simin, Pan Sichun, Yue Weihua, etc. Correlation analysis between non-oily particle filtration efficiency and bacterial filtration efficiency of medical surgical mask filter materials [J]. Capital Medicine, December 2013 (bottom): 8-9.
Bernadette Fuhrer. Using virus surrogate bacteriophage phi6 and human coronavirus 229E for determination of viral reduction efficacy in air and on surfaces[C].FILTREX 2022.