At this point in time, I’ve published around 50 mask reviews on Breathesafeair. While each of these reviews is unique and covers a different product, a few things appear in almost every article. Today I want to discuss one of these recurring topics.
That topic is PFE, BFE and VFE tests. When reviewing masks, I almost always provide test results from each of, or a combination of, these three tests. They demonstrate the particle, bacterial and viral filtration efficiency of a given mask or filter media. However, despite referring to these tests in so many articles, I have never taken a deeper look at what these tests are.
The idea to write this article crossed my mind recently because I had some questions regarding the different test guidelines and methodologies used for PFE, BFE and VFE tests. A lot of people might not know that these three test types do not follow universal standards. Instead, one form of PFE test can vary significantly from another test.
I wanted to address a few questions that I commonly get asked regarding each of these test types. Specifically, what are each of these tests, and how do they differ? Further, I want to discuss why PFE and BFE tests are not directly comparable between devices.
With that being said, if you read through this article and still have questions, please feel free to reach out to me by commenting on this post or by contacting me via the contact form on this website. I’ll do my best to reply as soon as I have time!
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What Are PFE, BFE and VFE Tests?
PFE, BFE and VFE tests are designed to test a filter media and/or mask’s filtration efficacy against three common particle types – particle (NaCl or latex beads), bacterial and viral.
PFE – Particle filtration efficiency
BFE – Bacterial filtration efficiency
VFE – Viral filtration efficiency
However, these three terms have caused a lot of confusion. There is a multiplicity of standards when it comes to masks and filters. and PFE, BFE and VFE are NOT standards. Instead, they are types of tests that must be carried out following a standard. The standard that a PFE test follows defines the testing conditions and methodology. For example:
For example, in the case of PFE testing, there are many variables in play. While most labs will follow the ASTM F2299 standard, which defines the testing conditions, there is no set ‘PFE’ standard. Rather, the term PFE simply refers to a type of test that can vary in testing methodology and specifications. Below is an image of a Latex Particle Challenge Final Report from Nelson Labs. On the report, you can see the test method employed highlighted in red.
What makes comparing these standards so difficult, especially in the case of PFE, is that not all labs will test PFE using ASTM F2299. Other labs may instead test for PFE following the European EN 149 standard. This creates issues because there is a range of variables that make these two tests different. Flow rates, particle sizes, laboratory conditions and more can vary.
This leads to issues because it means that in many cases, PFE test results are not directly comparable. Devices can achieve different results from different standards due to the variables involved. While far from an exhaustive list, below is an example of the different PFE standards:
- ASTM F2299 (part of ASTM F2100)
- TEB-APR-STP 0059 (used to test PFE on N95 devices. Part of 42 CFR 84)
- EN 13274-7 (used to test PFE on FFP1, FFP2 and FFP3 device. Part of EN 149)
Since each of these tests varies, it’s always important to check which standard PFE, BFE and VFE tests followed. Variables such as flow rate, particle diameter, and temperature can change between tests and not all PFE, BFE and VFE tests are equal.
TEB-APR-STP-0059 (N95), EN 13274-7 (FFP-series) and GB 2626-2006 (KN-series). Source
When it comes to particulate filtering efficiency tests, there are two standards that are usually followed. The first of these is NIOSH’s TEB-APR-STP 005x (where the x represents the class of respirator) in accordance with 42 CFR 84. 42 CFR 84 is the procedure used to certify N, R and P series respirators such as N95, R95, and P100 devices. The full testing procedure for TEB-APR-STP-0059 can be found here.
The Standardization Administration of China’s GB2626 standard for KN-class devices also closely follows the NIOSH standard. As the table above shows, testing conditions are identical to NIOSH’s TEB-APR-STP 0059. Therefore, GB 2626 and NIOSH devices are considered the same in this section.
The second commonly used PFE testing standard is ASTM F2100 substandard ASTM F2299. ASTM F2100 is a standard used to certify medical masks that are recognised by the FDA. ASTM F2299 is called upon by F2100 to test the particle filtration of the device. While ASTM F2299 also tests PFE, test conditions are different from the NIOSH standard. Generally, the NIOSH test methodology returns more conservative results compared to the ASTM F2299 PFE test. This is due to the smaller particle sizes, particle charge status, and test velocity which is stricter in the NIOSH test (source).
Another, more recent PFE testing standard is ASTM F3502. This standard was introduced to address the lack of standards between ASTM F2100 medical masks and NIOSH-certified respirators. ASTM F3502 follows the NIOSH testing procedure for N95 devices (TEB APR-STP 0059). Therefore, although ASTM F3502 only requires devices to have ≥ 20% filtration (Level 1) and ≥ 50% filtration (Level 2), the PFE testing methodology is the same (with minor modifications) to that N95 devices undergo (source).
For anyone living in Europe or purchasing devices from Europe, you will likely encounter the EN 149:2001 standard. EN 149:2001 is a European Standard set by the European Committee for Standardization. This is a standard for respiratory protective devices designed to protect against particles. This is a thorough standard that is used to certify devices FFP1, FFP2, FFP3 devices. As part of EN149, devices must undergo a PFE test in accordance with EN 13274-7. While the testing conditions and methodology are not the same as 42 CFR Part 84, this standard certifies the European Union’s ‘equivalent’ respirators to NIOSH-certified devices.
While this list isn’t conclusive, as there are other PFE testing standards around the world, these standards are the most commonly used. If you are familiar with TEB-APR-STP-0059, ASTM F2299, ASTM F3502, and EN 13274-7, you will be aware of the key differences between the PFE tests that the majority of masks and respirators undergo.
Now that we have identified the key PFE standards, let’s take a look into the key differences. ASTM F3502 closely follows the guidelines set by NIOSH in 42 CFR 84. As such, the main comparison in this section is between ASTM 2100 (ASTM F2299), 42 CFR 84 (TEB-APR-STP-0059), and EN 149:2001 (EN 13274-7).
NIOSH 42 CFR 84 compared to ASTM F2299. Source. CMD = count median diameter. MMAD = mass median diameter. PSL = polystyrene latex spheres. GSD = geometric standard deviation.
Between ASTM F2299 and 42 CFR 84, the NIOSH standard is the more conservative. Due to the smaller particle size, particle charge neutralisation, and higher flow rate, 42 CFR 84 is considered a ‘worst case scenario’. Devices tested at both standards typically perform worse under NIOSH testing conditions (source) and therefore 42 CFR 84 can be considered the more stringent PFE standard.
All devices tested in the study performed at their worst in NIOSH testing conditions as opposed to under PFE conditions set by ASTM F2299. While the ASTM F2299 is a relatively stringent standard in itself, NIOSH’s guidelines are designed to test respirators and PFE at their most extreme circumstances.
In the most extreme example, a NIOSH-certified device filtered just over 98% of NaCl test particles. The same device showed a filtration efficiency of well over 99% when tested under ASTM F2299 conditions. This was a worst-case scenario, and most NIOSH-tested devices performed more closely to their ASTM F2299 counterparts. However, in all cases, NIOSH-certified device filtration results were lower than ASTM F2299 results.
When compared tested under NIOSH and EN 13274-7 conditions, the NIOSH test will often show lower filtration efficiency. This is due to the fact that NIOSH requires particles to be neutralised, something that neither EN 13274-7 nor ASTM F2299 requires. Since many respirators and filters rely on electrostatic attraction, this makes the NIOSH test more challenging (source).
While there are other variables to be considered, studies have shown that NIOSH’s PFE testing is the most stringent and returns lower particle filtration results on tested devices. While devices that perform well under ASTM F2299 and EN 13274-7 can also perform well under the NIOSH conditions, they tend to lose some filtration efficacy
When it comes to BFE standards, there are thankfully fewer tests that we need to consider. While there are other BFE tests around the world, the vast majority of medical masks are tested under one of two standards. The first of these standards is ASTM F2101 (as part of ASTM F2100), and the second is EN1683 Annex B.
ASTM F2101 is recognised by the FDA for bacterial filtration efficiency testing. On the other hand, EN1683 is the BFE testing standard usually used within the European Union and the U.K.
Mask Testing Overview from MakerMask.org. The middle column, medical masks, compares ASTM F2100 (BFE as defined by ASTM F2101) and EN 14683.
While ASTM F2100 and EN1683 differ in other specifications, the BFE testing conditions outlined in ASTM F2101 and EN1683 Annex B are identical. This means that BFE results obtained from tests carried out under either of these standards are directly comparable. However, it’s important to reemphasise that the masks certified under the standards are not directly comparable – on the BFE testing conditions are.
On top of this, other medical mask standards such as AS 4381:2015 used in Australia, also accept BFE testing carried out under the ASTM and EN standards. Where there is a lot of variation in PFE testing methodologies and conditions, BFE tests are far more streamlined and the two most common BFE test standards are equivalent.
VFE is the most simple filtration efficiency test on this list to explain. This is due to the fact that VFE tests are a test method adapted by Nelson Laboratories from ASTM F2101 (the BFE standard discussed above) (source). While Nelson Labs modified the standard from ASTM F2101, many laboratories now offer VFE testing. These labs include Nelson Labs, SGS and Eurolab.
Although currently, VFE is not a recognised standard test method, it’s a test that you will commonly encounter when viewing mask websites. Since the common VFE test is a modified version of the BFE standard ASTM F2101, the testing conditions are the same but bacteriophage phiX174 is used in the place of bacterial particles.
Since VFE is not currently a recognised standard test method but follows the same testing methodology across most big labs, these test results are equivalent. VFE tests even across labs should be directly comparable.
It’s worth noting that viral particles are usually far smaller than bacterial particles. However, in VFE tests 2.8µm particles are usually used – the same as in the BFE standard, ASTM F2101. These particles are not representative of naked viral particles, but of virus-containing water droplets (source).
Differences Between PFE, BFE and VFE
Now that we’ve covered the differences within PFE, BFE and VFE tests, we will take a look at how these tests differ from one another. These tests differ greatly, and they aren’t easily comparable. However, since I often get asked about the differences between the three tests, I will do my best to compare and contrast them.
The most obvious difference is the particle type – PFE tests usually rely on latex spheres, whereas BFE and VFE tests rely on bacterial and bacteriophage viral particles respectively. However, past these key differences, there are other minor and major differences between the test methodologies. The table below does a great job of illustrating many of them.
Since I’ve spent so long discussing how not all standards are the same, especially in regards to PFE, I need to define the standards I will be comparing in this section. As such, I’ve chosen the most common PFE test – ASTM F2299 to compare alongside ASTM F2101 and the modified ASTM F2101 that is used for VFE testing. Further, I also want to include NIOSH’s 42 CFR Part 84. This is the most commonly referenced respirator standard and it provides a good point of comparison.
Comparison of filtration test methods. Source.
The biggest difference between these four test types is the particle size and type. NIOSH uses a NaCl particle at a count median diameter of 0.075μm. The most common PFE test, ASTM F2299 uses latex spheres at 0.1μm to 5.0μm depending on the test taker’s request. However, even at ASTM’s smallest particle testing size, 0.1μm, the NIOSH test uses smaller particles.
While this difference may seem relatively minor, it can have a significant impact as 0.075μm is closer to the most penetrating particle size (MPPS) of 30-40nm (0.03 – 0.04μm) of N95 respirators that rely on electrically charged fibres (source). Further, the NIOSH test uses neutralised charges which make it a more challenging test for charged fibres. While ASTM F2299 recommends using neutralised particles, it’s not essential.
ASTM F2101 uses far larger particle sizes for BFE. Since the Nelson Lab’s created VFE test is based on ASTM F2101, both of these tests are identical in particle size. Both tests use particles with a diameter of 3.0μm, and the big difference is the particle type. In the case of the BFE test, staphylococcus aureus is used, and in the case of VFE, PhiX174 is used.
Another key difference between the three standards is the flow rate (or face velocity) used. While it’s very hard to compare face velocity to flow rate due to the fact that it varies from mask to mask, the ASTM F2299 standard can either prove easier or more difficult for masks than the NIOSH standard. This is made more difficult by the fact that ASTM F2299 does not define a set face velocity, but rather a range.
According to the study quoted for the graph below, most labs test ASTM F2299 at 5 cm/second. If this is the case, most ASTM F2299 PFE tests are carried out are more lenient than the NIOSH test which requires a flow rate of 85L/minute. Both BFE and VFE tests require significantly lower flow rates, with rates of only 28L/minute being used.
Face velocity vs filtration efficiency (source).
Overall, the NIOSH method is the most conservative (source) and a device that has undergone the NIOSH PFE test according to TEB-APR-STP 005x should perform better in PFE (ASTM F2299), BFE and VFE (ASTM F2101 & modified ASTM F2101) tests. Current studies have shown NIOSH’s tests are the most stringent.
In turn, ASTM F2299 tends to be more stringent (in regards to filtration) than ASTM F2101. Assuming the particle size for ASTM F2299 is 0.1μm, face velocity 5cm/second and particles are neutralised, ASTM F2299 is a more challenging filtration test than its BFE and (modified) VFE counterpart.
In fact, the CDC released a study stating that the difference between living and infectious particles plays no role in how well they will be collected by a filter. The study compared PFE and BFE, and it suggested that BFE tests are not essential on devices that have displayed high PFE filtration. Since PFE filtration is carried out at smaller particle sizes (ASTM F2299, TEB-APR-STP 005x), at higher flow rates, and against more challenge particles (charge neutralised), a device that performs well under these circumstances should also display high BFE.
Bacterial organisms and viruses are unable to move without assistance, and filters adhere particles preventing them from moving. Therefore, a device that performs well in PFE should perform similarly or better in BFE. This, in turn, means that devices that perform well in BFE tend to also perform well in VFE due to the similarity of the tests.
PFE, BFE and VFE are relatively simple concepts, but the different testing standards and specifications make them far more difficult to understand and contrast. Thankfully, the standards used to test BFE and VFE are similar and very often comparable.
PFE tests, on the other hand, vary far more. Luckily, many standards have similarities or are entirely based on one another. The NIOSH PFE standard is the most stringent, and devices tested under this standard typically display higher filtration efficiencies when tested using other PFE standards.
If you have any questions or comments regarding this post please feel free to comment down below. I’ll do my best to reply to all comments as I best can!