The standards for Surgical Masks vs N95s
Is it true that surgical mask material can never be as good as N95 material?
Last Edited: 10/27/2020
What’s the difference between SM and N95s?
As you can see below, while N95s have an extra layer, filtration is not the extra layer’s main function.
The melt blown fabric layer is the part that does the bulk of the filtration, as you can see from the relative scales of the spun-bond vs melt blown fabrics.
The data on surgical masks being tested to the NIOSH standard varies widely. Some studies have proven that some surgical masks can pass the NIOSH 42 CFR 84 test method:
“one electrostatic surgical model in this study and one dust mask model tested in the previous study (Rengasamy et al. 2008) obtained from different manufacturers, showed <5% penetration level when tested similar to NIOSH respirator certification test conditions at 85 liters/minute flow rate.” (source)
But, others show that some surgical masks are incredibly inferior.
How can both standards guarantee the same filtration, but clearly surgical masks perform so much more variably when tested to the NIOSH standard?
- Hypothesis 1: The NIOSH fixture is not designed for testing flat filtration materials.
- Hypothesis 2: The ASTM standard allows for a lot of variability in the test set-up
Let’s take a deeper look.
Hypothesis 1: TSI 8310 Test Set-Up Variability
At the time of a lot of these publications, the NIOSH test method was to put a mask on a flat plate with a hole in the center and seal the edges with beeswax, as shown. The test set-up is favorable to testing masks that can hold their own shape, like molded N95s. Because this is only a positive pressure test, testing a surgical mask in this test-setup would cause the surgical mask to be pulled down into the hole, and result in a much lower area to be tested than what is practically used. A surgical mask filter would be more easily overloaded in this test set-up than a molded N95 mask. This issue with the test fixture was not updated until the TSI 8310a fixture was introduced in 2017, which is after a lot of these papers were published.
Conclusion: This is a huge problem and is likely a reason why surgical masks have historically not been able to pass 42 CFR 84. It’s critical that equivalent areas of materials be tested when comparing them side by side.
Update 10/27/2020: We’ve tested masks with an appropriate fixture, data can be found here. Spoiler alert, certain surgical masks are as good as N95 masks.
Hypothesis 2: Test Method Variability
*ASTM F2299 is a subset of F2100 that specifically tests PFE.
The Test Methods
The test set-ups for both the ASTM and NIOSH/CDC method look quite similar, see image below:
But let’s look into the actual test method differences. The CDC lists a resource comparing the different standards. Let’s break this table down by column.
Particle Filtration Efficiency (PFE)
The test methods that matter for COVID-19 relate to PFE, Particle Filtration Efficiency, not BFE or VFE. Why? COVID-19 is a virus, not a bacteria. Bacteria are much larger than viruses. Bacteria ~ 3μm while viruses ~0.025μm in diameter. So BFE, Bacterial Filtration Efficiency, does not apply. VFE, Virus Filtration Efficiency sounds misleadingly correct, but this is not a standard test method. Why not? Likely because we don’t want to be subjecting our test engineers to aerosolized viruses on a daily basis.
Doesn’t it matter if we’re testing a biological particle versus a non-biological particle? No. The CDC states, “Whether the particle is “living” or “infectious” plays no role in how well it will be collected by a filter. Once a particle is collected it will remain attached by electrostatic and van der Waals’ forces.” They also created this video to emphasize their point. This means the particle material doesn’t matter, and PFE is a valid test.
The ASTM and FDA use the same material — latex spheres, while NIOSH/CDC uses NaCl. While material should not matter, the particle’s densities are quite different. This means that at the same speed of airflow, the latex particles will have less momentum when hitting the filter.
Conclusion: Latex particles are less aggressive of a filter material than NaCl.
You’ll notice that the ASTM/FDA once again are roughly the same, while NIOSH tests to a slightly smaller particle size. But what particle size actually matters for COVID-19? The research here is inconclusive.
As stated, COVID-19 is a virus. A virus can be 0.025μm in diameter, way smaller than what these tests are testing for. Isn’t that a problem? No. That’s because viruses cannot live on their own. In order to be transferred between people, they need to be suspended in an aerosolized liquid droplet.
What is the smallest diameter of that aerosolized liquid droplet? Research suggests that 42% of a cough is spewed out in particle sizes of <1μm.
Conclusion: The NIOSH method tests with 0.075um particles. The ASTM standard tests to 0.1um particles. There is not enough research to definitively say which particle size is the ‘correct one’ for COVID-19, but both methods seem to be using particle sizes that seem relevant.
This one is important. The melt blown fabric layer of masks are often charged to improve the filtration capability without reducing the breathability. That said, the charge wears out over time. As a worst case, the masks need to be tested as if they were neutralized. To mitigate this, a good test method will neutralize particles to eliminate the effect of the electret.
Conclusion: ASTM and NIOSH/CDC are better test methods than FDA.
Because the ASTM and FDA standards are the same from here on out, we have excluded the FDA test for easier reading.
You’ll see here that the units of concentration are different between NIOSH and ASTM. We looked up the density of latex and converted the the results into the same units. We got that NIOSH = <200 mg/m³, while ASTM = 5mg/m³.
Conclusion: The NIOSH test method tests a much denser cloud of particles than the ASTM standard.
They use different methods for counting the amount of particles. We presume the difference between these detectors is related to the choice of aerosol type. Regardless, we think it’s valid to assume that both of these detection methods go through standard GRR and are trustworthy.
Conclusion: No important difference between these test methods.
NIOSH uses a volumetric velocity while ASTM uses a wind speed.
NIOSH shoves the same volume of air through each filter that they test, which allows them to assess filtration independent of breathability.
ASTM tests face velocity, meaning that less-breathable filters will perform better. If you look into the literature, surgical masks are more breathable than N95 masks. This means that testing them to the ASTM standard for face velocity would be a worst case.
That said, the ASTM method varies substantially. In order to convert from face velocity to volumetric flow rate, you need to know the area of the subject. The ASTM test method tests a 50mm — 150mm diameter circle of material. This means that the flow rate tested can vary from 0.588 L/min to 265.05 L/min.
Conclusion: The ASTM method has substantially more variability than the NIIOSH test method.
The CDC found that at 85 L/min, maximum penetration of some masks is reached at only 1 minute of testing (source). “A previous study in our laboratory found that efficiencies obtained by measuring initial penetration (average of the first minute) of N95 FFRs were comparable to those done at maximum penetration at full loading conditions.”
That sounds concerning when you think about health care workers wearing masks all day, until you realize that 85L/min is a worst case flow rate. I measured my own breath rates, and they were as follows.
So the CDC test runs for about 1 minute, while the ASTM test runs from 1–5 minutes. Depending on the flow rate — that can make a big difference.
Conclusion: The ASTM test has a much more variable set of criteria than the NIOSH test.
Conclusion: These are the same.
Sample Type (size)
NIOSH tests the full mask assembly.
The ASTM method tests the bulk material of the surgical mask, not the full mask assembly. The area requested, though is really variable, and likely contributes to the variability of SM performance.
Conclusion: the ASTM method allows for a lot of variability while the NIOSH method is really strict. Some versions of the ASTM method can be more aggressive than the NIOSH method, but it seems that a lot of surgical mask vendors opt to test the easier versions of the methods.
We need to test surgical mask material and N95 mask material of the same area in the same test set-up to truly understand whether or not a surgical mask is good enough. If we could find the surgical masks that pass the more aggressive versions of the ASTM test method, we could unlock a mask that could outperform N95s at a much lower cost.