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?

Source: https://www.youtube.com/watch?v=jRdTsH6l6zs 10:03, https://www.nap.edu/read/11637/chapter/4#23

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.

Photo by: Cameron Blackburn at MIT Media Lab. https://camblackburn.pages.cba.mit.edu/filter_media/

The Research

“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.

The Research

  • 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

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 F2100 (link)

ASTM F2299 Filtration Efficiency

CDC/NIOSH (link)

42 CFR 84 Filtration Efficiency

The Test Methods

ASTM F2299 vs 42 CFR 84 Test Set-Up

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.

Break down between the different test methods

Particle Filtration Efficiency (PFE)

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.

Aerosol Type

Aerosol Type

Conclusion: Latex particles are less aggressive of a filter material than NaCl.

Particle Size

Particle Size

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.

Particle Charge

Particle Charge

Conclusion: ASTM and NIOSH/CDC are better test methods than FDA.

Particle Concentration

Particle Concentration

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.

Aerosol Detector

Aerosol Detector

Conclusion: No important difference between these test methods.

Flow Rate

Flow Rate

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.

Test Time

Test Time

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.

Max Efficiency

Max Efficiency

Sample Type

Sample Type (size)

Source: https://www.youtube.com/watch?v=jRdTsH6l6zs

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.

Summary

The latest data and information on the Essential Mask Brace and masks in general.