Decoding the N95 Mask Filter for DIY Mask Material


SUMMARY POINTS

  • Masks commonly used by healthcare providers N95, FFP1, P2, or surgical mask
  • What makes filter masks so effective is that there is no direct path through the fibers
  • Fibers in a mask are essentially a maze for small particles with an exception
  • It is not the largest or the smallest particles that are the hardest to trap but the particles that are greater than 0.1 micron and less than 0.6 micron
  • Mask seal to the face is the most critical element in protecting the lungs aside from filters.
  • DIY mask makers should consider producing various sizes for maximum seal

Medical Mask Filtration Background

What makes filter masks so effective is that there is no direct path through the fibers that make up the filtration material. Therefore, the airflow and the particles it carries must turn and weave their way through the tortuous maze of the filter media. In general, there are 3-4 types of masks commonly used by healthcare providers, such as the N95, FFP1, P2, or surgical mask.  They are used for various purposes, such single-use face masks, surgical masks, and respirators.

The Science Behind How Filters Work

When it comes to viruses, large particle greater than around 0.6 um in diameter are usually captured when the particle can’t make the turn around a fiber due to its inertia and it impacts on a fiber. The random movement of very small particles (around 0.1 microns in diameter), because they are too small to be carried away in the airflow, cause them to accidentally come into contact with fibers and get trapped. Therefore, it is not the largest or the smallest particles that are the hardest to trap but the particles that are greater than 0.1 micron and less than 0.6 micron. These particles are large enough to be picked up by the airflow yet small enough to travel with the airflow around most fibers. Particles of 0.3 microns are therefore considered to be the most difficult particles to trap and the object test size for the most stringent requirements for NIOSH certification of an N95 Respirator. 

Medical Grade Testing Protocols

In researching materials to use for DIY masks, considering there are therefore three (3) testing protocols that evaluate medical grade filtering masks for filtering efficiency.

  1. The Bacterial Filtration Efficiency (BFE) test for filtering masks challenges them with bacteria that are 3 microns in size (and around the size for most bacteria) to see whatpercentage of large particles they trap. All of our masks trap at least 99% of large particles.
  2. The Particle Filtration Efficiency (PFE) test for filtering masks challenges them with small latex particles that are 0.1 micron in diameter. Here as well, all of our masks trap at least 99% of these small particles. These test results were performed by Nelson Laboratories, an independent test laboratory.
  3. The NIOSH (N95) test challenges the masks with NaCl that are 0.3 micron, the most difficult to trap. To receive N95 Certification, a mask must filter 95% of these particles and even N95 masks can only assure that less than 5% of these particles can pass through the filter media.

Conclusion

The standard mask with the lowest requirements on filtration effectiveness is the single use face masks (not to be confused with surgical masks). Surgical masks have higher requirements, and respirators have the highest requirements. Respirators also usually fit tighter around they face (data shows they score higher on fit effectiveness) than surgical masks and single-use face masks.

DIY Masks should focus on being sealed around the face, as filtration specifications are completely dependent on there being no leak around the mask.  The seal to the face is the most critical element in protecting the lungs aside from filters. Additionally, a single size of mask cannot not effectively seal to the faces of all users, masks should come in multiple sizes to fit children to large adults.

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