HOWTO: Sterilizing a Mask For Private, Non-Medical Use

akiba 2020-03-27

Disclaimer: This article falls under the Good Samaritan Law. Although we’re trying to take every precaution we can and use only trusted sources for our research, we can’t take responsibility if anything goes wrong.

Since I’ve been working on the UV sterilization project, I’ve noticed that there’s a lot of focus on masks and protection equipment for medical professionals, as there should be. But what about for normal, private citizens that are wondering if it’s safe to re-use their masks?

At the moment, there’s no definitive answer, as all the previous literature assumed an infinite supply of masks and adamantly said that masks shouldn’t be reused. Unfortunately that model no longer holds up as can be seen with all the mask hoarding. So rather than asking the question “is it safe to re-use a mask?”, let’s rephrase that in a different way. “If I have to re-use my mask, what is the safest way I can do it?”

In reusing a mask, there are three things that  health professionals worry about:

  • Will the sterilization method increase the pore size in the filter media?
  • Will the method degrade the structural integrity of the mask rendering it unsafe?
  • For N95 masks, will the technique affect the charged filter fabric inside the mask?

First, a bit of background on N95 masks. Along with having multiple layers of filtration media, the N95 also has a charged electret that will attract particles to it. If you think of the old CRT TV screens and how they’d always attract dust, the same principle applies. An N95 with uncharged filter media has a filter efficiency of around 25%, but by including the charged filter layers, the filtration goes up to 95%, hence the name.

The increase of the FE is ten folds compared to that not charged, e.g., the efficiency will go up to 95% for an uncharged media having FE of 25% suggesting that ten plies of the uncharged have the same FE as that of one ply of charged of the same media. The increase is 20 folds to 99.8% by the above novel charging technology. – Peter Tsai, Inventor of the N95 mask

That’s why health professionals are worried about sterilization techniques decaying the charge on the electret inside the filter media.

Fortunately, there’s been some literature that has come out that provides a bit of guidance. Stanford Medicine has conducted tests in partnership with 4C Air and released this guidance on mask reuse:

So this points us to three techniques that can be used to sterilize a mask if you will be re-using it anyways: dry heat, UV, and steam. Additionally, Peter Tsai, the previously mentioned inventor of the N95 mask, has authored a paper on sterilization of the masks. It seems to be a few days old as of this writing, so I haven’t been able to find links on the internet yet. I received it from health sterilization professionals. I’ll post it on Google Drive and just hope that I don’t get sued for copyright infringement.

Here’s the tl;dr

Both Stanford and Dr. Tsai emphasize that masks that use the dry heat method should not be done in home ovens. They should be industrial ovens specifically for sterilization. Anyways the dry heat is too fiddly since you have to keep the temperature constant for long periods of time.

UV sterilization for mask reuse is still under investigation. It’s also hard to get UV germicidal bulbs at the moment in most places except Japan and China where they are manufactured domestically.

This brings us to steam. The simplest, cheapest way to sterilize a mask, even an N95 mask, IF (and that’s a big IF)  you were going to re-use it anyways, is to steam it. Tests have already been done and steam won’t degrade the charged filtration element. It also maintains its filter integrity and does not degrade the mechanical structure. So grab those double boilers, bamboo bun steamers, or cool folding vegetable steamers you have sitting in your cupboard because they can be of use to sterilize your own masks (for private, non-medical use). Also, Dr. Tsai tested steaming for three minutes while Stanford tested it for ten minutes with no mask degradation. You may want to consider using the ten minute interval if you’re conservative about sterilization.

Hope you enjoy and stay safe, from all of us at hackerfarm.

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