Why can't I clean my N95 orphan mask in the microwave?
April 12, 2020 9:23 AM   Subscribe

I have a lonely N95 mask I bought a long time ago for a project that involved plaster dust I didn't want to breathe in, but I never used it. So now I have an N95 mask. I've used it once to go outside in this pandemic (NYC).

I would like to use this mask again. For example, on a short bike ride, where I might be downwind from another bicyclist.

Everything I read says you can't reuse them and only hospitals have proper ways to disinfect these masks. But why can't I put it in the microwave and disinfect it that way? I just read something about this -- that you CAN -- and then the news outlet retracted it, WITHOUT SAYING WHY.

I know -- I'm not supposed to have one of these masks. I should have donated it but I didn't want to go outside or figure it out some other way so I didn't. So now I have a slightly used N95 mask that I would like to use again.

THE QUESTION: Is there ANY way for me to make this mask safely reusable? Is it, perhaps, weeks after using it once with an extremely low risk it got contaminated, ALREADY reusable? Am I being told it's not reusable because we're not supposed to be using these masks outside of a medical setting and they don't want to "encourage" us to use them? Is there really NO way to disinfect this (most likely not infected) mask to use again?

*HERE are some answers I would not be receptive to:

- "you're bad; you shouldn't have that mask" (MY RESPONSE: I know, I know, but this is my current reality)

- "here is a 30-minute video telling you all about different masks and when and how to use them" (MY RESPONSE: I've probably seen it already, or seen the information in it 1000 times in 1000 other videos and articles and yet my question has NEVER been answered to my satisfaction; that is why I'm asking this question!)

- "you can't wash an N95 mask in water" (MY RESPONSE: I KNOW you can't; that's why I'm asking about e.g microwaves or other methods that are NOT immersing in water or some other liquid)

- "why don't you just forget about this N95 mask and make a mask out of a tee-shirt? or this shower curtain? or this vacuum cleaner filter? or this piece of used muskrat fur? or just wrap a dress around your head? And here are 100 videos showing you how to do that? That's what WE'RE doing; Why must you be different from us? Why must you ask stupid questions? 'They' say don't reuse N95 masks, so JUST DON'T!'" ***

***(triggering all the times as a student I was told I asked too many questions and was belaboring things "everybody" already knows -- at least until I was in a Ph.D. program, where, it turned out, it was considered FINE to question things other people just take for granted)

(MY RESPONSE: I HAVE some masks that I made out of tee shirts. The N95, believe it or not, is more comfortable AND is, at least in its "pure" state, a better mask (I think -- BUT I DON'T WANT TO DISCUSS the relative merits of the N95 versus any other masks -- that's not what this question is about!!!))

So, PLEASE: only answer if either YOU have actual scientific information about this in your own personal brain, OR if you can quote somebody who actually answers this question in a scientific, factual way. THANK YOU!!!
posted by DMelanogaster to Health & Fitness (18 answers total) 7 users marked this as a favorite
There's a study out of Stanford that determined the best way to clean an n95 mask is half an hour in a 170 {F) oven.

My sole n95 makes breathing difficult; I can't imagine trying to exercise with it. I wish you better experiences than me.
posted by Dashy at 9:28 AM on April 12, 2020 [5 favorites]

Best answer: Most N95 masks have a strip of metal you can pinch to tighten the seal around the nose. Does yours? Can't put metal in a microwave without risking a fire. It heats so much the material beside it ignites if it is combustible. Once set fire to some embroidery that had a pin in it that I didn't know about by microwaving it so it would dry faster, after I dampened it to get creases out.

Some plastics cannot go in the microwave because they are too dense, like melmac. While the result is not likely to be a fire the dense part of the plastic absorbs too much of the microwaves so that the matter that is not dense plastic gets uneven and insufficient amounts of microwave to sterilize them fully.
posted by Jane the Brown at 9:29 AM on April 12, 2020 [13 favorites]

Best answer: Have you done much searching on Google? There's a zillion medically plausible sounding strategies. Dry Heat Ovens Can Effectively Disinfect N95 Masks (bake it in an oven). What are my options for extending the life of my N95 mask?. (Rotate through several masks, bake them). Doctors scramble for best practices on reusing medical masks during shortage. These mostly describe medically rigorous procedures that may be implausible to do at home. I doubt you have a HEPA filtered oven. But they also suggest partial measures are still helpful.

You should consider three different problems. Dealing with contaminants on the outside of the mask (others germs). Dealing with contaminants on the inside of the mask (your germs, but very moist). And keeping the structural integrity of the mask.

Disposable N95 masks aren't designed for re-use. But in desperate times you're probably better off using a not-perfect mask vs no mask. Best of all for Covid-19 may be a cloth mask that's not medically secure but keeps the spit from flying in and out of your mouth and nose.
posted by Nelson at 9:32 AM on April 12, 2020

Best answer: Some N95’s (not sure if all, but the ones I’ve had at home do) have staples. Some also have metal in the nosepiece. Metal sparks in the microwave. Don’t microwave them if you don’t want to test your smoke alarms and extinguishers.

The “it’s not great but it’s all we have” technique being used by local EMT’s is to take a used mask, put it alone into a paper bag (to dry out any droplets), and to leave it there for as long as possible. This is in no way a recommendation, but it is what I am doing on the weekly grocery run where my main concern, having an EMT in the house, is that I am an asymptomatic carrier and not that I am likely to catch it from someone else. Your mileage and local risk factors may vary.
posted by tchemgrrl at 9:38 AM on April 12, 2020 [1 favorite]

Best answer: The masks degrade, so you can end up with a mask that's less effective. I would use it as little as possible instead, giving the virus time to die out between uses.
posted by pinochiette at 9:45 AM on April 12, 2020 [5 favorites]

Best answer: I had a nurse tell me that you can reuse an N95 if you use it with a cover, so put a T-shirt mask (or whatever) over the N95 and wash or dispose of the cover after each use. This will allow you to extend the life of your N95. Combined with weekly or biweekly cleaning of the N95 (as per Dashy's comment) you should be safe to continue reusing your mask indefinitely.
posted by irisclara at 9:47 AM on April 12, 2020 [1 favorite]

Best answer: Putting non-microwave-safe plastic in the microwave is a bad idea, generally speaking. Not to mention the metal pieces. Also, part of what makes the n95 work is the ionic charge of the material, and I'm not sure whether that would be affected by the microwave.

Here is a reference exploring the various methods. The safest and least likely to destroy/degrade the mask is actually to just leave it for 7 days in a dry place. The oven might work but I would make sure your oven temp is consistent, many ovens have trouble holding a low temperature accurately.

The article mentions hydrogen peroxide vaporization but I wouldn't trust that spraying it with hydrogen peroxide at home would have the same effect, the vaporization particles are much smaller and would penetrate more effectively.
posted by 100kb at 9:53 AM on April 12, 2020 [2 favorites]

I'm a scientist but not a public health expert. The issues with reusing N95 that I'm aware of mainly apply to a healthcare setting with frequent exposures.

After you intubate someone with COVID there are likely LOTS of contagious droplets on the outside of the mask, which require advanced sterilization. And if reusing it degrades the fit by... Let's say it's still 80% effective per exposure, if you're dealing with 10 exposure incidents a day in a hospital ward, it's now only 10% effective per day.

But if you're having less than 1 exposure per day, 80% is still pretty good!

This is assuming the mask use here is widespread, according to the principle/saying that "my mask protects you, your mask protects me". Most of what your mask is doing in that scenario is keeping *your* germs on the inside - so by reusing them, you're mainly risking being reexposed to... Your own germs.

I'd still prefer to have a few days between uses, and leave a mask outside or in sunlight between (to get some UV exposure and speed the breakdown of the virus, which should already be fairly quick on a porous surface like the mask or filter medium), to take care of any infectious particles that happen to be on the outside. But I don't think laypeople should be terribly concerned overall about reusing masks, especially if you're considering a cloth mask as a viable option.
posted by Lady Li at 10:00 AM on April 12, 2020 [16 favorites]

This document seems to support the 70C / 170F in an oven for half an hour idea, with some caveats:
For optimal results, a heating protocol should both completely inactivate viral particles on the mask, and preserve the filtration efficacy of the mask. Heating of the mask at 70C for 30 minutes has been suggested as a method of decontamination which adequately preserves the filter integrity for re-use.[5] However, it is unclear whether there is a specific humidity required during this process to completely inactivate SARS-COV-2 viral particles. Due to the dearth of specific data on a protocol to achieve both aims, the N95 Decon group does not recommend this method.[6]
Here is the study they cite.
posted by jordemort at 11:51 AM on April 12, 2020

Response by poster: Thank you for all the WONDERFUL answers! Duh I totally forgot about the METAL in the nose piece! Anyway I have some great ideas here as well as warnings, etc., that are all so useful! (sorry about generally hysterical tone of my question -- well, y'know)
posted by DMelanogaster at 12:23 PM on April 12, 2020 [4 favorites]

I think hospitals are disinfecting them to be used by multiple workers, i.e. in between each worker.

I have one too, that I wear when I go to the grocery store. I see no reason to disinfect it.

For civilians, it’s mostly there to keep our mouth and nose “spray” or “mist” in, so it doesn’t drift and land on others. Worn by someone who has never had an official fitting and worn more than once, it probably won’t be tight enough to reliably keep others’ “mist” 100% out. It’s to protect others when you wear it; and their masks protect you.
posted by amaire at 12:56 PM on April 12, 2020

Here's a set of "decon fact sheets" that I found for decontaminating N95 masks, which may help with reuse:


There's all kinds of quackery on the Internet, but this seems legit enough, as far as I can tell. I've read of hospitals using UVC light to sanitize rooms. Hydrogen peroxide seems to be useful, as well, at least at sufficient strength:


It might help you get a bit more mileage out of what you have until more become available.
posted by They sucked his brains out! at 1:31 PM on April 12, 2020

A couple of doctors I know use a towel warmer with a germicidal UV lamp inside to sterilize their masks.

If you want a 170F oven with precise temperature control, no reason you can't sous-vide a mask, or all your masks inside an airtight bag. I wouldn't bother with the vacuum sealer here; since you won't be eating the mask a freezer-bag should be fine for this even at the relatively high temp. It'll heat them up to equilibrium temperature, and at the same time you can poach eggs in the shell (13 minutes at 167F, so slightly shorten that 170F, or not) or cook some potatoes at the same time. A water-oven will give heat more directly to the mask than a dry oven, bake it evenly, and make for weird photos to share on social media. You can't overcook a mask, so while it'll be done in 30 minutes, you can leave it in for hours if you need to do something else, or be somewhere else.
posted by Sunburnt at 2:32 PM on April 12, 2020 [1 favorite]

To address your title: From my understanding, microwaves heat water, so a dry mask won't heat well and is probably made out of non-microwavable materials. Low oven or UV are likely your best bets...
posted by token-ring at 4:52 PM on April 12, 2020

Just a thought, pre-heat the oven for a substantial time and check with an oven thermometer if you have one, that it's actually at 170. Ovens are very inexact machines.
posted by sammyo at 5:07 PM on April 12, 2020

I have been steaming mine between (infrequent) uses. I also stuff the damn thing into my UV cell phone sanitizer. I know this means it might degrade more quickly but I figure if it extends its life, it's still better than a t-shirt mask.
posted by egeanin at 5:24 PM on April 12, 2020

Is it, perhaps, weeks after using it once with an extremely low risk it got contaminated, ALREADY reusable?

posted by aielen at 11:43 PM on April 12, 2020

> you can't wash an N95 mask in water" (MY RESPONSE: I KNOW you can't

Alcohol will destroy the electrostatic charges that lets the masks trap particles, but water might be OK.

This page took down their FAQ, referring people to n95decon.org instead, but here's the wayback machine's version and it addresses sterilization with boiling water. The filter medium is melt-blown non-woven polypropylene, a "hydrophobic plastic material with zero moisture content, and a respirator can dry out in less than two to three days in a dry air environment."

It also says:

"Our investigation showed that sterilization using 125°C steam for three minutes has an unnoticeable effect on the charge loss on the electret. Note that it is important to be sure that the inner or the outer veil of the mask is not made of paper-like tissues – paper pulp or nonwovens bond by water soluble binder – which will either dissolve in water resulting in loose fibers in the veil or the loss of its strength after exposing to water."

The n95decon.org talks about high heat & humidity & doesn't really disagree with this. I'm steaming (in a stove top vegetable steamer) my Camp Fire-leftover mask. I'm shopping infrequently enough that it has time to dry.
posted by ASCII Costanza head at 9:30 PM on April 13, 2020

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