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With N95 and N100/P100 Particulate Respirator Masks in Short Supply, What Might Work to Disinfect / re-Use?

See also: Disinfecting Masks with Chlorine Dioxide (ClO2).

Stanford MedicineAddressing COVID-19 Face Mask Shortages at COVID-19 Service.

See the table “Can Facial Masks be Disinfected for Re-use?”. It appears that UV light as well as baking in an oven at 70°C for 30 minutes are both effective on E Coli, and thus presumably COVID-19. It’s unclear what intensity UV light or wavelength are needed, and thus it is unclear how effective sunlight would be.

  • Frontline health care workers across the United States report shortages of PPE ranging from gloves, protective gowns, eye wear and face masks.
  • It is unknown how wearing the same mask multiple times effects the fit of N95 masks [NIOSH]
  • NIOSH states“there is no way of determining the maximum possible number of safe reuses for an N95 respirator as a generic number to be applied in all cases” and advise to“discard N95 respirators following use during aerosol generating procedures.”
  • Some methods of N95 mask disinfection can maintain filtration efficiency. Their effect on mask fit is unknown, and these methods are not approved by NIOSH.


First of all, keep your immune system strong just in case you are exposed/infected. And hang in there—we are likely to see more infections and more death BUT with a lot of good news over the next week or two (e.g. drug treatments, rapid ramp-up of masks, ventilators, etc).

With N95 and N100/P100 particulate respirator masks in short supply, re-use is the only practical approach for those of us with a very limited supply (for example, I have only one per family member). I was lucky enough to have one for each of my parents also.

First of all, the idea that going bare-faced and unprotected is better than nothing at all makes no sense to me. Obviously it is better to have a stash of masks and use a fresh one each time, but that’s a luxury that only health care workers can take, and the masks are better used there.

The idea that the general public does not need a N95/N100/P100 mask is a bald-faced lie which translates to “in shortage and we don’t want people using them because medical professionals need them more”—fair enough on the latter point, but that people who already have masks are misled into not using them is disgusting and can only lead to more infections—counter productive.


  • Basing my assumptions on the white fabric 3M N95 and N100/P100 masks. Other designs and materials... dunno.
  • UV (ultraviolet) light kills many pathogens. How long it takes is unclear, and how far sunlight penetrates into the fabric of the mask are unknowns. However, water left in a clear plastic bottle will disinfect from sunlight for many pathogens.
  • Glass partially blocks UV light but plastic should pass most UV light.
  • Heat degrades most pathogens. Heat plus UV may increase the degradation.
  • Most trapped virus particles will be nearer the outside surface of the mask than the inside surface, hence the chance of UV penetration is at least promising.

Suggestion for disinfecting

If there is an expert out there who can find a flaw in my reasoning/assumptions, please advise specifically as to which item and why, with supporting factual evidence saying why it won’t work. Along with alternatives. At present, apparently no one in the world knows exactly such basics as how long COVID-19 can live on various types of surfaces and what environmental factors apply (heat, humidity, etc). So it’s guesswork, but I’ll take UV light and heat as a hell of a lot more promising than no mask, or sticking it in a cool dark place inside a paper bag.

These are my thoughts on what *I* would do for myself. Use at your own risk. Note that in no way I am suggesting that this process will result in a sterile mask since the masks I’ve recommended are not sterile even to begin with. I am merely suggesting that killing or weakening any COVID-19 might occur with heat and UV—you can either throw away perhaps the only mask you have or make some effort at reducing the risk in a way consistent with known facts.

As described above, my thinking is that the combination of sunlight/UV surely and the heat will surely degrade the virus to some extent.

  1. Wash hands thoroughly before removing the mask. This step is critical in case either mask or hands are contaminated. Ideally, wash hands then don disposable gloves before removing the mask. [if gloves were worn, remove them and put on fresh ones before removing mask].
  2. Place mask in fresh 1-gallon Ziploc plastic bag. Close the bag mostly but not entirely, so that water vapor can escape yet heat will be trapped. [MAYBE: seal bag and place silica gel in bag, to suck all moisture out of bag; this might be ideal if in fact near-zero humidity kills the virus].
  3. Wash hands again.
  4. Place bag with mask in outdoor location having direct sunlight all day, preferably in a wind-free and dark nook so that it gets hot. The dash of a car would be perfect, but the glass likely blocks too much UV to be ideal. If you have a small thermometer like this @AMAZON, then you can also monitor the temperature.
  5. When reusing the mask, wash hands thoroughly, then don disposable gloves before putting mask on again. Take care to touch the mask as little as possible, just in case the process was not fully successful.


  • Would placing the mask in an oven at 140°F or so kill the virus and leave the efficacy of the mask alone (and not damage the straps, etc)?
  • Could a UV-A lamp be used to disinfect the mask?

A home gamma-ray “microwave” would be handy for such situations!

David G writes:

I have washed the n95 mask with soap and water focussing most efforts on the outside of the mask. Then heat dry on my radiator overnight. Also thought of wiping both sides of the mask with 90% isopropyl alcohol. I would wear gloves for all of these processes. What do you think?

DIGLLOYD: could washing damage the fabric and thus affect its filtration? Is the fabric treated in any way such that washing would damage that treatment, or is it just fabric? I don't know.

As for wiping, could the alcohol make full contact embedded virus particles ? And does the wiping damage the fabric and affect its filtration? I don't know. I suppose that saturating the mask with alcohol would do it.

Expert view

Industry expert Mark R writes:

Here is an excellent summary of the attention to detail which must be applied to determine sterilization standards.

I worked with Anthony Faucci and other at the NIH and the CDC in 1980’s. Myself and many others represented Industry to help formulate the initial round of Universal Precautions published in 1985. The issues you are attempting to address here are extremely complex. One mistake with an AID patient in the 1980’s or early 90’s or Ebola or the 2019 Novel Corona Virus could mean death. Of course, I know that you fully and deeply understand this. Further, you are to be commended for having, very early on called on your readers to obtain the N or P 95’s and 100 class masks to prevent either dust or biological pathogens from entering ones airways.

I can address this issue only from the perspective of great gravitas. Let's consider a fact. Mount Sinai Hospital, in Manhattan, normally uses approximately 10-12,000 masks every 24 hours (I sold them masks for years so I know the usage rates). They reached out to me this week (I sold my dental, then medical supply company 5 years ago but my Latexx Partners account is still in their system). The buyer informed me that their daily mask usage hand topped 40,000 masks per day. They are facing critical shortages precisely because no protocols for mass of sterilization “used’ masks has ever been established.

It is one and done for good reason if the exterior wall or inner ply are bearing a pathogen and if they continue to use the same mask it could eventually be drawn into the airways of the medical professional or be blown back and shed on someone not infected. So to your line of inquiry. Again the tolerances under discussion are critical.

Surely heat can kill a virus please see the "heat plus pressure" paradigms discussed in my previous CDC link. I might add that the articles to which that validated science apply are generally reusable stainless steel surgical, dental or tattoo staples. Soft, multilayered, highly woven medical masks represent a very different matter. Today the CDC revealed that "A high proportion of asymptomatic infections could partially explain the high attack rate among cruise ship passengers and crew. SARS-CoV-2 RNA was identified on a variety of surfaces in cabins of both symptomatic and asymptomatic infected passengers up to 17 days after cabins were vacated on the Diamond Princess but before disinfection procedures had been conducted (Takuya Yamagishi, National Institute of Infectious Diseases, personal communication, 2020). Although these data cannot be used to determine whether transmission occurred from contaminated surfaces, further study of fomite transmission of SARS-CoV-2 aboard cruise ships is warranted”.

How longer can a virus live in a pleated corner of a mask? The heating of the woven used to form meshes used in masks would if it were to exceed 125 degree F would certainly cause an expansion and contraction which would invalidate the published mask permeability standard. Chemical or or Alcohol application to any mask can again easily degrade a masks woven structure.

The material most commonly used to make a mask is polypropylene, either 20 or 25 grams per square meter (gsm) in density. Masks can also be made of polystyrene, polycarbonate, polyethylene, or polyester. 20 gsm mask material is made in a spunbond process, which involves extruding the melted plastic onto a conveyor. The material is extruded in a web, in which strands bond with each other as they cool. 25 gsm fabric is made through meltblown technology, which is a similar process where plastic is extruded through a die with hundreds of small nozzles and blown by hot air to become tiny fibers, again cooling and binding on a conveyor.

If Alcohol can damage/devulcanize rubber lens rings what can it do to the bonds which holds these plastic particles together? The chemicals which are used to disinfect surgical suites or the Diamond Princess are surely destructive of such particulate bonding found within masks.

I simply wanted to reiterate how very difficult it is to clean or sterilize something so simple as a mask. One mistake and that which looks like a mask not longer truly functions as one.


“At Beth Israel Lahey Health in the Boston area, workers were told this week of “extreme shortages” and asked to wear only single-use protective equipment “for as long as they can tolerate during their shift.”

They were also asked not to discard any surgical masks, N95 respirators or eye protection but to place the used ones in special bins to be saved. “BILH is investigating potential options for mask sterilization and future repurposing,” according to a copy of the memo obtained by The Post. *** “This is not currently active but we are storing supply in the interim”.***

Called my wife’s Uncle who is a physician in Houston (two PhD’s and am MD)-- brilliant fellow. Originally from Taiwan. Amazingly, an article his friend, Lin Tzu Hsien friend had written on the subject at hand. Turns out to be dead on.

Filter quality of electret masks in filtering 14.6–594 nm aerosol particles: Effects of five decontamination methods

UV light can indeed kill bacteria in the air (provided their is continuous exchange of air in conjunction with light concentration density). It is also effective at reducing or eliminating certain pathogens on a planar surface.

I can, as of yet, find no more detailed analysis in the clinical literature that would allow us to offer a solid, replicable solution using such an approach. I am thinking that the density of the spun poly weave beneath wherein pathogens, which easily by pass first outer layer layer, would need to be exposed directly and for an extended period of time by a UV light source of x minimum strength. I think you will find the conclusions very much in line with the methods and consequences I broadly suggested earlier.

A more detailed analysis of these issues is dealt with in this article. In particular, why it would be so important that pathogen destruction efficacy be assured by any medium used. Please see, especially from, “Human Factors”.

DIGLLOYD: something may be better than nothing, or it may not. Too much for me to grok.

I am reminded of proposals to irradiate food with gamma rays years ago—gamma rays will destroy all pathogens very quickly. But whether any such facilities exist is dubious.

Physician James G writes:

Effects of Ultraviolet Germicidal Irradiation (UVGI) on N95 Respirator Filtration Performance and Structural Integrity

Evaluation of five decontamination methods for filtering facepiece respirators

One of my partners just sent the first article to me and that one led to the second. Talk about prescient. Thank God there are nerds in the world who think about this stuff. Both articles have useful tidbits. Among other things one presents data that show viruses are the most susceptible and sensitive to UV-C. Anthrax and similar spore-forming bacteria are the least sensitive.

The UV-C wavelengths appears to be the most effective of the UV spectrum, though not much UV-C makes it through the ozone layer. Here’s what wiki has to say about germicidal UV:

So sunlight UV could work but is obviously less effective than a germicidal lamp. The devil is in the dose. Sadly, nothing on hypochlorous acid sterilization. I would guess the data would be similar to the bleach in the second article, though without the odor and skin sensitivity issues.

DIGLLOYD: this makes sense to me on ultraviolet (UV). As I understand it, UV-C is the shortest wavelength (versus UV-A and UV-B), and thus the highest energy and most destructive to living organisms. This is why I mentioned gamma rays (extreme 'hard' radiation that destroys or severely damages living cells and is a major health risk to astronauts going to Mars, even when mostly shielded).

Bart van H writes:

First of all, thanks for keeping us up to date on your experiences in the Alabama Hills. Im sure that if anyone can stay safe, it is you. I envy the views you have, the wacky tea somewhat less...

I am lucky enough to live on the edge of the woods and dunes here on the coast so I can take long and lonely walks to stay healthy and sane.

The Dutch health authority has done some research into disinfecting masks for re-use an came up with a technique that is feasible to use in hospitals and allows 2 re-uses. It is not of much practical use to you (nor me), but it shows that degradation is a real issue, as expected. The document is available in English from our government website here:

“Summary The Dutch National Institute for Public Health and the Environment (RIVM) has conducted a pilot study and found a reprocessing method that leads to an acceptable quality of reprocessed face masks. The caveat is that only limited research has been done on the retention of particles by reprocessed face masks. This study shows that FFP2 face masks retained their shape and were able to retain particles in a ‘quick’ test after sterilizing once and twice with a short hydrogen peroxide process. In times of scarcity, FFP2 masks can be used three times when sterilized twice with hydrogen peroxide in between use.”

DIGLLOYD: hey, don’t knock it till you try it (tea)!

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