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A Study Looks at How to Disinfect Your Mask at Home

by Roland Yan, Steve Chillrud, Debra Magadini and Beizhan Yan |April 21, 2020

Due to the unknown numbers of asymptomatic people infected with the SARS-CoV-2, the Centers for Disease Control and Prevention has recommended that all citizens wear face coverings when in public. More recently, some states have mandated face coverings. Many people are wearing homemade coverings, but these mandates potentially increase demand for medical face masks, exacerbating shortages for first responders and medical staff.

One way to to extend the supply of disposable masks is to disinfect them and reuse them. We have just published a paper in the Journal of the International Society for Respiratory Protection that looks into whether disposable masks can be disinfected by heating them without compromising their effectiveness. We also compared the effectiveness of medical-grade masks with homemade ones, and looked into the feasibility of improving masks with homemade nose clips.

person wearing a face mask with nose clip

A new study suggests that disposable surgical masks can be disinfected with heat multiple times without harming their effectiveness. A homemade plastic nose clip, used here, may add another layer of protection. (Photo courtesy Beizhan Yan)

Prior work by others on disinfection of disposable masks has shown that heating for 30 minutes at 158 degrees F (70 C) or above can effectively destroy SARS, influenza and the novel SARS-CoV-2 coronavirus. This can be done in a home oven. As such, we did no testing with viruses. Instead we focused on whether repeated heat disinfection affected how well the masks worked for removing particles in the same size range as coronavirus.

To do this, we put masks onto mannequin heads, and rigged the heads to “breathe” through their noses and mouths, using a vacuum pump. We then exposed the mannequins to black carbon (i.e, soot) from a kerosene lamp, which generates particles that overlap in size with those of the coronavirus. We determined filtration efficiency by comparing black-carbon levels on both sides of the masks worn by the mannequins. We did this with two brands of disposable N95 respirators and one brand of disposable surgical mask, as well as with one design of homemade face covering. We tried this out repeatedly, and in a variety of ways.

First, to measure the maximum filtration efficiency and resilience of the disposable masks, each disposable mask type was tested while taped or modified to tightly fit a plastic mannequin’s face when new, and again after each heating cycle.  We found that one N95 brand (3M) and a surgical mask (HSl brand) stood up to the 10 cycles of heat disinfection and reuse, with no reduction in performance. Filtration efficiency was greater than 95 percent over all 10 cycles for N95 respirators, and greater than 70 percent for surgical masks. (In contrast, we found that the nose-pad of another N95 brand, the Moldex, was unable to withstand multiple cycles of being put on and off the mannequin, whether or not it was heated in between uses.)

mannequin wearing homemade face mask

One of three homemade face coverings tested out by the researchers. The silicone headform was a donation from Joshua Turi. (Photo courtesy Beizhan Yan)

These tests show the maximum filtration efficiency possible, but they are not representative of how people normally wear masks, where the fit can be much looser. So, for a second set of tests, we obtained a head form covered in soft silicone to mimic the pliability of the human face. We then assessed the effectiveness of the masks as they are commonly worn, by simply putting the elastic straps around the head or ears without additional tightening.  As expected, the filtration efficiency of all the disposable masks decreased substantially, to around 40 percent. This confirms that the effectiveness of such masks relies upon a tight fit, and this may be hard for many people to achieve.

We also tested the filtration efficiency of three homemade cloth coverings made following instructions on the CDC website. We made one from a cotton dress, one from a cotton sweater, and the third from polyester cloth.  All three were worn in a normal mode on the silicone head form as directed by the CDC. The filtration efficiency of the cotton homemade cloth coverings in normal use was 55 percent, while the polyester covering came in at near 40 percent—about the same as loosely fitted medical-grade masks. This suggests that homemade cotton masks might actually work better than loosely fitted disposable masks, while polyester might be about the same.

We heated up the homemade masks for disinfection, which appeared to not affect the filtration efficiency. The CDC recommends washing and drying such coverings at home and we anticipate negligible effects on efficiency from this as well. When disinfecting your masks at home, we recommend you to place masks in an oven bag or a pressure cooker during heating, rather than directly put masks inside of the oven (see the instruction video below for details, or click here for a step-by-step guide).

Finally, to see if we could improve the fit for the public, we designed a process that uses heat-moldable plastic strips to make homemade customized nose clips molded to an individual’s face. By adding the customized nose clip to a normally worn disposable mask on a silicone head form, the filtration efficiency of the 3M N95 returned to greater than 95 percent, and the filtration efficiency of the surgical masks was measured at 88 percent. The nose clips passed two five-hour wearing tests for comfort. But due to the use of heat moldable plastic, the customized nose clips cannot be disinfected with heat; rather, they must be disinfected by soaking in solutions of alcohol or bleach.

This work has certain limitations. For one, our tests were all done under static conditions at a constant flow rate of air similar to how an adult breathes when sitting. We did not take into account the increase in breathing, nor the reduction in fit that can occur when someone is talking or active.

Our study is just one of many looking into how masks may be disinfected and reused. Others have been carried out or are in progress using not only heat, but ultraviolet light, vaporized hydrogen peroxide, or soaking in ethyl alcohol or bleach solutions. Most of these are aimed at medical personnel using specialized equipment. The soaking methods have been shown to reduce the effectiveness of certain types of N95 masks. Ours is a relatively modest effort aimed at everyday usage. Far more work needs to be done, but everything we know so far suggests that wearing almost any kind of mask in public is better than nothing; that a tight fit is best; and that, with certain limits, many types of masks can be reused outside of medical settings.

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MiriamEliza BallotRay BieseDon MRuth de Jauregui Recent comment authors
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Travis
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Travis

Do we know how long the virus is expected to survive on the surface of a facemask if no steps are taken to disinfect it?

M Rodgers
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M Rodgers

3 days

Beizhan Yan
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Beizhan Yan

In one study published in Lancet, it said ” a detectable level of infectious virus could still be present on the outer layer of a surgical mask on day 7 (~0·1% of the original inoculum); thus it is critical to disinfect masks

Michael McDonough
Guest

Thank you for this extraordinarily useful information.
With regard to your excellent video:
Please do the conversion from Celsius to Fahrenheit.
US ovens do not have centigrade indicators
Just round it up to 175° Fahrenheit or let them know that if their lowest oven setting is 200°F that will work as well.

Steve Chillrud
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Steve Chillrud

Thanks for the comment- we caught that too, but our tech savy high school student who made the movie has been side tracked by school exams.

David
Guest

Very interesting with the use of the nose clip. We will have to test that out.

don mascilak
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don mascilak

Appreciate the information, but why did your study not include sanitizing using various C-Pap cleaning machines which many, many people use to disinfect their sleep apnea equipment. I can not find any information whether these machines are effective compared to heat, UV light or hydrogen peroxide treatment. I, and many others, would appreciate a response. thanks.

R Bohn
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R Bohn

It’s a good idea. Someone who owns such a machine should try it. One shortcut could be to look at the maximum temperature the machines reach. (Recording thermometers are now reasonably cheap, or just open a machine up in the middle of the cycle and quickly note the temperature.) This video says 160F for 30 minutes , but longer times and higher temperatures should be fine as long as nothing melts or catches fire. (Which it should not – that is a very low temperature.) Viruses are “fragile”, which is why this is easy. Please report back, here or elsewhere,… Read more »

Don M
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Don M

The primary disinfectant in these machines is Ozone. I did find information from an (unbiased?) ozone equipment company that addressed this issue. To quote from their web site: “When ozone comes in contact with contaminants, such as bacteria and pathogens, it causes an oxidation reaction that eliminates the contaminants and breaks the ozone down into two oxygen molecules. This reaction is what makes ozone a safe and effective disinfectant. For over 100 years, ozone generation has been used as a disinfectant in many industries, such as food production, water treatment, and many more. Over time, ozone generation has shown that… Read more »

Ruth de Jauregui
Guest

Because I have a limited number of paper surgical masks (purchased for fire season over a year ago), I’ve been spritzing my mask inside and out with hydrogen peroxide and then hanging it to dry until my next excursion outside (usually several days).

At some point I am going to make cloth masks. I intend to continue masking up when going out in public from now on, even after the COVID-19 risk is reduced. It helps with pollen/allergy issues too.

Ray Biese
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Ray Biese

A useful and practical application of the data currently available. Readers should be warned that simply putting a mask in the oven can easily and permanently damage the mask effectiveness. As an addition, a meat thermometer should be used in the oven. The thermometer accuracy can be checked at 100°C (212°F) in boiling water (slightly lower at high altitude). Your advice that ‘the fit matters’ is especially important. Wearing a mask properly with a good fit is critical, sometimes more important than the mask type. Users should note that the ‘oven bag plus closed paper bag’ is important to reduce… Read more »

Ray Biese
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Ray Biese

Follow up: Sorry for the previous ‘run on’ posting. Paragraph separation was lost. The weakest part of your dry thermal disinfection procedure is oven temperature control. Temperature measurement is needed to ensure that oven tempersture overshoot does not damage the polymer fibers inside the N95 and medical procedure masks. At least an in-oven type meat thermometer should be used. I tested the oven temperature control on a 55 yr old, self-clean, convection optional, electric oven. The normal ‘bake’ temperature control was poor at low temperature. Rapid heating and temperature overshoot well over 210°F (175°F setpoint) was seen with a wide… Read more »

Miriam
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Miriam

Thanks a lot for explaining all the details behind the rationale! Since there are so many articles one can become confused!

Eliza Ballot
Guest

Wash the reusable masks like KN95 or N95 face mask, sanitize them and leave them to dry under sunlight for 9-10 hours. Virus will not survive under a heavy heat for long time.

Miriam
Guest
Miriam

Thank you so much for your work! I have researched papers on disinfection procedures “high and low” but this is the only study I found useful for everyday life. Your noseclip idea is genius!
What do you think about including hydrogen peroxide 3%? Since its vapour is a favourite, could the masks be sprayed or moistened then put into a container to be heated in the oven?
Best regards