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.
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.)
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.
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?
3 days
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
It is “critical” to handle them “as if they are contaminated”, before attempting to disinfect as well as to dispose them. Masking Up and hand sanitising is necessary for this too!
Interesting. Is there information available wether these masks in the study were still considered infectious?
I have read some professionals are of the opinion that masks can be safely reused after 3-7 days (some are even saying 2 days or less but 3 days seems to be a common estimate), I even remember reading about such a system from a CDC source (recommended just in case of lack of single use respirators in pandemic situations).
Correct me if I’m wrong but I understand in most viruses the virus RNA/DNA can generally be found long after the virus is not considered transmittable any more. There also seems to be disagreement on how much of the virus needs to be left on the mask to cause an infection risk.
There are a lot of limitations with decontamination. Firstly, not everyone (especially private persons) have the necessary resourses and equipment. Secondly, the new guidlines by CDC state that respirators manufactured in China or respirators with an exhalation valve should not be decontaminated. This is a big limitation since most respirators available seem to be of these types.
Of course these are still all all just emergency solutions that would never be used under normal circumstances.
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.
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.
I have had melt spots at 80C and would suggest 70C -75C maximum for fibre masks. see post above.
Very interesting with the use of the nose clip. We will have to test that out.
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.
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, what you find. Most of us don’t have those machines and therefore can’t test them.
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 it is an improvement to many processes that conventionally use harsh chemicals as disinfectants.
How does ozone kill a virus? Typically, viruses are small, independent particles, built of crystals and macromolecules that multiply only within the host cell. The new coronavirus is considered an “enveloped virus.” Enveloped viruses are usually more sensitive to physico-chemical challenges. In past studies, 99% of viruses have been damaged or destroyed after 30 seconds of exposure to ozone. Ozone destroys viruses by diffusing through the protein coat into the nucleic acid core, resulting in damage of the viral RNA. At higher concentrations, ozone destroys the exterior protein shell by oxidation.
When disinfecting large areas, ozone gas has been proven to kill the SARS coronavirus. Because the structure of the new 2019-nCoV coronavirus is almost identical, it is somewhat safe to believe that it will also work on the new coronavirus. Currently, there are more than 17 scientific studies that prove ozone gas can destroy the SARS coronavirus.” Since I am not a scientist I cannot comment pro or con on their statements, but, if true, would indicate a potentially safer non-chemical product for covid-19 usage. But as Dr. Fauci might say: ” There are no definitive studies proving this and annectotal evidence is informative but not conclusive.” (my words) Thank you Dr. F.
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.
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 the risk of permanently damaging the masks. Hand washing is important to prevent cross-contamination.
It is critical that N95 and 3-ply medical procedure masks not be overheated as the (usually polypropylene) fine fiber structure can be permanently damaged. I gave not seen data on a safe maximum (dry) temperaure. A guess would be 100°C~105°C (212°F~221°F).
Getting this type of mask wet will also degrade performance. Your approach of dry disinfection at 80°C (176°F) for 20 minutes should be just fine as long as overheating or underheating (below 70°C (158°F) is avoided. Your ‘double bag’ approach should help avoid overheating damage from temperature cycling above and below the setpoint in modern ovens. Temperatures below 70°C (158°F) should not be counted towards your 20 minutes.
Please remember that the useful life of ‘disposable’ masks is not infinite but using a ‘new’ mask for occasional use (like every trip to the grocery store) is quite probably not necessary.
Wear and tear from use can degrade the filtering layers. Streching of elastic ear loops can affect the fit.
However, reusing masks can create a virus contamination hazard. So non-damaging disinfection between uses is warranted.
Extra detail:
Liquid water can degrade N95 and medical 3-ply mask performance (but to a much lesser degree than alcohol). (I suspect that condensation is the reason behind mixed results for steam disinfection.) Even condensation from breathing getting the mask ‘wet’ will eventually degrade the efficiency.
Do not try to disinfect N95 or medical grade surgical masks with alcohol (either ethanol or IPA). This will permanently reduce efficiency.
There is a microfiber layer inside which has been corona treated to create an electric surface charge on the fine fibers to enhance small particle collection. Alcohol neutralizes this surface charge quickly. Water and water based disinfectants (such as bleach solutions) will degrade this surface charge.
The paper bag will reduce radiative heat transfer directly to the masks. So users should note why this is important and not use plastic bags instead. Using a convection setting might help and a ‘grill’ setting should be avoided.
Placing the oven bag inside a preheated (turkey) roasting pan (containing some water) to reduce oven temperature fluctuation is another trick but should be avoided as an unnecessary complication and scalding hazard.
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 hysterisis. The maximum temperature seen during setpoint overshoot was high enough to damage polypropylene fibers in N95 or medical surgical masks.
Both overshoot and hysterisis were much better using a ‘convection bake’ setting.
Luckily, the oven cooled very slowly. After cooling to ~190°F, it took over 20 min to cool from ~190°F to ~175°F (if kept closed). If the oven bag containing the masks (in paper bags) were loaded into the oven after cooling to ~190°F, this would avoid the overshoot and still perform the thernal disinfection.
I should note that meat thermometers come in two types. One is used to check food temperature only outside the oven (dial and digital). The second (dial) type can be left in the oven during cooking. Make sure the thermometer is appropriate for your use.
Thanks a lot for explaining all the details behind the rationale! Since there are so many articles one can become confused!
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.
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
A Breville Mini oven with digital controls including “IQ” for even heating, was used.
#1_15/8/20 _at 175F/45 mins_1x P2V Charcoal mask, inside brown paper bag, inside oven bag.
*Unknown whether heated on oven rack or on enamel bake tray on oven rack.
RESULT_ no deformation of any construction materials.
#2_16/8/20 _at 175F/45 mins_1x Surgical Mask, inside brown paper bag, inside oven bag.
*Unknown whether heated on oven rack or on enamel bake tray on oven rack.
RESULT_ no deformation of any construction materials.
#3_23/8/20_at 80C/45_4 x different masks (P2V charcoal, Kn95, Surgical) in individual paper bags “stacked” inside 1 oven bag.
*Unknown whether heated on oven rack or on enamel bake tray on oven rack.
RESULT_ 1/4 (P2V Charcoal) mask sustained melt damage on outer fibre layer.
Reason Unknown_Hypothesis #1_80C too hot, #2 stacking resulted in heat retention due to position (which was not recorded), #3 combination of both, #4 Oven temp. or function = inaccurate.
#4_18/9/20_at 75C? / 80C?/45 mins x3 Surgical masks, in individual paper bags “stacked” inside 1 oven bag.
*Unknown whether heated on oven rack or on enamel bake tray on oven rack.
RESULT_ no deformation of any construction materials.
#5_20/10/20_75C/ 45 mins_3 x different masks_P2 charcoal, Kn95, Surgical Mask_ “stacked” inside oven bag
Heated on wire oven rack only.
RESULT_1/3 (Kn95) sustained melt damage* on outer fibre layer and bonding to paper bag (which was able to be separated without causing any (actually, further) physical damage.
*The the melted areas (x3) was consistent with spacing of the wire rack. (However, again the positioning of the bagged masks in the stack were not recorded. eg did the ‘vertical’ (due to space limits) stack’s weight/pressure affect the bottom mask and create the melt along the wire rack contact??
Reason undetermined for the melt damage to the 2 separate mask and batch conditions.
Conclusion thus far:
Take Caution #1. re stacking of masks . The advice to include multiple bags may not have considered the effect/s of temp accuracy on space around the oven bag nor inside it. Eg.
My oven being much smaller than the one shown, and having digital accuracy and convection fan may have reached greater consistency in temp control and thus overheated some of the contents. (?)
An opposing point is whether the outer bags in the “stack” effectively insulated the inner one/s and also whether those inner one/s were then effectively disinfected with appropriate heat levels. (I did observe that ALL the paper bags felt warm on removal though.)
Caution #2. the the contact surface of the oven bag, to be considered re. ability to distribute heat to prevent “hot spots”.
The considerations of temperature & time likely increased the “possibility” of damage and that reducing temp to 70C as per other research findings* since and also correcting my mistaken timing back to the recommended 30 mins, are prudent actions.
*see also
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7260521/
http://www.imcclinics.com/english/index.php/news/view?id=83
At this point I will conduct a final trial with a multiple stack of 3.
I do concur that “dry heat” as prescribed, did not damage the P2V or surgical masks when disinfected “singly” as described. And that 10/12 masks in total survived the various conditions without incurring any damage.
As a further precaution, all have/will have been quarantined for 2 weeks, and used for single outings only whether 10 mins or 2 hours.
Please comment re possible insulating effect of placing multiple bagged masks inside oven bag for single disinfection period. Does every mask receive the set temperature/30 mins?