Myth: masks can only be used once

Fact: Many masks can be reused, but not all methods work

Note: You must know that, in a normal situation, health authorities recommend to renew the surgical masks approximately every 4 hours and the respirators (N95, FFP2, etc.) every 8 hours. All of them are considered single-use.
The methods described in this article are appropriate for an emergency, like the one we live in, in which there are not enough masks for everyone. The information I show here is based on recent scientific papers. I hope it will be useful, given the lack of masks.
If you decide to recycle your masks, you should read the post carefully and follow the guidelines as shown. Any action you take upon the information presented here is strictly at your own risk. The author assumes no responsibility for any damages in connection with the use of this information.

any people don’t know what to do with their used masks, and for a good reason. We hear contradictory advices.

What to do with used masks?

  1. If you have spare masks, it is better not to reuse them. In theory they are single use.
  2. If you have a few, say about seven, use them in order. Try to maintain at least seven days of “quarantine” with each mask. Viruses that may be in it will lose their ability to infect cells.
    Please do not put them in closed plastic bags, Zip type, or in airtight tupperware. If the mask is damp, it is a breeding ground for bacteria and fungi. It is better to air-dry the masks or keep them in paper bags. Let them “breathe”. You can quickly build a board to hang the masks. This board will make it easier to reuse masks in order.
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Fig. 1. An idea to hang masks in an orderly way. Materials: board, wooden tweezers, silicone gun.

If you don’t have as many masks or need them quickly, you can disinfect them. I am going to summarize in this article the recommended guidelines for disinfecting masks at home. I will base myself on what the best scientific papers say about it. I include the latest published.

Two main factors limit the decontamination and reuse of a mask:

  1. The stability of the filter’s electrical charge. The filtering material of both N95, FFP, etc., and surgical masks is electrically charged. This charge acts as a velcro to which the particles that we want to block are attached. If the electrical charge is lost, part of the filtration capacity is lost.
  2. The fit of the mask to the face. A mask that leaks around the edges is not very efficient. Although the filter material is excellent, it will protect us little.

Note: Masks visibly stained with respiratory or nasal secretions should be discarded and not reused. Likewise, masks that are damaged (rubber or, broken nose piece, scratches, holes, etc.), or those that cannot fit properly (deformed masks) should also be discarded and not reused.

N95, FFP, etc., masks (PPE, respirators, or self-filtering masks)

The best method for disinfecting FFP2, FFP3, N95, etc. masks at home is humid heat.

There are two ways to do it:

  1. In the oven, at 70 ºC, 1 hour, with a little water (the water is not in contact with the mask).
  2. In the microwave, with steam.

Microwave disinfection

We can use the microwave to disinfect masks at home. It is not a new method: the microwave has been used to decontaminate materials for at least twenty years. For example, it is used to rapidly decontaminate syringes and filters to prevent infections with viruses (HIV and hepatitis C virus) in people who inject drugs. But keep in mind that:

  1. If there is no water, the temperature will not be homogeneous. That means that when you put the masks in the microwave, some areas will heat up much more than others. Consequence: the microwave could break down or, worse still, could start a fire.
  2. Some materials cannot be heated in the microwave. Important: DO NOT use the microwave oven grill stand to support the mask. YOU CAN CAUSE A FIRE.

Several models of N95 masks have been tested. The results of nine studies were analyzed in this recent review.

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Fig. 2. Most proven N95 mask models in heat decontamination studies, both in the microwave and in the oven. https://www.sciencedirect.com/science/article/pii/S0195670120304035. Only masks without an exhalation valve should be decontaminated.

This is the method that Harvard researchers recommend in a recent paper: pour about 50–60 mL of water in a glass or plastic container with a wide base (about 6.7 by 6.7 inches). Place a mesh of fruit or vegetables on the container with water, held with a rubber band (fig. 3). Suspend the mask over the net, with the outer part down. Heat the whole in the microwave for 3 minutes at maximum power (1000–1100 W), with the plate turning.

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Fig. 3. Microwave disinfection of the N95 mask, according to the method described at https://mbio.asm.org/content/11/3/e00997-20?rss=1.

The researchers applied about 10 million viable MS2 viruses to different regions of the N95 masks. (This virus is used as a model in disease transmission studies since it does not cause disease in humans.)

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Fig. 4. N95 respirator used in the experiment. Each numbered box indicates the area where viruses were deposited. The researchers tested microwaves with different powers, and the results were not affected.

Microwaving the mask using the method described removed 99.9999% of the viruses (6 log reduction, the researchers say). The MS2 virus is more difficult to destroy than SARS-Cov-2, since the latter has a lipid membrane. For example, dry heat (even at 100 ° C) is completely ineffective against MS2 placed on masks, while SARS-CoV-2 is destroyed at 70–80 °C. That means that if the described method kills the MS2 virus, it will also destroy the SARS-CoV-2.

I have made a small modification. Instead of the mesh, I use a silicone net that fits the container (Fig. 5). I think this is more practical.

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Fig. 5. In this version of the method, the silicone mesh is fitted over a rectangular Pyrex glass container.

The idea is that the mask is suspended and in contact with the water vapor.

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Fig. 6. The curved metal part (nasal clip) of the FFP or N95 mask models similar to those in Figure 2 does not pose a problem. No need to remove staples.

The researchers showed that both filtration efficiency and fit were maintained for at least 20 disinfection cycles in all mask models they tested.

«We measure the filtration efficiency of masks after the microwave treatment, according to OSHA* guidelines. The fit and function of N95 respirators were intact, even after 20 sequential microwave steam decontamination treatments».

*Occupational Safety and Health Administration.

Besides, it is unnecessary to remove the flat aluminum strip with which the mask fits the nose, nor the metal staples if they had them.

«We have tried 50–60 times, and everything has been perfectly safe».

According to the researchers, we should be concerned about arcing or sparking in the microwave if:

a) metal has pointed ends, which can act as antennas for the voltage they collect;

b) there is no other material to absorb heat/energy.

In this case, since the aluminum part of the mask is curved, and there is water (and steam), there is no risk of sparks.

However:

The microwave method is invalid when the metal strip for nasal adjustment is not flat and wide, but thin.

When the wire contained in some masks is exposed to microwave radiation, the surrounding area melts. If that metal strip is not removed, we damage the mask.

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Fig. 7. We cannot use the microwave to disinfect masks with a wire or a thin metal strip unless we remove it first. To do this, make a small cut in the mask at one end of the strip, and remove it. After microwaving the mask, put it back in through the small hole where you took it out. This step is not always necessary since in some masks that piece is not made of metal but plastic. It is also unnecessary in masks with a flat and wide aluminum strip, such as those shown in fig. 2.

Disinfecting masks with wire in the microwave can be dangerous. It could spoil the microwave or even cause a fire.

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Fig. 8. Example of a surgical mask with a thin nasal clip (wire), subjected to microwave treatment. In this case, the microwave method is not valid.
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Fig. 9. Another example of a mask with wire. The accumulated heat around the wire causes the plastic material surrounding the metal to melt. The microwave method is unsuitable for these kinds of masks.
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Disinfection in the oven

We can disinfect an FFP or N95 type mask in the oven, at home. The researchers concluded that it is possibly beneficial to have a little humidity while the heat is applied.

“The best conditions to inactivate SARS-CoV-2 in N95 and FFP2 type masks are: temperature between 70 and 85 ºC ( 158 to 185 ºF) with a relative humidity (RH) higher than 50 %, for 60 minutes.”

It is an accessible method, which can be implemented by the general public at home.

«It is recommended to keep the humidity level below saturation, around 60–85 % RH (less than 1 mL of water for a ~1.25 quart size container), to avoid steam generation, which negatively affects efficiency and fit».

This video shows how to do it (starting at 6:22). The mask is placed in a closed, heat-resistant ~1.25 quart size container, along with a piece of paper towel impregnated with about ten drops of water (0.5 mL). This achieves a RH of roughly 60 %.

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Fig. 10. Preheat the oven to 85 ºC (185 ºF). Meanwhile, place the mask in an airtight, heat-resistant container of ~1.2 L (1.25 quart). Place a piece of paper towel moistened with 0.5 mL (about ten drops) of water alongside the mask. This will provide the proper amount of RH, ~ 60%.

Microwave irradiation and moderate heat (up to 90 °C), both in wet and dry conditions, have shown to be effective decontamination methods of viral pathogens from FFP / N95 masks. The latest studies confirm that these treatments alter neither the performance nor the masks’ function.

Once we have disinfected the mask by any of the thermal methods described, we should ventilate it immediately to reduce the risk of growth of pathogenic microorganisms.

Do not store the mask in a closed plastic bag until you use it again. It is best to leave it to air-dry or store it in a paper bag. Moisture favors the growth of bacteria and fungi.

Label your masks, and don’t share reused masks

A good practice to reduce the risk of spreading any disease is that each mask is worn by the same person, rather than sharing reused masks.

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Fig. 11. Example of a mask labeling. On the left, the name and department of the user. On the right, date of first use. In the center, number of disinfection cycles. Source: https://consteril.com/covid-19-pandemic-disinfection-and-sterilization-of-face-masks-for-viruses/

When labeling masks:

  1. We reduce the risk of cross-contamination.
  2. We extend the mask’s life (each person has a different face, so if different people use the same mask, it will deform more).

Some disinfection methods destroy viruses but damage masks

Decontamination of masks can lead to changes in the filter, in the rubber bands, in the material of the nasal bridge, or in other accessories. All of this can lead to:

  1. A worse fit;
  2. Poorer filtration efficiency;
  3. A worse breathability (we breathe worse).

The number of disinfection and reuse cycles that can be applied is limited both by the disinfection method and by the number of times we put on and remove the mask.

The fit is lost when we put on and take off the mask repeatedly

Rubber bands or elastic bands can become self-defeating so that they can no longer generate enough force to create a tight seal between the mask and the face. Poor sealing will allow unfiltered air to enter the breathing zone.
For this reason, the CDC advises that an N95 mask be reused no more than five times, unless it can be proven that the fit has not been altered.

If you notice that the mask fits worse or you find it more difficult to breathe with it, throw it away.

Washing N95 masks with soap and water or alcohol decreases particle capture effectiveness

Never immerse an FFP or N95 mask in soapy water, or spray it with alcohol or bleach. Soap, alcohol, and bleach remove the electrostatic charge from mask fibers, reducing their filtration efficiency.

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Surgical masks

Most health agencies recommend not recycling surgical masks. These masks are generally treated as single-use disposable items. As with the FFP/N95 masks, it is discouraged to wash or disinfect any part of the surgical masks with water, alcohol (including disinfectant wipes containing alcohol), bleach, dishwasher detergent, hand soap, or any detergent.

The electron microscope examination shows that surgical masks are damaged when washed with soap and water at 60 ºC. The same happens after spraying them with a 75 % alcohol spray (hand sanitizer). They lose their resistance to water. The fibers in the middle layer shrink and deform, negatively affecting their filtration ability. Part of the protection is lost.

In Taiwan, the disinfection method recommended by the Food and Drug Administration (FDA) is dry heat, with an electric rice cooker traditionally used in many Asian countries. The Taiwan FDA Director General and her Health Minister demonstrate the disinfection procedure in this video.

They place the masks in a rice cooker with dry heat (149–164 °C) for 3 minutes, without adding water or any other liquid. Then they let it rest for 5 minutes.

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Fig. 12. Disinfection of a surgical mask in a rice cooker.

The masks could be disinfected by this method a maximum of 4–5 times. This method is better than autoclaving, bleach, and alcohol.

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Cloth masks

The best method to disinfect cloth masks is to wash them with soap and water. Soaps and other liquid detergents effectively inactivate all types of viruses, particularly when you factor in the long exposure times and agitation provided by a normal wash cycle in the washing machine.

You can wash the masks with the rest of the laundry. Use regular laundry detergent and water as hot as the fabrics allow. Wash them daily if you use them a lot. And let them dry completely.

How often should we disinfect the masks?

It depends on the use. If we are in frequent contact with people or spend a long time in poorly ventilated places (in the subway, at work, at school), it would be convenient for us to disinfect them every day. If we are less in contact with people and use them from time to time (to go to the supermarket, for example), it would be enough to disinfect it every 2 or 3 days.

Written by

María I. Tapia holds a PhD in Biochemistry and Molecular Biology, with broad experience in basic and applied research.

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