Everyone’s being told to wear a mask now, but the problem is: there are no masks to be had. You could make your own mask, but even the best homemade masks won’t be as good as an N95 respirator. The problem with disposable N95 respirators is that they’re intended to be worn once and thrown away. But can their life be extended? Doctors, engineers, and scientists say, yes, they can be baked and reused, but there are some serious caveats.
Here’s what you need to know:
- Studies show that 30 minutes at 75C (167F) can possibly kill or deactivate SARS-CoV-2, but it’s not recommended by health authorities.
- Cooking a respirator will eventually reduce its effectiveness, and the heat might warp the mask to prevent a tight seal.
- If you cook your respirator anyway, you should use a separate oven if you can.
- Immersing your N95 in soapy water, alcohol, bleach, and other cleaners is not recommended.
Since surfaces can be contaminated by the SARS-CoV-2 virus (as well as by everyday bacteria, mold, and viruses,) masks can be a potential infection vector. That’s why it’s important to follow careful procedures when taking them on and off, and to have a system in place to decontaminate them.
You could simply set your mask aside for a few days. This is fairly easy if you don’t have many occasions to wear a mask or have a stockpile you can cycle through. Unfortunately, healthcare workers don’t have those luxuries, so they’re turning to some desperate measures.
Researchers at Michigan State University have found a way to decontaminate N95 respirators in a commercial oven based on research from Stanford University by using forced air from commercial ovens. I should say up front that neither Stanford or MSU recommend baking N95 respirators in a home oven due to potential contamination.
A document published by Stanford Medicine states: One area of concern was a misunderstanding about using home ovens 70C to disinfect masks and other equipment. None of us should take any contaminated materials home or leave them near food or drinking water as they present a risk to family and loved ones.
Norman Beauchamp Jr., MSU executive vice president for health sciences told the Lansing State Journal: “The process we’re using, we’re not going to sterilize masks and then go back to cooking food. You want to keep those processes separate. The risk is you contaminate the oven.”
But I’ve contributed to The Prepared enough now that I know you guys are going to do it anyway. After reading my package disinfection guide, some of you threw your packages in the oven, sometimes melting the contents.
So let’s look at what’s recommended, and how you can stake a balance between disinfection and melting down your precious supply of respirators.
The Goldilocks mask: not too hot, not too cold
There are a number of factors to consider when using heat to disinfect N95 respirators:
- Heat: The oven needs to be hot enough to kill the coronavirus, but not so hot as to impair the effectiveness of the respirator or deform it and prevent a good seal.
- Humidity: Preliminary studies suggest that high humidity works against the coronavirus, as it did against the original SARS, but the extra humidity can reduce respirator lifespan.
- Time: How long do you need to bake the respirator to kill SARS-CoV-2 without destroying the mask?
In my research, I found a few different numbers, and some prevailing patterns:
- Stanford recommends 75C (167F) hot air for 30 minutes, which maintains respirator integrity for 20 cycles.
- N95DECON, a scientific consortium formed to address the N95 shortage, also recommends 60-75C for 30 minutes or 70C at 85% humidity for 30 minutes.
- The World Health Organization doesn’t address respirator cooking specifically, but it does mention that coronaviruses are “thermolabile,” which means they’re killed off at regular cooking temperatures, specifically 70C.
- The Society of American Gastrointestinal and Endoscopic Surgeons mentions heat as a respirator decontamination method, but does not recommend it, because “…it is unclear whether there is a specific humidity required during this process to completely inactivate SARS-COV-2 viral particles”
There is one outlier: an article in Newsweek claims that 92C (198F) for 15 minutes is required. However, that’s based on a study that has not yet been peer-reviewed, so take it with a grain of salt for now.
N95DECON cautions that its data is based on related coronaviruses and not SARS-CoV-2. Both Stanford and N95DECON specify that the high humidity method reduces respirator effectiveness after 5 cycles.
So, our recommendation is to not do this to your respirator, but if you insist, here are the guidelines we suggest:
- Only use this method with disposable paper N95 respirators. Using it with a reusable mask or mask filter could melt the plastic.
- If you can, use a separate oven. A toaster oven or even a clean electric smoker could work. (Take out the wood chips unless you want a hickory smoked respirator).
- Heat the oven to as close to 167F as you can. In an informal poll of my colleagues at The Prepared, most ovens only go as low as 170F or 175F.
- Use an s-hook or wooden clothespin to hang the mask off a rack so it’s not touching a hot oven rack. This will help with even heating and reduce the chances of warping and melting. Place one rack at the top of the oven, hang your mask from it, and either move the other racks to the very bottom or remove them entirely.
- You may consider a pan of water in the bottom to increase humidity, which might make this method more effective, but will also reduce the mask’s lifespan. Personally, I would keep it dry.
- Only do one mask at a time, in case the mask gets damaged from the heat.
- Cook the mask for 30 minutes.
- Consider a decontamination procedure for the oven afterward. That could be as simple as letting it sit unused for a week or running the oven’s self-cleaning cycle. Much is still unknown about SAR-CoV-2, but we don’t see how the virus could survive 500F for 30 minutes.
Baking your masks is a desperate move, but at least there’s evidence to show that it works. Let’s take a quick look at methods that we know do not work.
How to not decontaminate a respirator
Both Stanford and N95DECON mention methods that have been found to be ineffective in decontaminating N95 respirators: soapy water, alcohol, and bleach immersion can degrade N95 filtration efficiency.
However, N95DECON reports that “wiping [the mask] 3 times with a fresh bleach-containing wipe (0.9% hypochlorite) has been shown NOT to cause damage to multiple N95 models, and can decontaminate for at least one model pathogen.”
N95DECON also says that overnight storage is not effective, however, the Society of American Gastrointestinal and Endoscopic Surgeons suggests that quarantining a mask for 72 hours could be an effective reuse strategy: Assuming there is no soiling and minimal to no viral contamination to the outside of the mask, the CDC suggests that masks can be hung to dry or stored in a breathable container in between uses. The following strategy has been suggested by multiple organizations based on the fact that coronaviruses lose their viability significantly after 72 hours.
Stanford echoes many of these recommendations.
Have you tried disinfecting an N95 respirator in an oven? If so, tell us about your experience in the comments.