While masks are in short supply during the coronavirus pandemic, the shortage is prompting people to ask if UV light disinfects masks. This is a critical question for frontline healthcare workers, as they scramble for ways to safely re-use masks.
Here’s the problem. We know that UV-C light kills bacteria and viruses, but we also know that UV-C light degrades fibers. That means it’s crucial for healthcare workers to know whether UV light reduces the effectiveness of masks.
Putting It to The Test: Does UV Light Harm Mask Performance?
Researchers got hard data on this question by testing four N95 masks before and after UV sterilization.
They tested four models of N95 masks: the 3M 1860, 3M 9210, Gerson 1730, and Kimberly-Clark 46727.
They exposed masks to five different doses of UV light (also called “UVGI”) from 120 to 950 J/cm².
UV Light Disinfects Masks, But at What Dose?
These are high doses. Even the lowest dose is 66 times higher than a dose that researchers found is sufficient to deactivate 99.99% of viruses on masks (1.8 J/cm²).
To test how well the masks could capture particles, they shot tiny pieces of salt from 0.02 to 0.4 microns at the masks and measured what percentage penetrated the masks. This range of particle size covers the size of flu, ebola, and coronavirus particle sizes.
Related: Are UV light air purifiers effective at disinfecting viruses? →
They tested 10 different sizes of particles and used the data for whichever sized particle penetrated the mask the most.
UV Light Harms Mask Effectiveness Less Than 2%
The data showed almost no harm to performance. For most masks and most dosages, the degradation in particle capture was less than 1%.
In only three tests was the harm above 1%. And even in the worst case, the UV light caused degradation of under 2%.
UV Light Sterilization Harms Straps More Than Filtration
However, capturing particles isn’t the only things mask have to do. They also have to keep their shape and fit to the face. Thus, the researchers also tested how the UV affected the structural integrity of the masks.
One test they did was to pull the straps as hard as they could until they broke. The tests found that, at higher and higher doses of UV light, less and less force was needed to break the mask straps.
Besides the effect of UV light, the data also showed that masks with rubber band straps (3M 9210) broke far more easily than masks with elastic straps (3M 1860). This strength is also important for people who need to re-use masks. Masks with fabric straps will last longer on average than masks with rubber band straps.
A Simple Way to Tell If Mask Needs to Be Disposed
If healthcare workers are going to use UV light to disinfect masks from COVID-19, they’re going to have to solve another question: how many times can I disinfect the mask? One dose–even a strong dose–had only a small effect on performance, but surely repeated doses will have a cumulative effect.
The researchers suggested the answer lies in integrity. They found that, besides the straps, the UV light “dramatically affected” the structural body of the masks. The researchers suggested that (at least for some masks) masks will fall apart before they lose their ability to filter particles.
This is important because healthcare workers don’t have laser particle counters to continually test how well their masks are performing. So how do they know when it’s time to throw them out?
The scientists suggested people can use the structural degradation as a signal. Put simply, when the mask starts to fall apart, that’s a sign that they should throw the mask away while knowing that filtration performance has likely not dropped severely.
UV light disinfects N95 masks with less than a 2% harm to filtration effectiveness, allowing them to be re-used quickly. Bottom Line: UV Light Disinfects Masks
Better Options Than Disinfecting Masks With UV Light
This finding is similar to results of washing masks with soap and water, which also harms performance. Instead, research shows that simply drying and waiting as better options for disinfecting masks.
Read more: This proven disinfection method deactivates viruses with zero harm to mask performance.
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Can I use UV light to initially clean a carbon filter before first use in a mask respirator?
Hi Tim. For a clean before the initial usage, we haven’t done test on this but theoretically it’ll work without harming the carbon filter’s performance if you control the UV amount and use it within the instructed time period. Carbon filters are for gaseous pollutants and their porous structure allows the VOC to be adsorbed inside. UV light can only disinfect materials by deactivate the virus or kills the bacteria. However, it doesn’t make the molecules disappear.
An excellent website. Thank you for all your hard work.
What if you sprayed an N95 mask with Lysol disinfectant and then hung it out in fresh air and sunlight?
Hi Tinmaster, Liz here from Smart Air. I won’t recommend using sprays because we do not have any data on how that will effect the filtration efficiency. But the good news is leave the mask there dry can help to disinfect it: https://smartairfilters.com/en/blog/disinfect-clean-n95-mask-virus-coronavirus/
What about putting it in the dryer?
What if you spray an; N95 mask with; Lysol disinfectant and then hang it out in fresh air and sunlight?
Thank you for this clear and sensible information. Hard to find info that doesn’t leave gaps or questions open. A job well done! You deserve a raise!
How about disinfecting a mask by leaving it outer surface up on the car dashboard, being irradiated by sunlight and also exposed to the heat from sunshine building up inside the car? How many hours might this take to render the virus non-viable? I am doing this currently with the surgical masks I use whenever I go out – when I leave the car to go into my house, I leave the mask sunning itself on the dashboard.
Hi Rosanne, Liz here from Smart Air. Good question! I looked it up and most cars windshield glassed have been treated with UV protections. So they can block out most UV light, although we always feel the heat accumulated on the front deck under the windshield. I would recommend to leave the mask on your porch or balcony where there is enough sunlight and good airflow instead.
How long would you recommend leaving the mask in the sun for?
Hi Daysi, the UV daylight dose in most countries varies from 30-70J/cm2. The researchers found is sufficient to deactivate 99.99% of viruses on masks (1.8 J/cm²). So the sunlight should be powerful enough for disinfect the mask. However, I’m not sure for how long to leave it under the sun. Here is an article published on U of Toronto’s website: https://www.utoronto.ca/news/can-uv-light-help-hospitals-disinfect-masks-and-gowns-u-t-researcher-explains The professor recommend to use UV treatment as one of the disinfect method. So I would leave my mask under the sun dry for at least three days, and the UV light would be an additional secure disinfect method!
good morning,
and thank you so much for providing all that valuable information!
Wonderful to have people like you.
I see cheap small UV-light-desinfection boxes being sold at Amazon.
However, can those itty bitty LED lights that some of them use be trusted?
What is your oppinion? Are you planning on researching them like the air filters? Could they be made @Home? Could I just lay my masks in the sun on my balcony or a sunny south facing windowsill inside? or use my old tanning uv lamp?
thx
Hi Sabine. Song here. Sunbathing would be great. See “The Best Way to Disinfect Masks Without Destroying Them”: https://smartairfilters.com/en/blog/disinfect-clean-n95-mask-virus-coronavirus/. When you decide which UV lamp to use, remember that common UV lights aren’t great at killing COVID-19, and please find more information here: https://smartairfilters.com/en/blog/uv-light-kill-covid19-virus-uvc-disinfect-sanitizer/
Thanks for publishing
this study seem to contradict your conclusion. They tested several decontamination methods, including vaporized: hydrogen peroxide (VHP), 70-degree Celsius dry heat, ultraviolet light, and 70% ethanol spray.
Asbtract:
https://medicalxpress.com/news/2020-04-nih-validates-decontamination-methods-reuse.html
The study followed by interesting critcal comments:
https://www.medrxiv.org/content/10.1101/2020.04.11.20062018v1
That’s a great review, thanks Thomas for your usual excellent piece! I did my own UV-C review on my blog (link is here http://www.myhealthbeijing.com/illness/coronavirus-covid-19/uv-c-lamps-can-kill-covid-19/) I am a family doctor near Seattle and I have started to use UV-C at home and in my clinic using off the shelf products from Amazon, less than $50. I think it’s important to remember that since the goals are to irradiate items each time with total 1J/cm2, it’s crucial to look at that strap strength article, let’s say maximum we can use a mask 50 times, that’s still a tiny bit on the far left of this graft, far less even than the first line of 500 J. It’s almost a misleading graph, no one would ever use a mask that many times even in Beijing for air pollution, the strap would break anyway after a few uses, we all know that from using N95 masks in China. So I totally agree with their conclusion that people can essentially keep reusing them until they break. The VA goal was 50 times re-use; in the real world even 5-10 times re-use would be extraordinarily useful for us front line covid-19 workers.
Not taking drying times into consideration, “5-10 times re-use would be extraordinarily” – Is “times re-use” used to indicate days or incidents of possible exposure? Example: You work in a grocery store at the register 5 days/week. This means after 2wks it’s best to throw the mask away? or re: incidents of exposure – as a MD, you see 10 Covid-19 patients/day. Would you throw away your mask at the end of the day?
Hello! You all are doing a great job with all this valuable information you are providing the world. Would you please consider testing the efficacy of ozone to sanitize masks & at what milligrams & amount of time would be effective with lowest degradation of the face mask?
Also, there is a company called Battelle who is steaming masks with hydrogen peroxide up to 20 times. Could you also test the efficacy of hydrogen peroxide soaking & steaming the mask with hydrogen peroxide, of course air drying thereafter?
Thanks again for all you do!
Hi Nickol. Song here. Ozone, hydrogen peroxide soaking & steaming should be able to disinfect the mask too! But we haven’t tested it yet and not sure whether the disinfection method will harm the filtration efficiency. There is a simple way to disinfect your mask, not by microwaving, not by ozone, just simple leave you mask out and let it dry, here is the article with more details: https://smartairfilters.com/en/blog/disinfect-clean-n95-mask-virus-coronavirus/
Can you show us what is the best way to make a mask at home? I know there is a lot of Youtube videos, but none of them have tested them.