Can sunlight sanitise your N95 / KN95 / surgical face mask so you can reuse them for weeks or months?
Find out why this claim has resurfaced, and what the facts really are!
Claim : Sunlight Can Sanitise Your Face Mask For Weeks Of Reuse!
When the COVID-19 pandemic started, personal protection equipment (PPE) like N95 face masks quickly became scarce, so scientists looked at whether they could be sanitised and reused.
When a May 2020 study in the Journal of Infectious Diseases showed the sunlight could potentially kill the SARS-CoV-2 virus, people started sharing advice on sanitising face masks using sunlight.
Infectious disease physician Dr. Faheem Younus recently reignited this claim, when he reshared the same study on Twitter and offered his “pro tip” :
Buy two KN95 masks. Wear one today, leave the other in a paper bag, in your car. Alternate daily.
Keep alternating these 2 masks for weeks/months as long as they fit.
Sunlight kills ~90% of the SARS-CoV2 virus in 7 minutes!https://t.co/LA9VwOKvNQ
— Faheem Younus, MD (@FaheemYounus) September 9, 2021
People started sharing his advice on social media – Facebook and Twitter, and even on WhatsApp.
But is Dr. Younus correct? Let’s find out…
Truth : Sunlight CANNOT Sanitise Your Face Mask For Weeks Of Reuse!
Here is the short and simple truth – sunlight cannot sanitise your N95 / KN95 / surgical face mask for weeks or months of reuse!
And here are the reasons why…
Fact #1 : Study Used Simulated Sunlight On Simulated Saliva
The study that Dr. Faheem Younus shared was published in The Journal of Infectious Diseases on 20 May 2020.
That study showed that simulated sunlight “may rapidly inactivate SARS-CoV-2 on surfaces” – 90% of infectious virus in simulated saliva was inactivated every 6.8 minutes. That much is correct.
However, the study was a preliminary look at how natural sunlight “may” be an effective disinfectant against COVID-19.
The study does NOT actually prove that sunlight can adequately sanitise N95 / KN95 / surgical face masks.
Fact #2 : Study Only Looked At Non-Porous Surfaces
The study only tested the effect of simulated sunlight on non-porous surfaces – simulated saliva / culture media on stainless steel coupons.
The results of that study on non-porous surfaces cannot be used to deduce the effect of sunlight on porous surfaces.
Face masks may have a hydrophobic outer layer, but they all have to be porous to allow air to pass through.
That not only allows the SARS-CoV-2 virus to penetrate into the inner filter layer, it would likely make it harder for sunlight to penetrate into the inner filter layer to sanitise it.
Fact #3 : Car Windows Often Block UV Rays
Modern car windows often block UV rays, especially the front windshield. The sanitising effect of sunlight is therefore greatly reduced.
The UV ray blocking effect is even higher in cars that have window films applied to block solar radiation.
In short, placing your N95 / KN95 / surgical face mask on the dashboard may not adequately sanitise them.
Fact #4 : Paper Bag Blocks Sunlight
This may seem obvious, but it has to be said – paper bags block sunlight.
Dr. Younus’ advice to place the used face mask in a paper bag negates the sanitising effect of sunlight by directly blocking it!
Even without the UV-blocking properties of modern laminated glass and solar films, the paper bag would ensure that ZERO sunlight reaches the face mask inside.
Fact #5 : Face Masks Are Contaminated By Other Microbes Too
Even if you hang your face mask on the clothesline for sunlight to directly sanitise it for several hours, it may not be that safe to reuse.
When you wear a face mask, its inner hydrophilic layer readily absorbs moisture and microbes from your breath.
And when you speak, this same layer absorbs your saliva, tiny bits of food and of course, microbes from your mouth.
These microbes that were absorbed into the hydrophilic inner layer will multiply and could pose a health risk, should you keep reusing the same face mask over and over again.
Even hanging the face mask in sunlight for hours may not adequately sanitise the hydrophilic inner layer.
Fact #6 : N95 Face Masks May Be Safely Reused For Up To 3 Times
In April 2020, the US NIH tested four decontamination methods that would allow for N95 face masks to be safely reused, up to 3 times.
- ethanol spray : safely reuse once
- 70°C dry heat : safely reuse twice
- ultraviolet light : safely reuse twice
- vaporised hydrogen peroxide : safely reuse three times
There is simply no decontamination method that would allow any N95 / KN95 / surgical face mask to be reused for weeks or months!
I should point out that the US NIH is not recommending the use of sunlight to disinfect the N95 respirators, but a high-powered LED UV germicidal lamp placed just 50 cm away from the respirator for about an hour.
Even dry heat at 70°C in an oven takes about an hour to reasonably decontaminate the face mask.
For those of us at home who may want to “stretch” the use of our face masks, you can use a 70% ethanol spray to quickly decontaminate our face mask and reuse it once.
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