Can You Use HVAC Filters in Coronavirus Masks?

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Droplets from a sneeze can travel 18 feet across a room

Back in 2018 I wrote a series of articles on indoor air quality and filtration.  One of those articles, The Unintended Consequences of High-MERV Filters, has seen a flurry of activity in the comments over the past three weeks.  It all started when a reader named Pat wrote:

I have a question about merv 13 filters. People are using them in between fabric on face masks. I saw some are made with fiberglass in them. Is this safe?

That was the first of 33 comments (so far) on the topic of homemade coronavirus masks and HVAC filters.  So let's take a deeper look at this subject, including the reasons you should be wearing a mask in public, the issue of what kind of mask will help, and whether or not it's safe to use high-MERV HVAC filters in your mask.

Why you should wear a mask when you go out

The coronavirus, as you may have heard, is wreaking illness and death among humans (and apparently even showing up in tigers and lions, too).  For something that is arguably not even alive, this thing is quite adept at reproducing itself, with a little help from the cells in your body. Here's a quick review of what we know:

  • This new coronavirus (technically, SARS-COV-2) is very infectious.
  • Researchers haven't figured out all the ways it spreads.
  • Sneezing and coughing can send droplets across room.
  • Just breathing also puts droplets into the air when you exhale, but they don't go as far.
  • Large droplets can transmit the virus from one person to another through the air or when they land on a surface that is later touched by another person.
  • Small droplets can float around in the air for an extended period of time (hours? days?) and be breathed deep into the lungs.

There's still uncertainty about whether large or small droplets cause more problems, but it seems that maybe it's the larger ones since the infections seem to start in the upper part of the respiratory system.  Still, it's best to avoid contact with coronavirus droplets of any size. 

There's a difference in how to avoid large and small droplets, though.  If you're breathing air without any kind of mask, you're likely to breathe in droplets from other people when you're not alone.  You're also likely to put droplets into the air that others could breathe.  All it takes is one person infected person in a room to spread the coronavirus to others through those droplets.

Your mask options

When it comes to masks, you have three basic options.  You can use:

  • Homemade mask
  • Surgical mask
  • N95 mask

Wearing an N95 mask in public

As you move down in that list, the protection you get improves.  The best filtration of both large and small droplets comes from an N95 mask, but if you've tried to buy any lately you know they're not available.  (The N95 mask I'm wearing above was the last one I had at home, and I just broke it out on Saturday for my first time wearing a mask in public.)  Surgical masks are in short supply, too, and you should leave both of those types for medical professionals anyway. 

A research paper by van der Sande et al. from 2008 looked at the capabilities of these three types of masks and found:

  • N95 masks are better than hospital masks.
  • Surgical masks are better than homemade masks.
  • Protection for the person wearing the mask (inward protection) is better than protection for others (outward protection).  (See Figure 5 in this article for a summary of the van der Sande results on this.)

If you'd like to know more about this aspect of mask-wearing, see the excellent article, COVID-19: Why We Should All Wear Masks — There Is New Scientific Rationale.  That article also does a great job explaining the spread of coronavirus through droplets.

Using HVAC filter material in your homemade mask

A homemade mask made of cloth is definitely better than no mask at all.  Its outward protection reduces the small droplets that get out by about 10%, but the inward protection is impressive.  Only about one third of the small droplets outside the mask will get through when you inhale.  They do a good job with the large droplets in both cases.

So if you want to wear a homemade mask and also have it remove the small particles, what can you do?  One idea that has occurred to people is to put a piece of HVAC filter fabric between two pieces of cloth in a homemade mask.  And that's how the discussion in my article on high-MERV filters began.

A homemade coronavirus mask, made from a folded bandana and a couple of hair bands

The first question that comes up is about whether using filter material designed for HVAC systems is safe to breathe through when you put it on your face.  There are a lot of companies that make HVAC filters and  probably different materials used by different manufacturers.  What I've found out is that high-MERV filters, like MERV 13, are generally made out of polyester and cotton and do not contain fiberglass. 

If you talk to manufacturers, they'll probably warn you not to use them to breathe through directly because that's not what they were designed for.  3M, which makes the Filtrete brand of HVAC filters, has a page on this topic and says:

Our filters are designed to be used in HVAC systems, and the filter media has not been tested to be used as a face mask for respiratory protection. Altering any of our 3M Filtrete™ Air Filters is not recommended or supported by 3M or the Filtrete™ Brand. Customer safety is our number one priority.

It's possible they use some kind of binder or other material that offgases toxic chemicals.  If you call them up, they won't tell if they do.  They'll just give you a version of the statement above.

The online company Air Filters Delivered, however, is selling MERV-13 filter fabric for use in coronavirus masks.  They even have a page with instructions on how to make your own coronavirus mask with a pocket that can hold a tissue or a piece of HVAC filter fabric.

Anthony Grisolia, a friend of mine who works for building science firm Ibacos, and his wife recently posted a video on Youtube showing the homemade mask they made out of MERV-13 filter material.  Check it out.

The key points

Stopping the spread of this coronavirus is paramount.  As the article I referenced earlier states, we should all wear masks.  Assuming you don't have a stash of N95 or surgical masks, you need a homemade mask.  If you just wear a plain cloth mask with no extra filter added, you'll be much better protected than you are without a mask, especially from the large droplets.  (Of course, there are a lot of caveats about bypass and contaminated masks, too, so be smart about it.)

Going to the next level and adding a piece of MERV-13 HVAC filter fabric to your homemade mask improves its ability to filter out the small droplets.  Is it safe to breathe through HVAC filter fabric?  You'll have to determine what level of risk you're willing to take in that regard.  I personally feel OK with doing that, so once my last N95 mask is kaput, I'll switch to the homemade mask with a piece of MERV-13 fabric for my occasional excursions from home while the coronavirus is still spreading.

This virus is not something to mess around with.  John Prine died of COVID-19 yesterday.  UK Prime Minister Boris Johnson has spent the past two days in intensive care with it.  I've got two friends who are struggling to get over it, one of whom had been hospitalized.  It's not something I want to get.

 

Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard.  He is also the author of the Energy Vanguard Blog.  You can follow him on Twitter at @EnergyVanguard and pre-order his upcoming book at Publishizer.

 

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Comments

It is eerie how we operated in parallel - my first public masking was with an N95 at a HomeDepot - my N95s are leftover issue from my time in law enforcement.

My wife, superb with a sewing machine, has produced about 30 homemade fabric masks each with a pocket that holds a removable section of...drumroll...MERV 13 HVAC filter material I obtained as an HVAC contractor

She also inserted another layer - blue polypropylene cloth apparently similar to what the University of Florida has worked up from materials commonly already on hand in hospital settings.

Hello Curt,
I am in dire need for a well-made mask as you have described. Are any for sale?

Dear Curt,
I am a very experienced seamstress who is now making masks with pocket for filter. They are handmade in Pennsylvania with a choice of many Fabrics sizes are lg,med and child. If anyone wants to get in touch with me they can either get find me at erikarelyte@hotmail.com or my Etsy shop spunbymedesign. Thank you,Erika Relyte

Mine was one of those 33 comments. Thank you for an excellent and timely article!

Great article. It lead me to www.airfiltersdeliverd.com, with their own instructions for using the air filter material for masks. I noted that the washable filters were out of stock (We used them heavily aboard Merchant Marine vessels, particularly for crew and passenger staterooms.), and they recommended the MERV-13 as the correct filter for viruses, but that brings up the issue of breathability. I've used twin cartridge respirators for 10-hour workdays, 8 days in a row, removing lead paint, and it was exhausting. Probably one more reason lead paint removal is expensive. I needed filters for my air handlers, too, but just MERV-8. My neck of the woods doesn't have much choice, or i wouldn't have added the URL to this post.

A 2009 article in "Annals of Occupational Hygiene" on the effectiveness of sterilizing N95 masks. Baking them at 160 or 170 F for half an hour was one option. However the most recent missive from Stanford said: "Current Evidence-based Reviews:
Do not use anything in your home to disinfect contaminated equipment. Please do not heat your masks in a home oven! Our reports do not advocate or advise on practices and suggest that you strictly adhere to your hospital's policies and procedures."

Stanford is disinclined to go against any recommendation of the CDC as is shown in this article:
https://stanfordmedicine.app.box.com/v/covid19-PPE-1-2

"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."

Keep in mind this was a recommendation being made specifically to health care professionals, and the stance taken was directed by the risk of spread of disease, not because the proposed method was deemed ineffective.

There's another benefit to wearing a mask as simple as a bandana (though not recommended of course). We're all careful about wearing gloves or thoroughly sanitizing hands when we do have to go out for food and essential missions, but those of us who are not medical professionals and not used to wearing masks daily, are finding out how difficult it is to not touch our face for an extended period of time.

Three weeks ago I went to Costco - the last time I ventured out - and took three pairs of gloves to change when needed. Little itches drove me crazy! The next time I go out I'll definitely have a face cover so I can touch my face without concern of transmitting whatever is on my gloves to my face.

Every disaster or major episode like this leaves a legacy. 9-11 left us TSA and the DHS. Katrina challenged FEMA and taught us bout IAQ in temporary housing among other lessons. After COVID-19, I suspect we'll become more accustomed to masks and shake fewer hands as has long been common in most Asian cultures. We'll probably be more aware of air filtration and hopefully pay more attention to the V in HVAC. We'll see how smart we are as a society as all this winds down.

So I decide to make my own protector and mask. This is Cheap stuff...
1 Clear sheet protector (from your office)
1 Shoe string
2 Sleeves from old T-shirt

Weave the shoe string thru the sheet to tie behind your back. Cut the 2 sleeves from an old T-shirt you can fit thru your head (Athletic t-shirts are better, and you can still wear them sleeveless). You could use more than 2 sleeves or attach additional fabric between two the two sleeves. You are done!

Allison
Bailes

Very clever, Armando!

Here's the photo you emailed me, for anyone who wants to see what it looks like:

Simple coronavirus face protector and mask, from Armando Cobo

I noticed the comment on filters, particle sizes, and the size of the SARS-CoV-2 virus. Filters, including the N95, pass particles the size of the virus, however, because the virus exists in a droplet, it is larger, and the var der Waals force attracts it to the filter media. It gets attached and the droplet evaporates, leaving a dead virus. Job done. 3M has published credible peer-reviewed third party studies confirming this.

Thought you would like to know. Cheers.

Sorry,Robert but I think if you read the Lancet and or CDC guidelines you will find that coronavirus lives even longer on mask material, n95 for example, than it does on glass, cardboard and paper. So your conjecture about the droplet evaporating and leaving behind a dead virus particle is quite mistaken.
Maker/wearer beware!

I'm worried about bypass with the MERV-13 filter. With the added resistance, it feels to me that air comes in around the filter which may negate its effectiveness.

"Breathe-ability" includes not just how hard it is to take and expel a breath, but how much leakage around the filter (and thus our reluctance to wear one). Home-made masks mostly don't seal well, even with the metal that forms it at the nose. That said, a home-made mask is STILL a lot better than none. Those N95s with the exhaust valves. Mine were in my workshop, and another box of ten in my CERT supplies (Community Emergency Response Team) for the certainty of the next volcanic eruption and ash-fall. So they had the relief valve. Not good for people in range of an exhale, but still better than nothing, as it confines the exhale to a smaller, less forceful area. My next restocking of our CERT kit will have N95s with valves for non-medical purposes, and either the N95s without the exhale valve, or the surgical type - strictly for medical.
CERT just makes sense. Look it up. Join or form a local CERT. Sometimes a city or county has an Office of Emergency Preparedness, or something similar, and has a CERT Program. Ours is run by the Borough, and sends out automatic texts or robo-phone warnings in an event. It is volunteer, provides training, has several sub-catagories to suit different people and dangers. (Medical, structures, fire, forest fire, flood, paperwork, Incident Command System, etc.) Great people, mostly your neighbors.

This video has an excellent (IMO) mask that you fit to your face and you can make it fit well under your chin, cheeks, across the nose. It's easy to make as well. We chose a MERV 13 to use and we double it. I have asthma and have had no breathing issues. Go to the .40 sec. mark and see how the mask fits.https://www.youtube.com/watch?v=zm_NmpdgWWA .... East for a novice sewer as well. Thanks much!

3M electret technology where used in facemasks or HVAC filtration can have a positive effect on the filtration vs. Sp resistance dance. MERV rating performance(in HVAC)and actual COVID 19 / CDC guidance will be an important element before any actual next steps.

Current market and supply chains / decisions are in play now.

The material being sold by air filters delivered is not cotton/polyester. The media is treated with Polypropylene / Acrylic Fibers per the website and specifications link.

Thanks for this information, but it does drive me nutty that there is always this default to Home Depot. Just an FYI, the LOCALLY OWNED hardware stores, such as Ace, carry filters too. Thank you.

Fiberglass, really? That seems nearly as bad as inhaling asbestos. If you can get the kind that doesn't have fiberglass, okay. But under no circumstance would I advise anyone to inhale particulate matter the shape or size of fiberglass. You'll trade a possible COVID infection for lung cancer or pulmonary hypertension. Fiberglass can also lodge in your eyeballs & nostrils, causing bleeding & other painful issues.

Of course, we don't want to breathe fiberglass particles. But the scare videos and articles that have appeared regarding mask making have outlandishly exaggerated the problem. Firstly, because fiberglass is not an ingredient of the high-efficiency filters that are being suggested for use as masks. Secondly, because 100% of the filters under discussion are intended to have air blown through them, which will then be breathed by people. Whether in the furnace, vacuum cleaner, or in a mask, these filter products are created and sold with the intent that some of the filtered air will end up in people's lungs.

Could you reuse your n95 by leaving it out in the sun all day?

Stanford researchers have said putting n95 mask in over at 158 degrees for 30 min will sterilize it.

One approach being used is cycling through at least 5 masks and storing them in individual cloth or paper bags to allow them to dry and enough time for the virus to degrade. The virus will be trapped between plastic filter fibres, so I would imagine it will take a lot longer to degrade than the 3 days it remains viable on a normal flat plastic surface.

Some manufactures eg 3M advise against heat or microwaves to sanitise their masks as it will damage them, and alcohol apparently degrades the filtration badly. Spraying with a suitable dilution of hydrogen peroxide is another possibility.

Vapourised hydrogen peroxide is being used to sanitise single use respirators in some US hospitals.

There's a guide here about what's been tested:-

https://www.cdc.gov/coronavirus/2019-ncov/hcp/ppe-strategy/decontaminati...

Hope that's some help.

Please address protection of your eyes as well.
I never see articles or tv reporting on contacting the virus via eye ducts. Since it is extremely contagious than protecting your eyes should be just as important. And eyeglasses are not sufficient. Thank you Armando for addressing this.
Keeping your eyes and nose moist is important as well.
And I'd sure we all agree w Christopher in that every article, post, comment or newscast should mention supporting your LOCAL stores instead of large chains. Thank you for the great article Allison.

2 points 1) Ironic that we would filter the air for our homes with the same air filters that the manufacturers are leery of suggesting we use to filter the air in our masks. (I understand that cutting the material could produce unintended consequences). 2) I've noticed certain N-95 rated masks offer a one way pressure relief valve for the user to more easily exhale. This helps the mask wearer retain comfort but greatly reduces the effectiveness of the mask for others. This was mentioned above (sort of) but I've yet to see any discussion of this in honest anywhere up until now.

So I've looked up the specs for a BestAir MERV 13 filter. They have listed "synthetic" as the media material. Fiberglass is a synthetic material, right? Not sure what to think. Don't want to use a fiberglass product. Seems too vague a description. Do you think this could be code for fiberglass?

Yes,Fibreglass is a synthetic material.
So is Polyester, polypropylene, Kevlar, Rayon, nylon, Terylene, Acrylic...nedd I go on?
You're not assuming that a product made to purify the air you breathe would be made of a product that can kill you?

The New York Times printed a brief item on Friday 4-17 that a homemade cloth mask with a pocket, into which two layers of paper towel material was placed, intercepted 25 to 30% of virus-sized particles. Not a home run, but better than cloth alone.

I'm putting a sheet of copper mesh in mine as well, as copper ions do a number on viruses. The stack:

- microfiber cloth (water resistant)
- copper mesh
- MERV from a 2200 filter
- microfiber

Iggy

I thought homemade masks protected others from you. Have I gotten that wrong? I believe surgical masks protect you from large droplets, but not small (ie., viral size). N95s protect you and others, unless they have a valve. In the valve case, they only protect you. Have I got it?

Here's the most detailed reference to "viral load" I've been able to find.
Bottom line......no one knows for sure how much or how little of this thing is able to make you sick. BUT, so far, it doesn't seem to matter. If you "receive" ANY particles of it.....you're just as likely to be infected as if you were exposed to a "shit-ton".

I wanted to use the gauzelike material from an eltrostatic air filter in my homemade face masks. Would this be harmful?

Hi, seems the focus is on Merv 13 materials. Why not a higher Merv rated filter? Isnt more better after all? or is there some downside to consider?

I currently have two layers of cloth and a hypoallergenic pillow case material with advertised pore size of 3-4 microns. I'm in the process of making a pocket to add several layers MERV 13 ripped from a home heater AC filter. Using 4 layers of that was tough to breath through. I'll probably go three. Still not sure best way to sterilize the MERV 13 but I think I'll end up heating it in an oven bag to 200 degrees for 30 minutes - I think I heard that on a video from Texas A&M homemade masks. Comments appreciated.

Uv light is an effective way of sterilizing masks for reuse according to the cdc.

People on Etsy are selling Merv-13 mask sized filters that are advertised to be reusable (washable) and free of fiber glass particles. But now there's talk of chemical binders. I'm not sure I can trust someone else's assurances of non-toxicity. I'm looking into the fabric that medical instruments are sterilized in. It is supposed to keep out very small micron viruses. Expensive, but also durable.

I know I'm late to this conversation, and I have no affiliation with this website, but I found some great COVID 19 face mask Before this purchase, I have been having a hard time finding masks that are comfortable and actually stay on. The earloops are truly amazing. I hope you can protect yourself.

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