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Building Science

Using HVAC Filters in Coronavirus Masks

As people turn to homemade masks for protection, innovators are cutting up HVAC filters to include in them for extra protection

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 new 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 a 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

Allison Bailes wearing an N95 mask in public [Image by Energy Vanguard]
The author, Allison Bailes, 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 [Image by Energy Vanguard]
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 the 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 here.

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. Photos courtesy of the author.

14 Comments

  1. Trevor Lambert | | #1

    Where does a P100 respirator fit in on this list?

    1. Darren Adams | | #13

      Trevor, no personal expertise but simple web lookup: P = oil resistant, not a relevant consideration here. 95 = 95% particulate removal vs 100 = 99.8% particulate removal. So, P100 wins.

      Aside from looking a little extreme by comparison, I'm very happy I had a few spare sets of P100 cartridges for my two respirators.

      From our Chemical industry and Nursing backgrounds, as commented below it is all about tight fit to avoid bypassing. Shave, and if you have them, trot out your respirators when you go out. But when not, use whatever you have as something is better than nothing.

  2. Andy CD Zone 5 - NW Ohio | | #2

    Thank you for this article. Just a reminder, however, that it makes no sense in any risk analysis to be wearing an N95 mask in public, such as outside a Home Depot, as long as social distancing is maintained. One of the lesser masks is very protective. Any medical or nursing home worker would weep with gratitude if you would donate that N95 where the risk is exponentially higher.

  3. James Someone | | #3

    I believe Mr. Bailes was wearing the N95 to protect himself. Maybe it's overkill, maybe it's not. That's for him to decide. The message hasn't really been clear from our higher ups since the beginning of this mess. I OWN A FEW N95's, always believed in PPE working in the trades, while most guys didn't.

    I will protect myself first as I see fit, as I know no one else wilI.

    I wonder if breathing thru a filtered cigarette protects from silica exposure, that's what most masons believe.

    Let's keep GBA focused on building science not making a Gestapo out of it shaking everyone down for a mask.

    1. Andy CD Zone 5 - NW Ohio | | #7

      James, somewhere between your rugged individualism and the Gestapo lies empathy, which is most certainly an individual choice. Nobody's coming to pry the N95's from you.

  4. user-7520471 | | #4

    Well said!!

  5. User avater
    Kris Anderson | | #5

    According to this article, written by the author, a MERV-16 filter would match N95 particulate filtration:
    https://www.energyvanguard.com/blog/7-reasons-your-filter-isnt-improving-your-indoor-air-quality

    Why not use MERV-16 filter material?

    1. Andy CD Zone 5 - NW Ohio | | #8

      Kris, furnace filters may or may not match the particulate filtration of an N95, but the critical aspect is how well it fits your face. I've yet to wear a homemade mask that contours to my face as tightly as the N95 or the cartridge masks. No filter works much if there's bypass.

      1. Malcolm Taylor | | #10

        Andy,

        Up here anyway the construction industry lags behind others in safe practices during this virus. I've been very impressed by the measures my local stores and businesses have put into place, so was shocked to go to my regular lumberyard, see all the contractors arrive in one truck, and inside all lean on the sale desk exchanging plans and receipts cheek by jowl. They must experience some real cognitive dissonance when they go home and see the lives their spouses and children are living.

        1. Tyler Keniston | | #11

          I hate to say it, but my local hardware store might have lost my business for the foreseeable future due to an utter lack of any measures taken against virus spread. One wouldn't have had a clue anything was going on being in there (other than me wearing a buff over my face and feeling a lone fool). I've heard similar stories in automotive trades.

          1. Malcolm Taylor | | #12

            Tyler,

            Hats off to my local Home Hardware store in Sooke BC. They have a greeter maintaining a line up outside with everyone six feet apart. Only a few customers allowed inside at a time. Plastic screens at the cash, and constant wiping down of the counters. Not surprising. The businesses that contributed to the community in the good times, still stand out in times like these.

  6. Roger Berry | | #6

    The link "Covid-19 Why We Should All Wear Masks" is very convincing to me. The latest news about not sharing space with vigorously exercising people, while not terribly scientific, should also make one cautious about "social distancing" parameters. Someone made a similar observation about potential extended droplet range upon realizing that they were smelling the cigarette smoke of a person walking 20' ahead of them. At least part of the smoke they were smelling quite likely came out of the lungs of that person. What is the potential virus load in the droplets exhaled and carried that distance? I certainly do not know and have no desire to find out the hard way.

    I, for one, will be rigorous in wearing a mask when forced to venture out. I cannot psychically see who may have passed through a store aisle ahead of me. As a wood worker I know just how long the tiniest dust particles can hang in shop air. Applying that experience in guarding against Covid -19 make choosing the best mask a no-brainer.

    I am fortunate to have a few masks left from a bulk purchase 10 years ago, however as noted in comment #2, critical care staff should get all certifiable masks available. For the rest of the world that now finds itself trying to make masks that work, the following link may be useful. As for furnace filters, I would certainly hope that none are made of friable materials that would be injurious to lungs or health. The blue towels mentioned in the article are likely being sucked up as I write.

    Many of the mask patterns and suggestions out there apparently do not perform well enough despite good intentions. The people in this article at least have made an attempt to document their work efforts. The sooner we eliminate or sharply reduce the human pool of virus production the better. The non-symptomatic spreaders will be lurking for some time and masks for all for the months ahead will be critical in slowing infection and death rates.

    https://www.businessinsider.com/homemade-mask-using-hydro-knit-shop-towel-filters-better-2020-4

  7. User avater
    Fernando Pages | | #9

    I too have an N95 that I wear to the supermarket (and Home Depot) and feel a little guilty. But I had ONE and I used it BEFORE knowing to donate it to the hospital. So I use it still, spray it down with Lysol between outings. I have read about Tyvek WRB as an excellent facemask material, and there's plenty of that around. I have not tried it, but I wonder how easy it would be to breathe through it.

  8. PBP1 | | #14

    Careful: "since the infections seem to start in the upper part of the respiratory system".

    SARS-CoV-2 requires the enzyme ACE2 for entry. It is the lower respiratory tract that has an abundance of cells with ACE2. Some articles have speculated why nasal swabs can be ineffective at early detection because the URT is not the primary source of infection; rather the LRT. That is not to say that URT can be infected as well because the URT does include cells with ACE2.

    Article: "the predominant human receptor for the SARS S glycoprotein, human angiotensin-converting enzyme 2 (ACE2), is found primarily in the lower respiratory tract, rather than in the upper airway. Receptor distribution may account for both the dearth of upper respiratory tract symptoms and the finding that peak viral shedding occurred late (≈10 days) in illness when individuals were already hospitalized".

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