DIY air filter protects an entire room from COVID-19

Will this pandemic ever end? We finally have vaccines, and we still have breakthrough infections. We finally reach herd immunity from vaccination, then a new variant is 50% more infectious and brings the pandemic roaring back. We’re not even recovered from that when another variant comes along, even more infectious and a bit resistant to vaccines. At this point we need 150% of people vaccinated to reach herd immunity again, and 6 months after that another variant will set us back to the beginning.

The problem is that we’re relying too much on vaccines. They’re a great tool, but just not enough on their own. That’s why we need a layered approach.

Every COVID precaution has some failure rate or side effect that prevents it from being the magic bullet that fixes everything. That’s okay. We can make up for this by using several layers of protection. If one layer fails 20% of the time (vaccine) and another layer fails 20% of the time (N95 mask) then you can combine both precautions for a failure rate of only 4% (odds that both fail at the same time). If that’s still not enough, just add a third layer.

Here’s one more layer you can add, either for added protection or as a replacement when, for whatever reason, you can’t wear that mask.

One of the most overlooked precautions is also the easiest to keep up long term. The virus can only spread if you breathe it in. Cheap and effective ventilation systems remove viruses from the air, so that you can safely breathe that air without getting infected. With enough room air filtration, it becomes possible for people to share a room together without spreading infection, even if none of them are wearing masks. This is especially important for places where people need to remove their masks, such as dental offices or restaurants. It also makes a great backup layer for especially high risk environments, like classrooms or shared offices, where it’s difficult to wear a mask properly for many hours at a time.

Just build one of these filtration systems for each place that you want to protect. One unit is enough to hang out with friends, especially if you spread out a little with the fan placed between you pointed up. Three of them will provide supplemental protection for a typical, crowded classroom, in which everyone is still wearing masks.


The best part about layered defenses is that you can always afford to drop one of the layers, and the remaining layers will still take care of you. A filtration system is just one more layer you can use, so that you can safely drop any other layer if it’s getting in your way.

There’s a whole menu of layers that you can choose from:

  • Filtration System – keep the above air filter near you
  • Outdoors – because the ventilation is built-in
  • N95 Masks – for when you need to visit areas that you haven’t secured
  • Vaccines – 5 months of easy protection
  • Distance – just stand further away while talking

(BTW, hand washing and scrubbing doorknobs don’t belong on this list. They might help a little bit, but not nearly as much as any of the above. COVID-19 usually spreads through the air, not by touching surfaces.)

I always aim for at least two layers of protection, and sometimes add a third if I need to be in a crowded room. Decide how many you feel comfortable with – you definitely don’t need to use all at once.

And the best part about filtration systems? If one business owner installs filtration, every single customer is that much safer and doesn’t need to work so hard on protecting themselves. Bring your own filter, or ask your favorite businesses to take care of it for you. You and everyone around you will be that much safer.

Imagine if all of our public spaces had good ventilation. Everyone would be protected, even those that refuse to protect themselves. We might finally end this pandemic.

Until then, this is one more tool that you can use to protect yourself and your community.


  • Comments (39)

    • 9

      Air filtration is definitely helpful, but I wouldn’t rely on it as a sole fix. No matter how powerful your system is, if someone coughs or sneezes (or breathes heavy, or sings) in close proximity to you there’s a significant potential for exposure. I’m still wearing masks in public places. When with pods of people (like vaccinated family members) where masks come off, I still advocate for open windows when possible–but of course none of that precludes using good air filtration as well.  

      • 3

        Completely agree. All of these approaches work best in layers. You mentioned choosing a different set of two layers at home (vaccine + window ventilation) and another set of two layers in public (vaccine + mask).

        I’m also thinking about what would be more effective at a community level. If one business owner adds building ventilation, quite a lot of people get that protection without even needing to think about it.

      • 6

        I am copy and pasting a response from Ari Allyn-Feuer:

        “Stephanie is right.  This will help and it is a good thing to do, but it isn’t a magic bullet.
        Also in theory you could measure CFM values and so on, and find out how many air exchanges per hour this is doing, and compare it to hvac standards for airplanes and hospitals and so on.”

      • 3

        CFM sounds like a decent place to start. Rather than airplanes, I would start by comparing the CFM between this filter and human lungs.

        A person inhales about 5-8 liters per minute. Let’s just chose 7 liters per minute for simplicity. If we want to ensure that the filter traps 95% of the viruses before they get into human lungs, then the filter needs to move 20x more air than all the people in the room. So 7 * 20 = 280 liters per minute per person.

        The fan used for this recipe is rated for 2400 CFM. There are 28 liters per cubic foot, so the fan is moving 67,200 liters per minute. If we need 280 liters per minute per person, then this one fan pushes enough air through filters to protect 240 people.

        This is an oversimplification, based on the assumption that viruses are spread evenly over the room. As Stephanie pointed out, when someone breathes viruses straight into your face, the filter on the other side of the room won’t have time to help you. But combined with moderate other precautions like distancing, that filtration still makes a huge difference.

        I also pretended the fan would move air just as fast when filters were slowing the air down. If those filters slow the air down by 2x, then we drop from 240 people to 120 people, still great for such a cheap device.

      • 1

        Eric didn’t suggest air filtration as a stand-alone measure, but one of several, advocating the use of two or more at any one time. I think it’s a very good idea. We’re vaxxed and I got the booster on Nov 22, switching to Moderna from Pfizer. We’ve worn masks every time we go out in public since March 1, 2020, not stopping even when many others have stopped wearing them. I would feel more comfortable if the few places we go had a good air filtration system.

    • 6

      While I agree multiple layers of protection are great and that filtration does help with covid… I don’t know that I’d be comfortable going maskless in my workplace. Are there any studies showing that is okay? 

      Thanks for showing the kit and materials though Eric!

      • 3

        If you’re relying heavily on a mask for protection, just make sure it’s a high quality mask (N95 or similar) and that it seals tightly against your face. Here are two relevant guides about selecting masks and fitting them to your face.



        Most people are wearing cloth masks that don’t seal around the edges. Those are only about 10-20% effective, and people with such ineffective masks would be much better off switching to good ventilation.

        I’ll look for some details and references regarding ventilation effectiveness and post those later. I am definitely not saying that ventilation should be your only protection, but rather that multiple layers are needed and this particular option hasn’t gotten the amount of attention that it deserves. Masks are also just one of the options for a protective layer, and not strictly needed if you have some other layers in place.

      • 2

        Ah okay thanks for clarifying because I interpreted your original post to be along the lines of “tired of masks? don’t wear them anymore!” 

      • 2

        “I interpreted your original post to be along the lines of ‘tired of masks? don’t wear them anymore!’”

        The “no mask required” in the title needs some more explanation. Here’s where I was coming from…

        First, I love masks. I’ve been wearing both N95 as well as goggles since Feb 2020, well before anything like that was commonly recommended. But there are times when masks are just not an option. I haven’t been to the dentist in two years because I can’t wear a mask while someone pokes around in my mouth. I needed to adjust the mask slightly while getting fitted for new glasses, breaking the seal for a moment, and came down with a mild illness that evening (thankfully not COVID). There need to be other options for protection.

        Second, I know plenty of other people don’t share my love of masks, and I want to make sure they have options. The “no mask required” was deliberately to attract people that don’t agree with me. I’m pleasantly surprised that everyone who responded was pro-mask. Smart people on this site.

        So, was the “no mask required” too much? Feedback/advice on this is appreciated, and I’m certainly open to making changes.

      • 5

        I was just going to comment on this, but since you asked for feedback, here it is: the tone of your post is playful and it’s clear you’ve tried to be helpful, but some of the wording did throw me off as well. “No mask required.”, “Free at last!”, “you can relax and do what you want”, etc felt a bit like absolutes, especially since they were not backed up by things like studies that confirm these statements. I think I would have reacted differently if you had just said something like: “Here are instructions on a proven DIY method of filtration that can help as an added measure of protection, etc”. IMO there’s no need to change anything in your post, especially since it’s clear by all the comments that it was not your intention to give medical advice, etc.

        Having said that: I love the schematics, I didn’t know about this project but looked into it and it seems like a great idea for improving the quality of air in places with poor ventilation. And I love all the citizen-scientists projects! Kudos also in putting together the Kit! Thanks for this.

      • 3

        Eric, thanks for putting together the kit and linking to the schematics. Like many here I didn’t know that this option was available – since the beginning of the pandemic we have upgraded our home HVAC filters to filter viruses too for the very reason of trying to increase our protection. It probably does not compare to a filter that sits in one room (hence filtering more particles, I guess?) but both I and my husband work from home, rarely do anything in public, and when we do we take all the precautions possible so I feel it’s ok for us.

        And thanks for asking for feedback: my two cents are that I can see how some things could have been interpreted in the wrong way, but, as Main Pugh said I don’t think there’s any need for you to change the post at this point. 

      • 2

        Carlotta, I am already reasonably safe at home, similar to your situation, but would still like the added layer of protection. Even if it turns out to be less effective than a standalone unit, I like the idea of upgrading the home HVAC to avoid introducing a bulky piece of furniture. What was your approach for upgrading your home HVAC? Was it already strong enough that you could just swap in a MERV 13 filter?

      • 2

        Carlotta and MainPugh,

        Based on your feedback, I’ve edited the title and introduction to focus more on supplemental protection and resilience against new variants. Yes, I enjoy being playful, but sometimes I go too far. This is actually a serious topic, and I spend time on it because I want to contribute to ending this pandemic.

      • 2

        Thanks Eric, I have to admit that now it’s a lot clearer what you are trying to convey. I appreciate you caring about this.

      • 2

        “I guess MERV 12 it’s better than nothing, but I will look into the higher-rated filters, too.”

        Check the “Why MERV 13” section of this page, as well as the table above it. My impression is that MERV 12 is also a good choice. As you increase to the higher filtration ratings, the air resistance also increases, so you need to be careful not to increase beyond what your HVAC engines can handle.


        BTW, this topic seems to have been marked as spam again. I’m guessing that the spam filters ran again because I edited the post. Would you please mark it as “not spam” again?

      • 3

        Sorry Eric I think that was me who marked your post as spam by mistake! Sorry!!

        I just wanted to reply and say that that title does look better now, thanks for taking the time to change it. But no one can blame you for you using humor – I think a lot of people (such as myself) are just on edge as well as being wary of all the diverse, incredible-sounding, or straight up disinformation we are bombarded with nowadays. But I love that this community cares, and cares to get the facts straight! Even though I mostly lurk in here this site has been my refuge of sanity for the past two years.

      • 2

        No worries MainPugh, I thought that might have been a mistake and fixed it straight away 🙂

      • 1

        Phew, thank you!

      • 1

        Thanks for restoring my post. I’m also relieved to hear that marking my post as spam was just a misclick. After getting my post marked as spam on two separate days, I was starting to worry that TP’s spam filter just didn’t like me. 🙂

      • 1

        To be honest, sometimes our spam filter is a bit sensitive, especially if it’s a new user posting links etc. Please be patient if that happens, we try to fix things as soon as we can.

      • 2

        I actually just went to check and realized that our filters are only MERV 12, which our furnace is handling fine (they are the M3 Filtrete™ MPR 1550 Allergen, Bacteria & Virus Air Filters). When I got them at Walmart I didn’t know about the various types and just got the ones that were claiming to filter viruses. Now, thanks to this post, I am more educated!

        I guess MERV 12 it’s better than nothing, but I will look into the higher-rated filters, too. I see that M3 makes up to MERV 14 filters that are for residential use. Do you know by any chance what should I look for in my furnace if I wanted to use a MERV 13 or 14? The M3 website only mentions “residential use”, but can’t see any other obvious specifications. Thanks.

      • 2

        You might call your dentist’s office and ask. Everyone at our dentist’s office has been vaxxed, but in addition, the six or seven members of staff I’ve talked to since Covid started all caught the disease last year, and their children as well. They also observe a protocol to maximize patient safety. 

      • 2

        It’s really unfortunate that so many dentists caught COVID last year. It’s also not a surprise. They have a very high-risk job, spending substantial time just inches from a patient’s open mouth. Several of my coworkers had dental appointments and reported that dentists wore surgical masks for protection – not close to adequate. Even with infection followed by vaccination, they really need a combination of N95 (non-vented) and goggles.

        It’s true that I haven’t called to check. I haven’t seen a single other healthcare worker follow precautions that meet my standards, and was able to see doctors in other specialties only because I brought adequate protection of my own. Knowing that wouldn’t work for dentistry, I just focused on taking care of my own teeth.

    • 5

      Hey Eric, A++ for the effort you put into this forum post. I love the kit, great formatting, and you are sharing good ideas on how we can add another layer of protection and area of attack against this virus! Lets all do all we can to stop it from spreading and kill this thing off.

      I can also see someone maybe taking it on themselves to make a smaller version of this that they then could place on their desk at work to increase some protection around themselves. 

      I am not as covid savvy as many people on here but one thing I did hear is that being outdoors greatly decreases your likelihood of catching covid vs being indoors with stagnant air. Opening a window (gonna be harder to do come winter) or having a filtration fan like this will probably help more than having everything still and no air movement. 

      If anyone makes one of these, please share some pictures!

      • 2

        “I can also see someone maybe taking it on themselves to make a smaller version of this that they then could place on their desk at work to increase some protection around themselves.”

        That sounds like a good idea. The large filter pictured above is designed to spread its protection over the whole room. For a smaller, personal filter, I would aim the fan at the face of the person that it’s intended to protect. Someone gets a gentle breeze of clean, virus-free air that just went through a filter seconds before. I expect that some off-the-shelf products would do a great job at this. I’ll look around for some of those.

        “I am not as covid savvy as many people on here but one thing I did hear is that being outdoors greatly decreases your likelihood of catching covid vs being indoors with stagnant air. Opening a window (gonna be harder to do come winter) or having a filtration fan like this will probably help more than having everything still and no air movement.”

        Outdoors is great, not just because of air movement but because it’s so big compared to the number of people. If you let the viruses spread out evenly in the huge outdoors, no spot has enough viruses to hurt anyone. If you push viruses around fast indoors, it can actually make the problem worse, because you’re just moving viruses faster from infected people to non-infected people. What is helpful is either letting some of those viruses float out the window, or filtering them out of the air.

      • 2

        “If anyone makes one of these, please share some pictures!”

        Lots of pictures in this tweet from people that followed the same recipe.

    • 4

      I’ve looked more into this, and found some links that I found interesting and that might be helpful:


      Relevant statement: “When used properly, air cleaners and HVAC filters can help reduce airborne contaminants including viruses in a building or small space. By itself, air cleaning or filtration is not enough to protect people from COVID-19. When used along with other best practices recommended by CDC and other public health agencies, including social distancing and mask wearing, filtration can be part of a plan to reduce the potential for airborne transmission of COVID-19 indoors.”

      Results from a head-to-head test showing that a simple, taped-together DIY air purifier’s effectiveness is higher than a market-leading Swiss air purifier, IQ Air. Particles measured 0.5-micron: https://smartairfilters.com/en/blog/diy-air-purifier-effectiveness/

      More about its effectiveness: https://www.cbsnews.com/news/covid-air-purifiers-particles/

      This page claims that not all filters perform in the same way, so it might be worth keeping this in mind: https://encycla.com/Corsi-Rosenthal_Cube

    • 3

      It just occurred to me that this would also be perfect in case there’s a wildfire nearby, and you need to shelter in place! I thought that one could have the materials ready in storage and quickly put them together when needed. I’m going to link this to our wildfire guide 😁

      FWIW Experts suggest that if you are trapped at home or need to shelter in place, designate one interior room (pref. basement) to be closed off so no smoke can get in, but also to use air filters 💪

      • 1

        Taking shelter in the basement while the rest of the house burns sounds terrifying. If that’s the situation you’re thinking of using this air filter for, note that it’s very important to be able to run the fan on battery power, because the fire in the rest of the house will probably fry your house’s electrical system, and you may even lose power before the fire reaches your house due to power lines getting caught by the wildfire.

        For an immediate fire danger, I would choose a P100 half face respirator as my main air filtration, plus goggles for eye protection, and only consider something like a room air filter as a supplement. The respirator is portable, so also useful for bugging out, and requires no electricity. Their price is reasonable, and they’re ridiculously effective for filtering pretty much anything. A room filter would be a nice extra layer of protection, and also help with visibility, but would not be as flexible or as reliable as the respirator.

        On the other hand, if the fire is far away and just sending too much smoke as a nuisance, room air filters would be great for making that nuisance disappear entirely.

      • 3

        Carlotta Susanna mentioned wildfire.  We went through that last summer (western Oregon).  The fires never got within 7 miles of us, but the combined smoke of many wildfires throughout the state made it chokingly difficult to breathe outdoors for weeks.  Fortunately our house is “tight” and we were okay indoors without additional filtration.  But a drafty house would have been a nightmare. 

        I’m really impressed with this DIY project.  We usually have a big box fan directing warm and cold air in the house so the temperature is uniform throughout.  This project would be perfect for us – I’m sure we’ll pursue it.  Thank you for posting this project Eric!

    • 1

      Last year, Dr. John Campbell said in his online talks several times, that the UK motto was, “Hands, face, space, ventilate.” It surprised me that US sources didn’t mention the “ventilate” part very often. Thank you!

      • 2

        Yes, it’s odd that ventilation is so often overlooked. Respiratory viruses, like COVID-19, spread primarily through the air. Spreading via surfaces is also possible, but plays a much smaller role. Every business deep cleaning overnight was both wasteful, as well as a distraction from the changes that were really needed to stop this virus.

        CDC actually does advise ventilating buildings, and includes that advice in a wide variety of reports about how to protect various types of businesses. But I’m not sure that anyone reads those reports.

    • 2

      I just found a great visualization of how various precautions affect your odds of catching/spreading COVID-19. You can change the context (outside, small room, etc), turn different precautions on and off (masking, ventilation, plexiglass), and see how that affects each of the different ways that the virus can spread.


      It shows that high-quality masks, like FFP2, are very effective all by themselves, but that you can do almost as well by combining ventilation and distancing.

      Screen Shot 2021-12-02 at 5.53.43 PM

    • 1

      MedPage had an article on using improved air filtration in hospitals to prevent infection, and mentioned that the same would be helpful in other buildings as well. One of the commenters recommends replacing the filter with UV light to reduce maintenance costs, but unfortunately didn’t provide a reference for details.


    • 1

      “To stop COVID we must clean up our air”

      Eric Topol interviews Linsey Marr, an expert on using environmental engineering to prevent disease.


    • 2

      List of options for improving indoor air quality to prevent COVID, flu, RSV, etc. The DIY option I described in this post is listed first as the cheapest option to get started.